Bunch of Slate Questions [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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Mark Duddridge
02-19-2008, 05:53 PM
Hey everybody,

I just started the "Aldon Products" thread but now after reading a couple of other threads I have another question I was hoping you could help me out with.

I'm installing slate in three rooms.

1. 100 sq. foot front entry
2. 50 sq. foot bathroom
3. 50 sq. foot ensuite

The floor is 5/8" OSB.

The joists are 2"x10" P.T. Joists with 16" O.C. with X-Bridging mid-span.

I haven't researched this part yet, but I understand I'll need to add some sort of backerboard (3/8" concrete???) on top of my 5/8" OSB

My question is whether the combination of the 5/8" OSB and 3/8" backer board will be a sufficient subfloor combination to support the slate?

Thanks in advance for your help.


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Brian in San Diego
02-19-2008, 06:13 PM

In a word, no. Let's talk deflection. First of all, deflection is the enemy of a sound tile installation and it comes in two forms, deflection along the length of the joists and deflection between the joists. You have left a critical number out of your measurements and that is the span of the joists. How long is the longest unsupported span over which you plan to lay this slate. Use the numbers you have provided in the appropriate places using the Deflecto tool in the dark blue toolbar above. That will tell you if your joists are sufficient to install stone over. Once that's been determined then we can address the between joist deflection. In a nutshell, you will need a minimum of 1/2" (and 5/8" would be better) plywood over your existing subfloor and on top of that you'll need to install either a hardiebacker type product or a membrane such as Ditra from Schluter Systems.


02-19-2008, 06:16 PM
Keep in mind that the unsupported length is not the room size, it is what's underneath. Support could be a beam, an outside wall, or a load bearing wall.

Mark Duddridge
02-19-2008, 06:45 PM
Quick clarification question for using the Deflecto tool...

Is joist length the entire length of the joist, or just the portion under the floor in question, or just the portion of joist that is "unsupported?"

In my case, the length of the joists under the area I am laying the slate (the largest of the three rooms) is only 10'. However, the entire length of the joist is 30'. That 30' span is supported at 9' (under the area I am laying the slate) by a 3 ply 2x10 beam supported by two teleposts. So... I'm not sure whether to use 9', 10', or 30' as my joist length. Here are the "Deflecto" numbers I get for each:

9' = 0.100 inches or L / 1083
10' = 0.140 inches or L / 857
24' = 3.252 inches or L / 89 (24' since 30' isn't supported by Deflecto)

Since Deflecto states it is the longest joist length between supports, I'm guessing I should be using the 9' number. Is this correct?

Brian in San Diego
02-19-2008, 06:54 PM

The area you need to be concerned with is the area on which you will be laying tile. Let's use your numbers as an example. You have a 30' joist (that must have been an old tree) which has a support at 9'. If I'm reading you correctly that means you have 9' on one side as an unsupported span and you have 21' on the other side as an unsupported span. If your room was 13' wide, part of the tile job would be on the 21' unsupported span and for 2x10s that would not be a good thing. If the total area being tiled is inside the 9' dimension then you are golden, if part is in and part is out then that's not so good and some type of remediation would be needed IMO.


Mark Duddridge
02-19-2008, 07:02 PM
My area is "inside" the 9' area but, since it is 10' long, sticks over it by 1'.

Am I still okay since only 1' is sticking over the support?

Brian in San Diego
02-19-2008, 07:10 PM

That's a difficult question to answer. The problem that may arise is similar to a lever. When weight is put out in the middle of the "long" span the ends may want to act like a lever. I know over the support the joist doesn't actually end there, but there can be flexing at that point. I wonder if CX or Bob (bbcamp) are looking in, they would be far better versed in telling you yea or nay on that one.

What is under this long joist? Could support be put in somewhere between the 9' support and the 21' length?


Mark Duddridge
02-19-2008, 07:21 PM
Your question draws attention to a mistake I realize I made when describing our joists to you. The house is a bi-level, so the area in question (foyer) is actually it's own level. So... 10' represents that "floor" in it's entirety. The support at 9' supports that entire floor, and from that floor rises the stairs to the main floor and drops the stairs to the basement.

Sorry... novice's error in reading 2D blueprints! Bit stupid seeing as I've spent countless hours working on this project!

