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Old 02-04-2018, 09:57 AM   #1
ak6399
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Worried about porcelain tile cracking

I'll start by saying this is a long post, tldr at end

I would like to install porcelain 6x24 tile in our kitchen and I have a concern about too much deflection in the floor according to the calculator on this website I've seen recommended on about every topic I've searched on this subject. According to the calculator my floor has a deflection of L/258 and minimum deflection of L/360 is needed.

The house is a 1955 cape cod with a full basement. Right now the joist length from the basement wall to the center beam is 11' 6", 2x8s' 16" OC with 1x10s (these are truly 1" not 3/4") running at 45 to the joist. Above that is tar paper, then 1/2" plywood, then the existing flooring. I want to tear out the old 1/2 ply, and put down 3/4 OSB.

My concern is will this provide enough strength to put down my tile without cracking issues in the future. Im installing herringbone patterned 6x24s. Am I just crazy or is this a real concern? Will I need to put down another 1/2 ply over the 3/4 OSB?

tl:dr; 11' 6" joist length leading to possibly too much deflection. Will taking out existing 1/2 and putting down 3/4 OSB be enough or will I need to add another 1/2 of plywood

Thanks for the help and there are a ton of resources here!
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Old 02-04-2018, 10:05 AM   #2
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The deflectolator does not take in to account your subfloor. It only tells you if your joists meet the deflection rating.

You will need to sister the joists or something of that nature to improve the deflection and make your floor suitable for tile.

I think your subfloor is actually about good though. So no adding more subfloor won't fix it.
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Old 02-04-2018, 11:13 AM   #3
ak6399
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Thanks for the advice, I wasn't sure if the subfloor would make a difference or not. Should I sister every other beam or all of them?
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Old 02-04-2018, 04:11 PM   #4
Tool Guy - Kg
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Welcome to the forums, Kyle.

Yes, the Deflecto measures the deflection of the joist structure. The subfloor and underlayment are not structural and don't cut down the deflection an appreciable amount. If you're going to sister the joists, you need to do all of them.

But maybe you ought to hand tight for a second. Most folks have a hard time sistering joists because there's a lot of physical things in the way to install more joists. So, it's worth asking a few questions so that you don't have to do any more work than necessary.
1) Do you have any idea what your joists are made from?
2) Is the distance between the wall face and the beam face a full 11.5'?
3) Would you have an easy or hard time sistering joists?
4) Would you object to one of the easier solutions of installing a closet in the basement directly under the kitchen and perpendicular to the joists to effectively shorten the span of the joists?

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Old 02-04-2018, 07:15 PM   #5
jadnashua
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To achieve acceptable increases in joist strength, you have to sister at least the middle 2/3'rds and preferably more of the length.
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Old 02-05-2018, 07:20 AM   #6
ak6399
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1) Do you have any idea what your joists are made from?

I don't unfortunately and I can't find any markings on them. Doubt I will considering their age. I can PM a picture if that helps, can't post links yet.

2) Is the distance between the wall face and the beam face a full 11.5'?

That's correct

3) Would you have an easy or hard time sistering joists?

Wouldn't be too hard, just have to notch a small portion of one of them at the top because of electrical wiring.

4) Would you object to one of the easier solutions of installing a closet in the basement directly under the kitchen and perpendicular to the joists to effectively shorten the span of the joists?

I wouldn't, but the wife probably would lol.
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