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Unread 05-09-2020, 10:08 AM   #1
jimmartz
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Floor Structure

Hello,

I got a late 1800s home and was wanting to do tile, and
that is what I am figuring out now. But have some questions.

I see that there is a deflection calculator, but maybe the numbers
are a little low. I see that it looks like 50psf is used as the load,
in the calculator, is this correct?
From other stuff I read normally just the live load of 40psf is used.
So, for deflection of the floor beam for installing tile, should
just 40psf be used?

Moving on to stiffening the beam, I see that adding a 2x4
to the bottom of a current size 2x8 increases the moment of inertia
from 47.6 in^4 to 116 in^4 (provided it is glued/screwed solidly to the joist).
I was able to come up with those same numbers

But, unfortunately I got pipes mounted under the joist. And also I
have a lot of stuff(mostly wiring) going through the joist.
So, sistering or adding the 2x4 to the bottom would not be easy.

So, I got an idea, I can't fit a 2x4 on the bottom of the joist, but I could
fit a strap of steel on the bottom of the joist, between the joist and
the pipes. I see that Simpson Strong-Tie has a 14 gauge 1-1/4" strapping
(CS14). Using this I figure would increase the moment of inertia of a current
size 2x8 from 47.6 in^4 to 67.9 in^4, about a 42% increase.
Has anyone used this method to stiffen joists?

In my case, my joist are 1.75 to 2" by 7.5 to 8", yes somewhat random.
Using the lower size end, 1.75x7.5 with my span of 13.3ft, I can get an
deflection of about L/400 if using a load of 40psf. By adding the strap
I should get this up to around L/550. At least in theory...

Thanks for any help.
Jim
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Unread 05-09-2020, 10:29 AM   #2
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Welcome, Jim.

I've moved you to a project thread of your own from the general thread in the Hangout. There is some discussion in that thread, I believe, of using steel strapping to improve joist deflection.

There was one company that made a "system" using such a method bolted through the joist at each end that we (TYW) tested and found wanting due to creep after only a few months.

I can't verify your calculations because we don't know the species or grade of your joists, nor even the joist spacing, but your number sound a bit optimistic to me at a glance.

Are you planning to use a ceramic tile on this floor? If so, the deflection requirement is L/360 and if you've already got better than that according to your numbers, why are you making the effort to improve the structure?

Our Deflectometer, by the way, uses a combined live and deal loads of 40 and 10psf in its calculations. Somewhat conservative, but designed that way because most of our visitors are working on old construction rather than new.

My opinion; worth price charged.
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Unread 05-09-2020, 03:43 PM   #3
jimmartz
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Thank you for the quick reply.

Yes, my values was somewhat optimistic.
I understand why the TYW calculator is simplified and
conservative values. With it I can get L/382, but that is with
SYP and actual 2"x8". (16" on center, 13.5' length).
Because my joist are an odd size, 1.75x7.5 for the smaller ones,
I am using this calculator:
On "Canada Builds" website, canadabuilds dot com
It is not as simple, you need to convert psf to plf based on
the joist spacing and psf (I used 16" on center and 40psf = 53.3plf)
And it looks like I was using 1.6 for modulus of elasticity(optimistic).
If I use 40psf and 1.4 for modulus of elasticity I am right at L/360.
Add in a little bit of "butchery" done to the joist over the years, and
I am under the L360 mark. So that is why I feel it needs some
stiffening, and the thinking that if the joist are a little stiffer that it
may compensate some for the subfloor. (although I realize that is
only partly true).

The subfloor is 7/8"x5 to 6"(random width again) T&G planks
laying perpendicular (not diagonal). I would like to screw then down
good with good construction screws. Then flatten with mortar, not
level, that would be to much... And then 1/2 cement board embedded
with thin set. I know the only recommend thing that I know of is 1/2
of plywood on planks, but then the floor gets to thick. I still have not
figured out what I am going to do here, but ultimately put down those
6" x 24" or 36" porcelain tiles that look like wood, per my wife's request.