Brian in San Diego
02-19-2008, 07:31 PM

If that's the case then you should be o.k. joist deflection wise. You state these joists are PT...pressure treated? Do you know if they are KDAT...kiln dried after treatment? I want an engineer or home builder to weigh in if they could cause a movement problem as they dry (if they aren't kiln dried).


Mark Duddridge
02-19-2008, 07:40 PM
Not sure about the "KDAT."

I know they are "No. 2 or better." Would that signify KDAT?

Brian in San Diego
02-19-2008, 07:44 PM

No, the no. 2 or better would signify the quality of the lumber. You're sure they are pressure treated...kinda greenish in color with marks along the entire length that look like they were put there with the end of a straight screwdriver?

I pinged one of the mods to have a look but it may take until tomorrow.


Mark Duddridge
02-19-2008, 08:23 PM
I put down the "PT" because that is what was on the plan, not what I actually observed (I was at work, not the job site when I submitted the post). I did not realize "PT" meant pressure treated.

No... they are not pressure treated. The look like "normal" 2 x 10s to me.

Mark Duddridge
02-19-2008, 08:40 PM
The other issue is the other two rooms in question. Both of them are bathrooms, identically sized (5' x 10'). They will each have 5' x 7.5' of slate in them. One bathroom is located right next to the wall backing onto our attached garage, the other bathroom is right beside it (they share a plumbing stack). So... the furthest edge of the furthest bathroom is 10' from the exterior wall. The joists under the two bathrooms are 30' long and are supported by two beams, one at 12' and one at 25' (measuring from the "garage wall"). So... the longest unsupported section of joists under the two bathrooms is 12'. The Deflector tools gives me the following stats:

"For joists that are SYP or Douglas Fir, in good condition, 9.25 inches tall, 1.5 inches wide, 16 inches on center, and 12 feet long between supports, the deflection calculated is 0.258 inches. This translates to a deflection of L / 558. Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Ceramic tile, Congratulations!"

I suppose that means I could have problems in the bathrooms with slate as opposed to tile?

Even if the joists were adequate, increasing the thickness of my subfloor will make a really weird elevation change between the hardwood in the hallway and bedroom and the slate in the bathroom. Hardwood is 3/4" and your suggested 1/2" plywood plus Ditra (how thick is Ditra?) plus 1/4" tile means the tile will stick up above the hardwood as high as the Ditra is thick.

Thanks, by the way, for all your help this evening. You've been amazing!

Dave Taylor
02-19-2008, 08:56 PM
I did a rather quick review of your posts .... and paid some attention to the last.

Ditra is about 1/8" high installed.

Consider using ceramic slate look-a-like...... I've seen nice stuff from both box stores and flooring stores.

This may work better than natural stone with the flooring you have now.

Hope this helps,

02-19-2008, 10:16 PM
Welcome, Mark. :)

Not sure I understand all I know about this project, but it looks like y'all have sorted out most of the questions Brian invited me over here to shed darkness on.

I'm still wondering at these alleged thirty-foot long 2x10s. These are really single sawn boards of that length?

I agree with Dave about seeking out some ceramic tile for these rooms. Unless you're willing to get pretty serious about beefing up the structure.

For any natural stone you choose, you'll absolutely hafta install a second layer of plywood, as has been pointed out. And that layer would need to be a minimum of 5/8ths-inch to my thinking.

But even for ceramic tile your subflooring is the absolute minimum possible for a tile installation using any tiling substrate on the market. No way I'd tile that without some more subflooring. You could go down to 1/2" or even 3/8ths" (which I also don't like) for the second layer, but I'd want something more than you have.

And keep in mind that when the CBU and membrane manufacturers test their products to support their claim of subfloor requirements, they are testing over joists with zero flex, using new material, perfectly installed.

My opinion; worth price charged.

Mark Duddridge
02-20-2008, 12:03 AM

With regards to the 30' joists... I'd have to check tomorrow when I'm at the site. Good chance they consist of two joists that join together at the central beam supporting them.

Mark Duddridge
02-20-2008, 12:21 AM
I'm afraid I'm getting a bit confused with the terms being used here...


I've got 5/8" plywood right now... what do I need to add on top of it to be safe?

