subfloor for tile

[tom31415926]

I plan to put 12" tiles on a bathroom floor as part of a remodel. I'd like to find out if I need to stiffen the subfloor first. Here is a summary of the relevant information:

The subfloor is 2 x 8 T&G, house built in 1959. The floor girders are 4x6, about 4 feet apart, and the piers supporting the girders are spaced more than 4' apart (maybe even 8' apart).

I plan to install 1/4 inch hardibacker according to instructions. I will first make sure that the subfloor is level and smooth (maybe use floor leveloing compound). Then put thinset between subfloor and hardibacker. the hardibacker will have screws at 8" intervals.

In other parts of the house, where tiles were originally installed, the subfloor is doubled. That is, more 2x8 T&G below the subfloor, running parallel to the subfloor. The lower 2x8s are supported by ledger boards nailed to the girders.

Should I double the subfloor under the bathroom to be tiled? If I take the original construction as a guide, then yes. But they didn't have hardibacker; probably just thicker mortar (maybe with wire in it?).

I'd rather not take any chances and I don't mind getting dirty. But I would appreciate some expert advice to tell me how best to do the job.

Thanks,

Tom Matthews

[pushkins]

If your floor joists as you say are 48" apart then this needs to be addressed well before you lay backer board. I would add in 2x8 in the middle of these spaces (48").
The hardi backer board should be screwed closer than 8" intervals The boards are marked with little circle indentations where they should be screwed and that would be at 4" intervals.

So in answer to your questions :
Yes you should add some extra floor joists to the floor.
Yes you should use backer board screwed as per indentations on the board.

[tom31415926]

question

Thanks, Pushkins!

Just one thing about these added 2x8s - do you mean they should act as another floor joist, running perpendicular to the direction of the subfloor and between the existing floor joists?

Or do you mean that they should be added to "double up" the floor thickness as I described in the original post. But merely doubling the thickness would not decrease the 4-foot span, and it seems like that is a concern.

If I were to add more floor joists (to have 2-foot spans), how would the new joists be supported? There is no hope of a 2x8 reaching exterior walls with a reasonable span. Hence, I'd have to add more footings (piers?) as well as add some mechanism to keep the new joists from rotating. I suppose if I add more piers, I could use 4x6 joists like the original. But maybe these cost more.

Am I on the right track?


Thanks again,

tom

[pushkins]

Yes I do mean for you to reduce the floor joist 48" centers down to at least 24" centers.

You say that there is no way any new 2x8's could span the distance to reach a support wall, 2x6's can span FAR less than 2x8's and they must be sitting on some sort of bearer or foundation support.
Using this foundation or existing bearers the new floor joists can be added.

You mention the existing floor joists are 4" x 6" and that the dwelling was built in 1959, Are you sure of these measurements, these joists or bearer sizes are very unusual for a home built just 50 odd years ago.

If you could post a photo that would make this much easier.

[tom31415926]

more on floor

Hi again,

For some reason, I can't find my digital camera. I'll keep looking.

By the way, this is a one-story house, with crawl space underneath.

The floor joists are 3.5" by 5.5" (4x6). They are supported periodically along their length by "piers". Each of these "piers" comprises a concrete footing in the dirt floor of the crawlspace, on which rests a block of concrete with sloped sides (so it has a wider square for a base than it has for a top). On top of the block is a piece of pressure-treated wood (green) laying flat, then on top of that is a vertical section of 4x4 which supports the 4x6 joist. These "piers" support the 4x6 joists at a spacing of about 5 feet. The josts themselves are about 4 feet apart.

In this part of the house, in the directions the floor joists are running, it is at least 20 feet to a wall foundation. Hence, extra floor joists would need to have supporting piers constructed for them.

I was hoping that I could avoid adding extra joists by stiffening the floor from underneath. But it is beginning to sound like the first-class way to do it is to decrease the span from 48" to 24" by adding two more joists (there are two existing spans), and the assoicated new piers (I could get by with two new piers for each new joist and stay within the 5-foot guidelines.