Thanks for the help.
Jim
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Unread 05-09-2020, 04:56 PM   #4
jadnashua
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YOu cannot flatten your planks with mortar, nor can you (successfully, anyways) install cbu over it. YOu need at least 1/2" plywood installed over the planks first. IF the planks are cupped, you could plane them down to help provide a flatter surface. Any mortar you might put on the planks would likely shatter as you later tried to drive screws into it, and give you that wonderful crunching sound as you walked over it! IT works with cbu because you do it while the mortar is still pliable (i.e., not set).

Actually, installing the planks perpendicular to the joists is preferred as that makes the actual span shorter and makes the subfloor stiffer.

If you want to keep the subflooring height down, you could take out the planks, and true up the joists by either planing or cleats so it's flat (level is nice, but tile need at least flat (possibly making it easier to add some stiffening to them in the process), then glue new ply to them.
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Unread 05-09-2020, 05:30 PM   #5
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Might help if you'll put a geographic location into your User profile, Jim.

There is a dramatic difference in design deflection between dimension lumber that is a full 8" deep as compared to nominal lumber which would be 7.25" deep. Tough to really average the design deflection if you have different size framing lumber. And the species and grade can make a very large difference as well.

Are you in a position to add a support beam under the existing joists?

The only options in the US tile industry standards for tile over sawn board subflooring is to add the minimum nominal 1/2" plywood plus a tile substrate or add a reinforced mortar bed of a minimum 1 1/4" thickness.

What you are planning has been shown by CBU manufacturers not to work and they all, every one of'em, require their product be installed over plywood or OSB. But it's your floor and your choice.

My opinion; worth price charged.
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Unread 05-09-2020, 06:08 PM   #6
jimmartz
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jadnashua,

You make a good point, spreading regular mortar over my planks
would not stay long and probably crack apart just walking on it before
even getting the cement board down.
I thought I have seen people using something to flatten plywood subfloors,
I am not sure what off hand though. I need to do more research.
I am sure there is some epoxy that would not crack, especially if it was
reinforced with fiberglass or stronger, but that would get expensive.

I am not totally sure of the reason cement board can't be used on planks,
if the planks are screwed down tight and the cement board seems land on
and screw to the same plank. I guess it is because the planks would have more expansion and contraction with humidity then plywood would have.

I guess 1/4 plywood and 1/4 cement board would not be good enough either.
The planks go under the walls and I would have to cut it the whole way around the room and then add more joist where it goes under some non-supporting wall. Then that don't even make the floor flat, I would still need
to shim and trim the joist. There has got to be something I can do to lock
everything together and not have to replace the subfloor, or add more then
1/2" of thickness before the tiles.

Thanks for the help.
Jim
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Unread 05-09-2020, 06:44 PM   #7
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If you cut the planks out, you'd need to add blocking underneath that joint to support both any non-loadbearing walls and the end of the planks under them. If you're set on keeping the overall floor buildup down, that may be the only option.

There is NO method to attach tile to planks, every tested method requires plywood over it, and the testing has shown it needs to be a minimum of 1/2" nominal as described previously. Well, unless you want even more height and put an unbonded mud bed there (minimum 1-1/2" thick).

There are various materials that can be used to flatten your floor, but that needs to then be compatible with whatever you put on top of it, too. A membrane like Ditra is about 1/8" installed that could be installed over cement based products that you might use to flatten the floor rather than cement board. FWIW, on a floor, if using cbu, there's no reason to use 1/2" material UNLESS you want the thickness greater...IOW, all you need is the 1/4" stuff. When used on a floor, the thicker cbu really doesn't add anything except cost and height. A few of them can be used at 1/4" on a wall, but to keep things aligned with the more typical drywall, most use 1/2" there (and not all 1/4" panels are suitable for wall use).
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Unread 05-10-2020, 09:26 AM   #8
jimmartz
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CX,

Ok, I updated my profile.

