Tile over post & beam subfloor.

[William Kinzie]

Hi,
Been lurking here for a while now.

I have a job that requires me to tile a 100 sqft bathroom floor using a 5"x13" ceramic in a herringbone layout. I plan on using 1/4" backerboard screwed on the sub over modified thinset.

Problem is the room is supported by post and beam every 4ft with 4x6 joists running long. This leaves the 1 1/8" T&G plywood subfloor unsupported at the short seams.

Given there are posts every 4ft could just I put 2x6x4ft cross members under the unsupported seams?
This would create 4'x4' support squares. Do I still need to follow the basic 16" OC joist regardless? Also the L/360 rule (.200" @ 6') seems easy to attain but I want to do it right.

Better yet - do any of the TCA handbooks offer advice on this specific situation?

Note: I am a "do-it-yourselfer" on his second house and forth tile job but am looking for real advice from pros. (no more "just mastic over plywood is fine" from the other sites.)

TIA!
--
Bill

[William Kinzie]

Sorry. I just read the forum user rules and see it is not for homeowners. I'll try another site.

Too bad there aren't any good ones for us. : (

--
Bill

[Steve Price]

William

If you're going to go to the effort of installing the 2x6, you should make them 16"oc and glued/screwed. This is obviously relatively easy to do, and would give you far better support. If you consider the horrible scenario associated with your tile cracking because of minor flexing, it's well worth the extra effort. Also, I would go with at least 1/2" backer not 1/4". The difference in height is worth it. (mortar bed with an anti fracture membrane would be best of course)

ps, I'd have no problem who H/O's posting as long as they are polite and phrase their questions sensibly - you've done both

SJP

[William Kinzie]

Steve,

Thanks for the reply.

I should have mentioned I'm trying add the 2x6's from under the house (very tight space) without removing the subfloor. I planned on using metal joist hangers and glue then screw from the top. It's tough trying to maneuver about the sewer and electric though. I was hoping given the 4x4 grid w/ 1 1/18 plywood would be enough.

One thought that comes to mind (and perhaps what you were suggesting) is to cut the subfloor out to work from the top then glue/screw a new sub?

Your right though, I'd hate to have the tile fail down the road. One way or the other, I'll end up with 16" oc. Also noted is the 1/2" backer.

Thanks!

Bill

[Steve Price]

Yes Bill, definitely do it from above if you can. You will still need to use hangers (or squash blocks) and even with an open access it will still be a bit fiddly to get the joists where you want them. Patching subfloor is easy, as long as you install the correct blocking and glue/screw, and something we do on all our jobs - it's like having a crawl space access exactly where you need it. (Makes the plumbers job much easier, and cheaper too) Also, I wouldn't trust 1 1/8" on a 4x4 grid for hardwood floor, let alone tile. You'll be happy that you went with the 16" oc, imo

SJP

[Dick Seibert]

Bill:

I've run into this problem many times, and it is a real problem. This system was used because the building code calls for 12" clearance under girders and 18" clearance under floor joists, somebody got the bright idea that if they closed the girders down to 4' o.c., and used 2X subfloor (and later 1 1/8" plywood), they could eliminate the joists altogether, both saving money and lowering the profile of the house down by at least 6". If you add joists between the girders you have gone back to a joist system that requires a minimum 18" clearance to conform to code. What I do when confronted with this dilemma, is remove all the subfloor, pump a 2" concrete "rat-proofing" over the ground, and install joists like Steve describes between the 4x girders, the code allows you to reduce the crawl space if you have a concrete floor underneath. This becomes an expensive operation because all of the furnishings must be removed and stored, subfloor removed, concrete pumped in, and new joists and subfloor installed, but it’s the only way to install tile over such a floor.

[Steve Price]

good point Dick. If the job is getting inspected Bill could explain to the insp the dilemma with the flex in the floor and the fact that he's just trying to beef it up. Ratproofing will be a pain as you point out - what do you figure, around $500 for a short load + a pump/operator?...this becomes an expensive floor. Hopefully because he's not going below the 4x6 with the joists, the insp may give him some leeway, even if it is technicaly a code violation.

[Dick Seibert]

Steve:

I wouldn't expect an inspector to cut you any slack on this one, at least not around here anyway.

[Steve Price]

Yeah, you're probably right Dick

I'm curious, if the rat proofing adds +/- 2" then how can it be a clearance issue? (even though the code reads that way) If it's more about a potential rot issue, why not go with PT joists? Thats what we've done with similar scenarios, and in danville too!

[Dick Seibert]

Steve:

I think we are getting into uncharted territory, because when we install wood framing members in a floor structure over concrete they are requiring PT lumber, so someday some inspector is going to require me to install PT joists between the girders, but what about the girders themselves? I think this is going to be an AHJ's call.

[Bill]

Steve & Dick,

Wow! This is really a can of worms eh?

This section of the house was an addition from the 70's. The rest of the house is std 16" floors. The previous owner had carpet on 3/8" particle board in the bath as well as the master bedroom. Now I know why. I replaced the bedroom with floating laminate which worked out great. While I've seen it in the bath using silicone at seams - I'm not keen on it.

The job at this point is not permited as I only planned on tiling - sooo... don't ask, don't tell?

Even with a 12" clearance under the bathroom there would still be room to access plumbing as it's layed out.

Btw, my neighbor has 10 cats, I have 2. We don't have a rat problem around here. (g)

I'll move ahead with the 16" oc bracing.
Thanks for quick help guys.

--
Bill

[Steve Price]

You're welcome Bill, best of luck with the job.

SJP

[Bill]

One final note: I'm using pt lumber for the 2x6's just in case.

--
Bill

[Roger P.]

Bill, No need to fear the AHJ. The use of PT (or other rot resistant)joist is specified by the code in this situation.

Excerpt from 2003 IRC

R319.1 Location required. In areas subject to decay damage as established byTableR301.2(1), the following locations shall require the use of an approved species and grade of lumber,
pressure treated in accordance withAWPAC1,C2,C3,C4,C9, C15, C18, C22, C23, C24, C28, C31, C33, P1, P2 and P3, or decay-resistant heartwood of redwood, black locust, or cedars.
1. Wood joists or the bottom of a wood structural floor when closer than 18 inches (457 mm) or wood girders when closer than 12 inches (305 mm) to the exposed ground in crawl spaces or unexcavated area located within the periphery of the building foundation.
2. All wood framing members that rest on concrete or masonry exterior foundation walls and are less than 8 inches (203 mm) from the exposed ground.

[Dick Seibert]

Roger:

Good point, Steve and I are both in CBC country and know nothing about the IRC. I think Bill should check with his local building department before proceeding. It is not advisable to do something like this without a permit, because the owner will have to disclose this fact if he ever sells the house, and this could result in a significant reduction in the selling price (if he doesn't disclose then he's guilty of fraud and could lose the entire sale price of the house).