I just noticed that one of the new joists _could_ contact an exterior wall that bumps in at the entryway. So only three new piers, if I can find a way to attach the new joist to the exterior wall.

I infer that the construction sounds unusual to you. It's in Sacramento California if that helps.

I'll still try for a picture.

So, given the new information, do you still recommend adding new joists?

Tom

[pushkins]

Yes , if you want to stiffen the existing floor to help alievate the flexing in the floor then adding extra joists to cut down the span from 48" to 24" would be the best option.

BUT for a very close second place and a much easier fix the following could be an option for you.

Instead of using 1/4" backerboard use 1/2".

Another question just so I make sure I'm following you correctly.

you have 4x6 floor joists, spaced at 48" to each other, following your description these joists are supported by piers spaced at 5' intervals. This would mean that the piers would have to be 4' from the next row ?????

The basic anatomy of a floor foundation is as follows for pier foundation
1 concrete footer
2 block pier
3 Pressure treated lumber of some description to (stop moisture transfer).
3 bearer (takes the load from the span)
4 floor joists (resting on the bearers)
5 sub floor
6 Finished floor

A picture tells a thousand words, so if you could get a picture that would help a great deal.

[tom31415926]

Pictures - Finally

I haven't posted pictures here before, but here is my attempt. this is the same subfloor described in my earlier posts.





"Floor1" shows a pier, on top of which is the 4X6 floor joist (others have called this a "girder"). On top of the floor joists is the 2x8 tongue and groove subfloor.

"Floor2" is a view looking down into an area where I have already removed subfloor for access. (This was under the bathtub). You can see that the floor joists are 4 feet apart. Where it is still in place, the subfloor certainly does not deflect much when I walk on it (approx. 200 lbs.). I realize, however, that tile and grout need a very firm base in order to have a long, crack-free life.

"Floor3" is a view under the house showing parallel rows of floor joists, supported approx. every 5 feet by a pier. In the distance, you can see where the original builders "doubled up" on the subfloor underneath the original entry way tile. They added another course of 2x8 T+G _under_ the subfloor. This course is supported by ledger boards nailed to the floor joists (one of which is also visible in the picture).

Well, that is what I have. I am interested in stiffening the floor under the bathroom only. Addition of floor joists would require new piers, for as you can see there is no rim joist anywhere near. But I would do this. I suppose I'd rather just "double up" as in the original install. But with all the facts (I hope!) maybe I can get guidance as to what would be best.

Tom

[pushkins]

Okay, the pictures are proving the "thousand word " theory correct once again.

You could stiffen the floor by adding two joists (2x8's) spanning (left to right in your pictures) the 4' from one set of piers to the next set, one at each end of the to be tiled area (attached with joist hangers) , then on these new joists (at 90 degrees) add 2x8's spaced evenly attached with joist hangers. Basically what you will be creating is a header between the existing bearers and then adding joists (2) to these bearers increasing the joists supporting the floor at this area. Make sure you screw down the floor to these new joists, then using 1/4" backerboard you will have a nice solid and sound surface for your tiles.
Since what your doing is to add support to the above to be tiled area and not structual strength.
You will in essence be doubling the existing floor joists, this will be much stronger than adding more tongue and groove and MUCH cheaper.

[tom31415926]

Message received - thanks!

thanks, Pushkins!

I see what you mean. The new 2x8s will add a lot of stiffness, and will be supported by the existing piers. In two cases, I may have to hang the 2x8s on a 2x6 joist hanger (by notching the end of the 2X8). I think it will be OK to do so, since they will still be supported and will have the resistance to deflection of a 2x8.

BTW, I read that you have austrailian cattle dogs. My wife's sister and her family have one, and he is great. If we ever get a dog, that will probably be the breed.

In a while, I will probablly post asking about re-finishing a hardwood floor. I'll keep reading current topics for a while first.

Tom