There is a doorway at one end, so that would complicate adding a beam,
and there would be a head room problem.

I got more research to do before asking more about underlayment,
there are several products and methods. But from what I have read so
far agrees with that you are saying, the only proven thing it 1/2" minimum
plywood on planks. I have read of people using 1/4" plywood, or two layers
of 1/4" cement board on planks, but none of them seem to follow up after a
few years and give an update on there floor. But I need to do more research
on some of the underlayment products before asking more on that.

I will probably just suck up having a height difference in the floors.
Then add some threshold between the two floor heights. Make it look
like an added decorative element instead of something to cover up the
difference in floor heights.

Thanks for you help.
Jim
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Unread 05-10-2020, 10:40 AM   #9
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Ok, back to my original questioning about floor deflection.

So, I did some calculations and my floor is on the edge
of being L/360, so I need to stiffen it some and it should
theoretically be stiff enough.

But what about a quantitative measurement?
I could place a string across and place a known weight
(my fat behind) on the center of the floor and measure
the deflection. But that don't directly relate to the
equation used for deflection. For one, with 40psf and
the joist length of 13.3', that is 709lbs, I am not near that
big. Although looking at the equation there is a linear relationship
between weight and deflection, so if I put 3/7 the load I should
expect 3/7 the deflection. Also I believe the formula is for a
distributed load, not a load at one point (my feet).
And finally the equation is for a single beam, with a subfloor
installed some weight is transferred to adjacent joists.

So that being said, I would expect that the deflection with
a subfloor installed would need to be a fair amount better then
the calculated L/360.
Is there some rule of thumb on how much deflection is acceptable
on a floor with subfloor installed?
Something like adding a given weight to the center of the floor should
not cause it to bend more than a certain amount per foot?

Is there some other test for bounce?
Any suspended floor is going to have some deflection or bounce,
but that is subjective from one person to the next. How can one
make a real quantitative measurement of the bounce. Then re-test
after stiffing the joist to measure the improvement and know
that it is acceptable for tile.

Thanks,
Jim
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Unread 05-10-2020, 11:21 AM   #10
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Quote:
Originally Posted by Jim
I am not totally sure of the reason cement board can't be used on planks,
if the planks are screwed down tight and the cement board seems land on
and screw to the same plank. I guess it is because the planks would have more expansion and contraction with humidity then plywood would have.
That's a close evaluation, Jim, but the primary reason is because the CBU manufacturer says you cannot install his product in that manner. That trumps all building codes and other industry standards. The manufacturers of CBUs want you to use as much of their product as possible and they're gonna be quite willing to have you use it in as many applications as you can conjure up, but not installing it over sawn board subflooring.

You can overthink the joist deflection as much as will make you happy and try to figure as many ways to calculate the actual deflection of your particular structure, but the industry standards are based upon a design deflection calculation, which is based upon known and assumed attributes of specific lumber. If you prefer to base your assumptions on something else, or prefer to attempt to use actual measurements of some kind, that's entirely up to you. Your results might be just fine. They may not.

The subfloor deflection, generally the more critical of the two deflection criteria, is also based upon the design computation of known types and dimensions of natural and engineered subflooring types. You could also try to measure the actual deflection of your subfloor as well (that is, in fact, how the design standards are determined for subflooring), but, again, you're probably better off using known standards and criteria than trying to measure your own.

You're free to tile over anything that you're comfortable with. We can only tell you what the ceramic tile industry requires and where the smart money will be betting.