5/8" Plywood (existing) + 5/8" Plywood + Membrane (Ditra) + Thinset + Slate

Or can I get away with...

5/8" Plywood (existing) + 1/2" Plywood + Membrane (Ditra) + Thinset + Slate

Or are neither of these correct?

Mark Duddridge
02-20-2008, 12:22 AM
Oh yeah... and where does "backerboard" fit into the equations above?

I've got Tom Meehan's "Working With Tile" book sitting open in front of me... and I'm afraid it's not yielding the answers and definitions I need!

Brian in San Diego
02-20-2008, 07:09 AM

Your first scenario would be better than the second, but I am of the opinion the second would work, especially if you use Ditra.

Some definitions vary slightly but for all intents the first layer of plywood over the joists is called your subfloor. Some people call the second layer the subfloor as well but the term used by the Tile Council of North America is "underlayment". So in your case the 5/8" OSB is your subfloor and the layer you add to it will be your underlayment. Whatever you put on top of the plywood to set on would also be called an underlayment.

Substrate is the material to which the Ditra or membrane is being bonded (in your case, plywood). If your were on a slab, the substrate would be concrete.

Membranes are made by many manufacturers and have various features and benefits. They are used in place of "backerboard" or CBU (cemetious backer unit). One or the other needs to be applied over plywood substrates to facilitate the application of tile.

Hope that helps.


Mark Duddridge
02-20-2008, 07:52 AM
Thanks Brian. Your clarification certainly does help. I'll make my way to the jobsite today and figure out how thick I can go without running into door clearance trouble. If possible, I'll go with scenario #1 above.

Thanks for all your (and everybody else's) help. It's very much appreciated. You guys have an amazing forum going on here.


02-20-2008, 08:00 AM

If it was me, I would take one step back and check the plans and actual construction very close along with fire code information from the building department. You have refered to the plans, so it sounds like it may be an extensive rehab or new construction area. Many towns require 1.25 to 1.5 inches of wood to meet fire code. Our code states 1.5 inch total wood. So we have 3/4 of plywood with hardwood covering and 1.5 under carpet.

Also from my experience, I would always sister an extra joist, every other, for any joists 18 ft or longer. I also chose to do this for the floor joists under our master bath due to the weight of a large volume tub. The room has 15ft joists. The sistered joists reduce deflection.

If the rough carpentry is still "open", I hope this helps! If not, stow the info away for your next project.

Mark Duddridge
02-26-2008, 02:04 PM
Me again!

I'm getting some kick-back from my finishing carpenter who isn't happy with how high my slate is going to be with the different layers I'll be applying through installation.

Here is what I was planning on putting on top of my 5/8 OSB subfloor:

Plywood - 1/2"
Ditra - 1/8"
Thinset - 1/2" (to accommodate the varying slate thicknesses)
Slate - 1/2" (some are a bit thinner, but most are 1/2")

Total = 1-5/8"

That's really thick! Especially when compared to my 3/4" hardwood that is butting up to it.

The problem I'm going to run into is (a) in my front entry where I only have 1-1/2" clearance between my subfloor and the door and (b) in my bathrooms where not only will there be a 7/8" difference between the hardwood and the slate, but also an issue with trying to get my bathroom doors up high enough to clear the slate.

So... my question is whether there is any way I could reduce the total thickness of my layers to no more than 1-3/8" (1/4" less than I'm showing above).

02-26-2008, 10:23 PM
The half-inch of bonding mortar sounds excessive to me. If most of your slate is 1/2" thick and some thinner, there should be no need at all to have a half-inch of thinset under the thicker tiles. You could gain your quarter inch and more right there.

There should be no problem cutting the bottoms of interior doors to accommodate the tile installation.

My opinion; worth price charged.

Mark Duddridge
02-29-2008, 11:17 AM
Thanks, CX. I'll reduce the thickness of my mortar if necessary.

Appreciate (as always!) your help.


Mark Duddridge
03-19-2008, 10:29 AM
I am planning on laying slate in two bathrooms in a house I am currently building. The "stats" are as follows:

Area: 5' x 7' (per bathroom)

Subfloor: 5/8" OSB + 1/2" Plywood (1 1/8" total thickness)

Decoupling Membrane: Ditra

My problem is with deflection. Here's what I got out of the Deflecto tool:

"For joists that are SYP or Douglas Fir, in good condition, 9.25 inches tall, 1.5 inches wide, 16 inches on center, and 13 feet long between supports, the deflection calculated is 0.341 inches.

