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Old 08-28-2014, 09:08 AM   #1
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Question on preparing my subfloor for tile


Hi everyone,

I am planning my first tile project, have been doing a lot of research on the forum.

My joists are solid 2x8s 16" OC, my maximum span is 11 1/2 feet.

I used the deflection calculator and it gives me L / 337, below the L / 360 recommended for tile.

Current subfloor is 5/8 plywood.

This is what I have gathered I should be doing so far:

-Screw down the current 5/8" plywood to the joists to make sure there is no movement

-Add a layer of exterior grade plywood 1/2" in thickness installing it perpendicular to the joists, overlapping the current plywood by 1/2 a sheet.

-Screw down the 1/2" plywood using flooring screws making sure to hit only the original 5/8 " plywood and not the joists

I am planning on using Ditra as my underlayment.

My question is the following:

If I have dips in the floor, when do I correct them? Do I add some type of self leveler on top of the 1/2" plywood before the Ditra?

Thanks for your help.

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Old 08-28-2014, 01:41 PM   #2
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Question on preparing my subfloor for tile


Hi Vito,

The steps you outlined are fine to improve the subfloor, but how about the joists? In the end your joists' deflection will still be below the recommended L360 since you have done nothing to stiffen the joists.

You also left out important info about the joists. Need to know their species and grade. Without that info all we do is guess.

So, how are you gonna make the joists stiffer?

Jaz

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Old 08-28-2014, 03:19 PM   #3
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Hi Vito,

The steps you outlined are fine to improve the subfloor, but how about the joists? In the end your joists' deflection will still be below the recommended L360 since you have done nothing to stiffen the joists.

You also left out important info about the joists. Need to know their species and grade. Without that info all we do is guess.

So, how are you gonna make the joists stiffer?

Jaz
Hi Jaz,

The joists seem to be regular 2x8s made from pine. The house was built in 1973, not sure if that helps or not.

I will look tonight to see if I can find any kind of information that may be stamped on the joists.

Do you recommend sistering the joists to stiffen them? There is some bracing cut from 2x2s nailed in an X between the joists in a few locations from when the home was built, I am assuming this was done to stiffen them.

Does that mean if I would hire a contractor to install my tile floor they would be reinforcing it from underneath as well?
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Old 08-28-2014, 04:21 PM   #4
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I knew the joists would be a type of Pine, it would be helpful if we knew which one. Douglas Fir, Southern Yellow, Spruce + 25 more, and the grade makes a big diff too. Originally most joists are marked, so look, ask a neighbor with the same house or maybe your town's building dept.

X braces do help, but we assume every house is built that way and don't add for it, I would subtract if you didn't have em. Sistering will absolutely do the job, either that or building a wall under it, or a beam.

Quote:
Does that mean if I would hire a contractor to install my tile floor they would be reinforcing it from underneath as well?
That would be the right thing to do, or you do it. Many contractors would probably avoid the subject. Then they could blame someone else if something went wrong.

Jaz
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Old 08-28-2014, 04:31 PM   #5
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I knew the joists would be a type of Pine, it would be helpful if we knew which one. Douglas Fir, Southern Yellow, Spruce + 25 more, and the grade makes a big diff too. Originally most joists are marked, so look, ask a neighbor with the same house or maybe your town's building dept.

X braces do help, but we assume every house is built that way and don't add for it, I would subtract if you didn't have em. Sistering will absolutely do the job, either that or building a wall under it, or a beam.



That would be the right thing to do, or you do it. Many contractors would probably avoid the subject. Then they could blame someone else if something went wrong.

Jaz
OK so I will go the sistering route adding some 2x8s along the existing ones with some construction adhesive and nails.

When I put in the new joists, do I need to force them up really high so they apply lots of pressure to the original plywood from underneath? I was planning on adding the glue to the top of the new joist so that it makes contact with the plywood, just not sure if I really need to force it up there.

I know you are a true pro in the field and I really appreciate your advice.
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Old 08-28-2014, 06:44 PM   #6
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I looked at my joist bays from the basement, there are far too many obstructions to make sistering even remotely possible, so I guess I can't do it in my case.

Would it be a good alternative to put a beam underneath running perpendicular to the joists to pick up the load?

The only problem is I have a window in that area, so the best I could do is reduce the maximum span from 11 1/2 feet to 7 1/2 feet, I cannot put it directly in the middle.

Or I could possibly manage some cross bracing with some 2x6s or 2x8s with joist hangers.
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Old 08-28-2014, 06:59 PM   #7
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Originally Posted by VitoB View Post
I looked at my joist bays from the basement, there are far too many obstructions to make sistering even remotely possible, so I guess I can't do it in my case.

Would it be a good alternative to put a beam underneath running perpendicular to the joists to pick up the load?

The only problem is I have a window in that area, so the best I could do is reduce the maximum span from 11 1/2 feet to 7 1/2 feet, I cannot put it directly in the middle.