My opinion; worth price charged.
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Unread 05-20-2020, 09:43 PM   #11
jimmartz
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Underlayment over plank subfloor

Hello,

I am wanting to tile a kitchen.
The house is late 1800s with 7/8" plank T&G subfloor,
each about 5 to 6" wide and perpendicular to the joist.
There is to much involved to replace the subfloor, so I
am going to place the underpayment over it.
My plan is to first stiffen the joist(another post) then
add 2 screw per plank per joist. At that point I have
a stiff enough and tight plank floor that needs flattened.
The planks are cupped up to 1/16" each and the floor overall
has dips up to 3/8".
Here are the options I am thinking of:
1. Do nothing to the plank floor and screw 1/2" plywood to it
with some PL Premium. Then screw 1/4 cement board to the
plywood. Then use a cement based motor to flatten the floor
before installing the tiles.

2. Do nothing to the plank floor and screw 1/2" plywood to it
with some PL Premium. Then use a cement based motor to
flatten the floor. Then install Ditra before installing the tiles.

3. Remove the high spots on the cupped planks. Then screw 1/2"
plywood to it with some PL Premium. Then use a cement based
motor to flatten the floor. Then install Ditra before installing the tiles.

4. Remove the high spots on the cupped planks. Then fill in the dips with
shimming maternal. Then screw 1/2" plywood to it with some PL Premium.
Then install Ditra or cement board, before installing the tiles.

Please let me know your opinions of each option.
I would like to just use a leveler over the plank floor, put all the levelers
I have found are cement based and will probably crack when installing
the plywood. I seen some liquid rubber underpayments, but non of them
list as being used as a leveler on planks, and are pricey.

Thanks for any help.
Jim
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Unread 05-20-2020, 11:00 PM   #12
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Welcome, Jim.

You want to flatten the existing subfloor as much as possible. Renting a floor sander might be involved (can't see it from over here).

You must install the nominal half-inch plywood before you do anything else and you do not want to use any PL Premium or other construction adhesive.

Once the plywood is installed with lots of mechanical fasteners, you'll be able to decide how much and what type of flattening still needs to be done.

If you elect to use the CBU, you must install that before you do any non-structural flattening.

If you elect to use Ditra or similar membrane, you must do the flattening before installing the membrane.

My opinion; worth price charged.
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Unread 05-21-2020, 06:17 PM   #13
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CX,

Thank you for the reply.
Let me repeat what I think you are suggesting to make sure
I got it right. Adding a few details of my own.
1. Sand down the high spots on the existing plank subfloor.
2. Install 1/2" or thicker plywood, no glue, and screw every 6"
each direction with construction screws (GRK R4 maybe).
3A. If using CBU, install the CBU and then flatten, using a motor
based leveler. Then install the tile.
3B. If using Ditra, flatten over the plywood, using a motor
based leveler. Then install the Ditra. Then install the tile.

Would this be correct?

A few questions.
I see a lot of people shimming the low spots in the existing subfloor
with door skins, shingles, and roof felt. before installing plywood.
I think you are suggesting just sanding before adding the plywood?
If so, any reason for not shimming before the plywood?
Any reason to use CBU vs Ditra?
I do not have any preference/experience with either, but the
Ditra looks quicker/easier.

Thanks,
Jim
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Unread 05-21-2020, 06:34 PM   #14
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The issue with shimming the joists is that unless it's giving continuous support, and plywood doesn't have the same stiffness in both directions, you could end up with movement you don't want. If you did remove the planks, you could sister shorter 2x material to the sides to account for any low spots, and plane down any high spots. I did that for about 500 sqft on the first floor of my condo when I remodeled...it was way out of whack! I wasn't tiling, but the specs for the floating floor were as tight on flatness.

If you decide on a floor sander, make sure to run around first with a nail set and make sure the screws you install don't stick up, or you'll be buying lots of sandpaper!
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Unread 05-21-2020, 07:35 PM   #15
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jadnashua,

If I shim on the existing plank subfloor, I was assuming
I would shim everywhere, not just at the joist.
Screwing every 6" would probably pull the plywood into
a dip if there was any gap under the plywood.
I have a 4" belt sander that I can use to take off the
high spots.

Thanks,
Jim
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