This translates to a deflection of L / 458.

Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Ceramic tile, Congratulations!"

So... as per these results, I have too much deflection for slate.

At this point, I may be a bit too far along to switch to tile (I've already purchased all of the slate and can't return it). If it were possible, I would have simply reinforced the floor under each of the bathrooms by screwing a 2x10" to each existing 2x10" joist to double the joist thickness from 1.5" to 3". Unfortunately, there is so much plumbing and electrical running through that area that I don't have the room to do so.

My questions are as follows:

1. How much of a risk would I be taking by installing slate on a floor with this much deflection?

2. What is likely to happen if I do?

The orientation of the bathrooms relative to the joists (running vertically on the diagram below) is as follows:

| _________________
| Bath # 1
| _________________
| _________________
| Bath #2
| _________________

Again, the area for each of the bathrooms illustrated above is 5'x7'

Thanks in advance for your help.

03-19-2008, 10:56 AM
you can add another layer of 1/2" plywood and then the Ditra and you'll be fine.

Mark Duddridge
03-19-2008, 11:11 AM
If I add an additional 1/2" of plywood, my slate would be 1 1/4" higher than my hardwood (which it butts up against in the entrance of the bathrooms.

Hardwood = 3/4"

Slate = 2"
-Plywood #1 = 1/2"
-Plywood #2 = 1/2"
-Ditra (including thinset) = 1/4"
-Thinset under slate = 1/4"
-Slate = 1/2"

Difference = 1 1/4" - That's a really significant height differential - one that I would really like to avoid.

03-19-2008, 11:24 AM
I think Schluter would be happy with your 5/8" OSB and 1/2" plywood. Ditra is 1/8" thick when installed. That gets your height difference to a more manageable 5/8".

I don't think you can ignore the joist stiffness, though. Can you shorten the span by building a closet below the floor joists? One that's 2 feet deep and built like a load bearing wall will do wonders for your joist stiffness.

Mark Duddridge
03-19-2008, 11:43 AM
The proposed plan for my basement would include a wall under the joists in question, but I do not intend to actually finish the basement out before I sell the home in a year or two.

As shown in my diagram above, the width of the bathroom floor that runs along the joists is only 5'. Would there really be enough deflection along a 5' span to cause me problems (especially with Ditra and a 1 1/8" subfloor)?

Mark Duddridge
03-19-2008, 11:48 AM
Let's assume for a moment that I am unable to reinforce my joists beyond their existing state. If that ends up being the case...

Is there a pretty good chance I'll experience problems, or at a deflection of L/458 is there good chance I'll be okay?

If I where to experience problems, what problems are most likely to occur?

I know there are no "guarantees," but I'm trying to get an idea of how much risk I would be taking here.

Brian in San Diego
03-19-2008, 12:40 PM

Because I remember you from this thread (http://www.johnbridge.com/vbulletin/showthread.php?t=59613&page=1&pp=15) I'm confused as to what you are asking about now. Didn't many of these same questions get answered in the previous thread or is this a whole new house? Are you separating each area as separate projects now? If it's the same project then it should be on the same thread.

I'll defer to Bob on possible problems but I think it may involve cracking grout or perhaps tiles coming loose.


Mark Duddridge
03-19-2008, 12:56 PM

Yes... same house. Probably should have used my previous thread, but no, my questions specifically regarding the bathroom deflection problem were not answered in that thread. There were so many issues being addressed in that thread that I thought I would start a new one to specifically address the question of the deflection in my bathroom floors.

As an FYI... I later found out that the 10' by 10' front entry landing that I am putting slate down on is only supported by 2x6 joists, not 2x10 joists like the main floor of the house. I'll be solving that problem by placing an additional beam under the landing. That will leave me with only 5' of unsupported span which, according to the Deflecto tool, will leave me with more than enough support.

Mark Duddridge
03-31-2008, 01:50 PM
Hey everybody!

I'm installing slate in three rooms (front entry, bathroom, ensuite) in a house I'm building, and I'm up to the "laying" stage. So far I have...