Or I could possibly manage some cross bracing with some 2x6s or 2x8s with joist hangers.
Cross bracing won't change the deflection but a beam would be ideal.
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Old 08-28-2014, 08:17 PM   #8
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Would it be a good alternative to put a beam underneath running perpendicular to the joists to pick up the load?
Yes, that's what I said in #4 and Bud above in #7. Reducing the span to only 7.5 ft. will give you the stiffest floor on the block. Heck even just 18-24" would be good. I think I'm the only one who says this but.....It makes sense to me if the joists were forced up about 1/8" in the center when sistering or installing a beam. But no more than 1/8" if that. BTW, when sistering you do not have to sister the entire length and you don't have to use the same size lumber. You can use 2x6 - 2x4 or even 3/4" ply.

Jaz
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Old 08-29-2014, 08:21 AM   #9
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Originally Posted by JazMan View Post
Yes, that's what I said in #4 and Bud above in #7. Reducing the span to only 7.5 ft. will give you the stiffest floor on the block. Heck even just 18-24" would be good. I think I'm the only one who says this but.....It makes sense to me if the joists were forced up about 1/8" in the center when sistering or installing a beam. But no more than 1/8" if that. BTW, when sistering you do not have to sister the entire length and you don't have to use the same size lumber. You can use 2x6 - 2x4 or even 3/4" ply.

Jaz
What is the proper way to make the beam?

Do I put 3 2x8s together glued and nailed? Is that sufficient?

At the wall, do I use 3 2x4s or 3 2x6s to pick up the load all the way to the floor?
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Old 08-29-2014, 09:06 PM   #10
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I think 3 2x4's would be plenty for the supports. Use bigger lumber for esthetic reasons if you want.

I agree 3 2x8's should be enough in most cases, but it depends on the span, we don't know that yet. If you had the headroom you can decide on 2x10's.

Do some careful evaluation of how the joists curve, (if they do) and select lumber that will best fit any curvature. Most joists have a crown and of course you want the crown up. You may have to utilize small shims.

It's best to glue and bolt them together, or at least screw them. They also make laminated beams that are stronger than sin.

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Old 08-29-2014, 11:50 PM   #11
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I think 3 2x4's would be plenty for the supports. Use bigger lumber for esthetic reasons if you want.

I agree 3 2x8's should be enough in most cases, but it depends on the span, we don't know that yet. If you had the headroom you can decide on 2x10's.

Do some careful evaluation of how the joists curve, (if they do) and select lumber that will best fit any curvature. Most joists have a crown and of course you want the crown up. You may have to utilize small shims.

It's best to glue and bolt them together, or at least screw them. They also make laminated beams that are stronger than sin.

Jaz
The total span is 11 feet. I already have a big air duct right next to where I will be placing the cross beam and it drops 9 inches from the ceiling so in either case a bulk head will be needed if I finish the basement later on.

So if a 2x10 is 9 1/4 inches tall, I won't lose much headroom.

Is it better to laminate the 2x10s on the floor first? Or should I put them up one at a time, shim, then bolt/screw/nail and glue them?
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Old 08-30-2014, 12:58 AM   #12
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The total span is 11 feet. I already have a big air duct right next to where I will be placing the cross beam and it drops 9 inches from the ceiling so in either case a bulk head will be needed if I finish the basement later on.

So if a 2x10 is 9 1/4 inches tall, I won't lose much headroom.

Is it better to laminate the 2x10s on the floor first? Or should I put them up one at a time, shim, then bolt/screw/nail and glue them?
I would assemble the beam first then lift it into place. It may not contact all floor joists equally so be prepared to shim some joists.
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Old 08-30-2014, 09:40 AM   #13
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I would assemble the beam first then lift it into place. It may not contact all floor joists equally so be prepared to shim some joists.
Thanks Bud, sounds good.

So now I understand how to stiffen up my floor joists and where it needs to be done.

At which point do I make my floor flat if I have humps in it?

Do I fill in the humps with my modified thinset on my second layer of plywood before the Ditra?
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Old 08-30-2014, 12:24 PM   #14
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Thanks Bud, sounds good.

So now I understand how to stiffen up my floor joists and where it needs to be done.

At which point do I make my floor flat if I have humps in it?

Do I fill in the humps with my modified thinset on my second layer of plywood before the Ditra?
The floor needs to be plane before installing the DITRA and adding more thinset isn't the way to do it. Thinset doesn't make a good leveling product. It is too sticky to drag-out and get smooth, and it shrinks terribly as it dries.

Excessive deflection is your problem
not necessarily a floor that is out-of-plane. Let's address one issue at a time. Install the beam first then look at the floor flatness. There are acceptable tolerances.
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Old 09-01-2014, 10:15 AM   #15
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The floor needs to be plane before installing the DITRA and adding more thinset isn't the way to do it. Thinset doesn't make a good leveling product. It is too sticky to drag-out and get smooth, and it shrinks terribly as it dries.

Excessive deflection is your problem
not necessarily a floor that is out-of-plane. Let's address one issue at a time. Install the beam first then look at the floor flatness. There are acceptable tolerances.
I appreciate your advice and guidance. I won't be taking on the project for probably a few months, I am in the planning phase right now so the beam will not be installed until later and I won't be able to see how flat the floor is right now.

This is a big project, I am tiling my entrance, kitchen, laundry room and powder room so I am trying to gather a lot of information so that I know what needs to be done to work around any problems I can encounter along the way.

Assuming I am out of spec for flatness, what can be done? I would think that if my original plywood layer has humps, it will be the same when I install my second layer as well.

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