Need help to determine if floor can handle slate [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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02-17-2010, 04:02 PM

I am trying to determine if the floor in my laundry room meets the requirements for slate flooring. The room itself is small (8.8ft L x 5.2ft W) - we currently have a front loading washer/dryer in there with the washer creating quite a bit of vibration during the spin cycle. The room itself is accessible just off the mud room at the front of the house and drops about a foot below the main level. I have direct access to the floor joists under the laundry via the basement.

Some other notes:

The laundry room is in one corner of the house so two of the walls are sitting on the foundation of the house. The other 8.8L wall is sitting on partitioned wall in the basement which part of the stairs.

The joists under the laundry room run the length of the room. There are two of them under the laundry room floor.

The two joists are slightly smaller than the main floor joists and are:
o TJI 100C
o Approx 9ft in length
o 9.5inches in height
o 24inches from center to center

The subfloor is 23/32 OSB from what I can see from the basement. Haven't removed the lino yet to see from the topside.

Also, one end of the joists sit on the foundation of the house while the other end are in u-shaped brackets...joist hangars?

House was built in 2004.

I don't quite understand the L/720 rule which is why I am posting on the forums looking for help.


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02-17-2010, 04:23 PM
o 24inches from center to center

No way too would need to double the joist (12oc)and add another layer of 1/2 ply and then cement board to be safe..If you have 1/4underlay on top of the might be able to get away with not using the plywood(still need cbu though)

This should get you closer to l/720 but remember that spec is made to cover the weakest of stones. Slate being clefted horizontally makes it a pretty hardy stone.

02-17-2010, 04:30 PM
The joists are wood i-beam silent floor joists and quite bouncy now when the washer is going. I thought about putting another joist in there but there are all kinds of water lines, electrical wires running in the area directly under the room. [its over the furnace/hot water tank].

Can I get away with just another layer of 1/2 ply and then cement board? Or should I move away from slate altogether?

Slate is 1/2" thickness.

02-17-2010, 04:34 PM
You would need to contact the joist manufacturer to find out the deflection rating on the joists. I think one of the engineers here had some information that might be helpful to you.

The spacing on the joists could be another problem, but if the joists are too bouncy, it's a moot point. Find about about the joists first, then go from there. Slate is one of the softer stones and needs a very strong floor, so you don't want to take any shortcuts.

02-17-2010, 05:51 PM
Schluter says you can do slate with ditra or ditra xl on 24oc..but you need more plywood

about half way down.

02-17-2010, 05:53 PM
Thanks for taking time to respond to my questions. I'll check with the manufacturer of the joists and weigh my pros/cons after that.


02-17-2010, 09:40 PM
Your joist should be are close to the outside walls and have is the space between the joist where we are worried about.

What the longest unsupported span of the joist anyways? If I read rite and its 8 ft. don't even worry about that part. No since in confusing a confused secretary technical support personel.

John Bridge
02-18-2010, 07:10 AM
I think you need to strengthen the joists. I'm going to ask our resident engineer to help you. Please hang tight. He might be off scuba diving in Belize or hanging out in a bar somewhere. I'll page him. :D

02-18-2010, 09:49 AM
I would talk to the tech support folks. Their span tables say the maximum span for TJI-100 - 9 1/2 i-joists spaced 24 inches apart is only 12'-9" at L/360. I'm guessing that, at 8 1/2 feet span, your floor currently is about L/460-ish, since that's about the normal range for sizing I-joists. Since they don't publish span tables for L/720, a talk with a tech support engineer is necessary.

You definately will need more subfloor. A single layer of 3/4" OSB isn't good enough for slate. And, given your washing machine's impact loading, a second layer would be a must.

02-18-2010, 10:05 AM
What should the thickness be of the second layer? 1/2" ?

Thanks for everything so far!

John Bridge
02-18-2010, 11:34 AM
1/2 will work. 5/8 is better. And don't forget to talk to the I-Joist folks. :)

02-18-2010, 10:27 PM
Well, I contacted the builder and was told the floor will have no issue holding the weight of the slate as it was designed for it. Although, they did suggest cross bracing if I wanted to add more "rigidity". I've never seen cross braces on i-joists before?

They also mentioned there'll be K3 board under the lino which will have to come up and be replaced by a higher grade plywood (as suggested by you kind folks already.) I hear the K3 is a pain to remove as it is glued and screwed down...any tips for removal?

Regardless, I am hoping to reach the manufacturer of the joists ( on Friday to pick their brains as well.

02-19-2010, 12:18 PM
Spoke to an engineer at the joist company and he says the joists exceed the minimal builder requirements and will have not problem with the slate. He stressed my main focus will be on selecting an adequate subfloor to put over the existing OSB with a really good adhesive.

Been a good learning experience...thx

02-19-2010, 02:31 PM
You can use glue between the sheets if you want, but a full spread is recommended if you do. Most here simply screw them together with a lot of screws......I mean A LOT of screws.

No, really....A LOT.

02-24-2010, 10:04 AM
Last couple of questions:

Any particuliar type of plywood to use?
What size of flooring screws? 1 inch?
How should the plywood be cut/laid out? Does it need to be all in one piece? Or can it be cut in multiple pieces?
If I cut into multiple pieces, should I butt the ends together or leave a bit of space in between pieces?
Should the joins of the plywood run parrallel (sp?) to the joists or perpendiculiar?
When screwing the plywood to the osb subfloor, should I avoid putting screws along the joist lines?

Thanks again,

02-24-2010, 11:57 AM
Take a look at these two posts

I used 1 5/8" deck screws to attach my top layer of 1/2" plywood to 3/4" OSB, spaced every 6". I don't think it matters if the screws fall along the joist lines, but you don't want to be screwing second layer into the joists.

02-24-2010, 12:33 PM
1) Exterior rated plywood with face grades no less than C, or OSB Exposure 1.

2) Your screws should penetrate the lower layer by at least 1/4".

3) Layout the plywood so it spans at least 3 joists in as few pieces as possible.

4) Leave 1/8" gap between pieces.

5) The face grain of plywood and the long direction of OSB should run across the joists.

6) Yes, but don't sweat it if you hit one.

02-24-2010, 01:45 PM
Since those are I joists, you can add a lot of stiffness by placing a piece of osb or plywood on the face of the I-joist between the top and bottom web. I think on my I-joists the tech sheet said to cut the plywood 1/8" narrower than the space between the top and bottom web and then butt it up against the top web. Of course if you have a lot of pipe and wire penetrations thru the web, that would be a show stopper.

02-24-2010, 03:06 PM
Kent, was that detail intended to stiffen the web against crushing (like under or over walls) or to reinforce where something penetrates the web. It won't do much to improve bending stiffness.

03-15-2010, 11:02 AM
Hey all,

Thanks again for all the great tips and info!

Things have been moving along (slowly) over the past several weeks as I pick away at this project in between work and triathlon training.

I have to say, I really despise K3 board...that stuff was a major pain to tear up with it being stapled and glued to the floor. I just took my time, and managed to get it all off without destroying the osb subfloor underneath.

Finished laying my new 5/8 plywood subfloor this past week. I get excited during the building phase!

One additional question, should I put some type of water resistant coating on the plywood before I start laying my slate? Or will coating the slate/grout with water resistant stuff be enough?


03-15-2010, 11:45 AM
You need some sort of underlayment between the plywood subfloor and your slate. Backerboard, membrane, etc. The plywood or OSB should be Exterior rated or Exposure 1 rated, so it is waterproof enough for the thinset you need to install backerboard or Ditra. Noble's membranes use an adhesive, but it still needs a proper substrate.