1. Reinforced my subfloor to get a sufficient "deflection" rating
2. Added 1/2" plywood to my 5/8" of OSB
3. Layed Ditra on all respective surfaces using a modified thinset

Before I get started on the next stage, I have a few questions I was hoping I could get clarification on:

1. Do I need to seal the joints between strips of Ditra? I know you need to seal the joints in a high moisture area (shower), but do I need to seal the joints if the surface will just be occassionaly wet (walking in with muddy shoes, stepping out of the shower, etc.)? If I do need to seal the joints, what should I seal them with?

2. What is the recommended spacing between slate? 3/16, 1/4, 5/16?

3. Given the varying thickness of slate, I've heard you should use a thicker bed of thinset when laying the slate (in addition to backbuttering). I've heard some guys use a 1/2" moon shaped trowel, and other guys use 1/4x1/4 notched trowel. Which should I use? I'd prefer 1/4" notched because I'd like to keep my height down, but I'll use a 1/2" moon if it's necessary.

4. How thick/thin should I mix my thinset to? I've heard comparisons from mayonnaise to oatmeal to peanut butter... all of which have different consistencies! What's the best way to figure out how thick/thin you should be mixing it to?

5. Should I be using a sanded, unsanded, or epoxy grout?

Sorry for all the questions, and thanks in advance for your help.



03-31-2008, 04:00 PM
i'll answer a coupleof your questions.

#2 i just did a slate shower with 1/4" grout lines. look in the Pro section there are some pics.

#5 1/8 " or less unsanded greater than 1/8" use sanded grout.

I sealed my slate prior to grouting. helps in keeping things clean.


Scottish Tile and Stone
03-31-2008, 07:17 PM
1. Not necessary.
2. The quality of your slate will dictate grout spacing. Lay a few from several boxes out to get you grout spacing.
3. Use atleast a 1/4x3/8 trowel with granite and marble thinset. It will help keep them level to each other.
4. When you mix your thinset, trowel some on the floor, the ridges left by the trowel should stand up and not sag.
5. Sanded grout..

Brian in San Diego
03-31-2008, 09:30 PM

This is the third thread you've started for the same project. The forum has a "one project, one thread" protocol. Please don't start new threads when you get a new idea or question, please. Ask all your questions here from now on, O.K.?


03-31-2008, 09:49 PM
Mark, I've combined your project threads here. As Brian points out it's a lot easier for our people to keep up with what you're doing and what's already been asked and answered if you just ask all your questions on a single thread. Bookmark this one so you can always find it again. Threads never get too old that they don't rise to the top of the queue with each new post. :)

Mark Duddridge
04-01-2008, 08:23 AM

My apologies. Most of the online forums I participate in operate under a "new-issue-new-thread" policy... especially when the previous related threads get old. I'll make sure any future questions are posed via this thread.

Mark Duddridge
04-01-2008, 08:32 AM
Thanks for all your answers, Hammy and Scott!

I've got some more questions that popped up when I was at the site yesterday...

6. The slate in the bathrooms butts up against my bathtubs... can I leave about a 1/4" space between the slate and the bathtub and then just fill in that space with grout, or is there something special I've got to do against the bathtub?

7. Is it better to first lay every piece of tile out, do all your cuts, and then start laying, or is it better to do your cuts as you lay (ensuring you don't spread too much thinset at once)?

8. I was at HD picking up a grout float, and they sold ones that had a soft foam rubber base and then some that had a plastic base with a layer of spongy cushion between the plastic and the handle. Which is more appropriate for sanded grout?

04-01-2008, 09:11 AM
6) Leave the gap, but fill it with caulk.

7) It's a matter of preference. However, if your cuts must slide under undercut door casings or similar situations, you'll want to have the cuts ready before you set the field tile.

8) The hard floats are for epoxy grout.

04-01-2008, 09:28 AM
Most of the online forums I participate in operate under a "new-issue-new-thread" policy... especially when the previous related threads get old.Is the same here, Mark, but we treat your current project as one "issue." The history is helpful and keeps our regular helpers from having to ask a lot of questions for clarification that have already been covered.

Threads never get any older than your project. Some take years, which we fully understand, but the history is still applicable. :)

8. Some of us like those hard "epoxy floats" for a lot of other grout applications, too.

My opinion; worth price charged.

Mark Duddridge
04-01-2008, 09:40 AM
So with respect to my grout float, which is typically preferred for applying grout to slate (uneven surface), the softer foam floats or the harder "epoxy" floats?

04-01-2008, 10:35 AM
Teh softer float will allow you to remove more grout and follow the undulations in your slate's surface. The harder float will tend to give you fuller joints.

04-01-2008, 10:36 PM
Are you going to seal your slate before grouting? Hammy

Mark Duddridge
04-03-2008, 08:29 AM

I am going to be using a product called "Grout Easy" from Aldon (http://www.aldonchem.com/pr-grout-easy.htm.


04-03-2008, 04:42 PM

I am going to be using a product called "Grout Easy" from Aldon (http://www.aldonchem.com/pr-grout-easy.htm.


Good deal. I think youll be glad you did. Hammy

Mark Duddridge
04-08-2008, 10:59 AM
So... I've got the slate laid in two out of three rooms and just started on the last one last night.

In places where I was unable to get the slate's edges to line up due to thickness inconsistencies, is it a good idea to take an angle grinder and grind down the edges so they match better?

I'm assuming the grout will help "soften" the transition from one piece of slate to another, but I was wondering if I should help soften a couple of the pieces with a grinder.

Anyone have any recommendations regarding this?

Mark Duddridge
04-08-2008, 12:56 PM
Another question...

If any of you recall, earlier in this post I asked some questions about some concerns I have regarding the deflection in my two bathrooms. The Deflecto results I get are as follows:

For joists that are SYP or Douglas Fir, in good condition, 9.25 inches tall, 1.5 inches wide, 16 inches on center, and 13 feet long between supports, the deflection calculated is 0.341 inches.

This translates to a deflection of L / 458.

Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Ceramic tile, Congratulations!

Since I couldn't see any way of reinforcing the joists through sister joists or a supporting wall, I decided to take the risk and install the slate with the L/458 deflection.

Then... after spending hours laying all that slate, making it look beautiful, and falling in love with it... I realized that I couldn't stand the floor failing, and I decided to build a wall underneath it, even if it would look unsightly and awkward in the basement (not good when trying to sell the home).

But then... I found this thread: http://www.johnbridge.com/vbulletin/showthread.php?t=54371

Solving my deflection problem by simply gluing/screwing 2x4s underneath each of my joists would be ABSOLUTELY IDEAL. Is there any reason this approach wouldn't work for my situation?

Assuming Plainrider is correct in his calculations... my deflection would increase from L/458 to L/917... more than enough to meet the L/720 required for natural stone (slate, in this case).

Is this my "magic bullet?"

04-08-2008, 01:54 PM
Mark, that method will boost your deflection rating, but by how much, I don't know. What you would be trying to do is to reproduce an engineered I-joist in the field. There are a lot of variables that you have to deal with, some are totally within your control, others are not. In your control: wood species in the new wood, choice of glue, number and spacing of nails/screws, floor loading during installation. Not in your control: wood species of existing joist, condition of bottom edge of joist, floor loading during installation (if your have the slate installed, you cannot un-load the floor for fear of cracking something).

To achieve the 100% increase in joist stiffness, the wood species in the 2x4 must be at least as stiff as the old joist. Given the condition of the 2x4s you normally find, that's probably not gonna happen. The glue joint must be able to transmit all of the horizontal shear that will be applied when the joist is loaded. If the bottom of the existing joist does not present a suitable glue surface, then your are relying only on the screws. Metal fasteners in wood will slip as the load is applied and wood fibers become crushed. This may take some time to be realized, and can be abated by using plenty of screws to spread the load. However, if the joist has a bad case of "wane"
(bark or lack of wood in the corner of a board), then you won't have a good place to attach the 2x4. Unloading the joist is, in my opinion, necessary to ensure that the new 2x4 adequately takes up it's portion of the load. To do that effectively, the joist must be jacked up slightly and held in that position until the glue sets. When the jack is removed, the 2x4 becomes loaded with some of the dead load the joist was carrying, so that when live loads are applied, the composite joist is uniformly loaded.

All this is not meant to discourage you from using this method for stiffening your floor. I just want you to understand that there's no real way to quantify the effect you'll get from doing it this way.

Mark Duddridge
04-08-2008, 02:24 PM

Thanks for your comments. With respect to them...

Wood species in new wood - I'll be sure to get #1 2x4s.

Condition of "old" joists - The house is brand new (still under construction), and the joists are "#2 or better." I've taken a look at them, and they look to be in perfect condition.

Choice of glue - Plainrider recommends PL Construction Adhesive, so that's
what I'll use

Screw spacing - Plainrider used 10" spacing. I'll do the same.

With respect to floor loading, I don't think the potential benefits of relieving the floor loading are worth the risks (cracking the existing tiles or causing cracks in my drywall). Here is what Plainrider had to say about this when Brian (Brian in San Diego) asked about the issue of floor loading:

Unloading the joist before adding the bottom flange would provide limited improvements - even though it would be easy to do.

Adding a bottom (2x4) flange to the joist before removing dead loads on the floor tends to "lock in" the existing floor deflection (which would be due, in part, to the existing dead loads). Because floor joists are stressed to a relatively small portion of their strength limit from dead loads (generally), floors are not unduely deflected downwards by the existing dead loads i.e. for typical situations. The bottom flange re-enforcement of the joist immediately increases the stiffness of the floor joist to the same extent whether or not the dead loads have been removed. It is this floor "stiffness" consideration (not strength or prior deflection) that is the important issue regarding eventual ceramic tile or stone cracking.

However, if for any reason the existing floor is noticeably curved downward (e.g. due to dead loads) then the removal of the dead loads before adding the flange will permanently help to reduce or eliminate the (unsightly) floor deflection. This is an esthetics consideration.

I don't actually think there is a lot of floor loading yet. The only thing that's been laid down is the slate (less than 50 square feet in each of the two bathrooms). There's no furniture, cabinets, and very limited foot traffic.

I don't really need to necessarily "quantify" exactly how much floor stiffness I'll get, I just need to be reasonably certain that this method (given the conditions I describe above) will give me enough stiffness to meet the L/720 rule and thus help avoid floor failure in the future.

Mark Duddridge
04-10-2008, 10:19 AM
Just wondering if anybody had any advice on my question back on April 8th regarding grinding down sharp/uneven slate edges:

"In places where I was unable to get the slate's edges to line up due to thickness inconsistencies, is it a good idea to take an angle grinder and grind down the edges so they match better?

I'm assuming the grout will help "soften" the transition from one piece of slate to another, but I was wondering if I should help soften a couple of the pieces with a grinder.

Anyone have any recommendations regarding this?"

Mark Duddridge
04-10-2008, 10:36 AM
Another question... this one just out of curiousity...

I know the cost of installation would vary dramatically from one region of the country to the other, but can someone give me a sense of what the pros usually charge to lay tile or slate.

For an example... what would a typical (if "typical" exists) cost per foot be for my project (2 bathrooms -- 50 sq.ft. each plus front entry -- 100 sq.ft.)? My project has required that I lay down an extra 1/2" of plywood, lay the ditra, lay the slate, and then grout and seal it.

Mark Duddridge
04-21-2008, 11:02 AM
With the help of the wonderful people on this forum, my project is progressing nicely!

Last night, I finished grouting the last of the three rooms. Instead of presealing the slate, I used a product called "Grout Easy" from Aldon Chemicals (http://www.aldonchem.com/pr-grout-easy.htm). The product appeared to work very nicely as I had no problem getting all of the grout off the surface of the slate. I paid special attention to ensuring no grout remained in all of the many uneven areas of the slate. The process I used was as follows:

1. Applied two coats of Grout Easy (30 minutes dry time each coat)
2. Applied the grout
3. Sponged the grout
4. Went back and removed any residual grout from cracks with a brush

So... my grout lines look terrific, and there is no grout on the slate's surface, but the slate's surface is covered with grout haze. I have several questions regarding the removal of this haze.

1. When should I try removing it? Should I give the grout time to completely harden (72 hours, perhaps) so I don't damage the grout lines by scrubbing the haze off?

2. Should I just use water, or should I use some sort of cleaning solution (chemical, vinegar, etc.)

3. Should I wash the surface gently (to help protect the grout), or will I need to really scrub it?

Thanks in advance for your help!