Hi,

I am new to this forum, have tried some others with varying answers.

We are in the process of finsishing our basement and one thing I would like to do before drywalling the ceiling is address the potential issue of our main floor not being able to hold tile. Basically our basement is divided into 3 sections, everything is 2x8@16" OC. first 2 sections have 12' spans, and 3rd section has an 8' span.

My first idea would be to sister the joists before drywalling, however sistering with 2x8's would be hard, if not imposible on some areas because of plumbing/gas lines, etc.

2nd idea would be to laminate 3/4" plywood as a sister to the joists which would prove easier to install but as far as I can tell will not increase my deflection rating much.

3rd idea would be to add blocking which as far as I can tell will help with weight distribution but not necesarilly deflection. Note: I would add blocking if I sistered.

Another thought would be, based on the Ditra website, just rely on the ditra when installing tile upstairs, perhaps adding a 2nd layer of plywood.

Another insteresting side note is the floor upstairs is slighlty un level in spots I would want to put tile. Could I use self leveler on this and if so would I do that after adding to second layer of plywood?

Any advice would be great, I know there are alot of questions. Basically for now all I want to know is what I should do with the floor joists from below to prep for tile upstairs. Likely we will go with ceramic not stone on the main floor.

Thanks

Dave

Could you fit 2x6's in there to sister up the joists? The 2x8's are just a little overspanned at 12', the area that spans 8' would be fine as is.

A second layer of plywood is always a good thing. What is the thickness of the plywood/OSB up there now?

Hi there...thank you for the response. Not sure what the existing subfloor is. It is OSB not plywood, I would asume 3/4 or 5/8....but I am guessing you knew that.

As for the 2x6 sisters...I heard that the plywood would be better than undersized lumber sisters?? Can you comment on that? With 2x6 sisters how would I install them? Crown up and press them into the subfloor above??

Thanks again!

Dave

It's not going to take much to get the 12' spanned joists up to par. An 8' 2x6 glued to the joist, then screwed with 3" screws about every 8" inches top and bottom will suffice. Center the 2x6 on the 12' span. They don't need to be against the subfloor. Since plywood is comprised of several layers and only half of them are oriented correctly, you don't really gain that much using, and 2x6's are pretty cheap.

With OSB, I would definitely add a layer of 1/2" ply to reduce the deflection between the joists. 3/4" OSB is the bare minimum for a single-layer subfloor, so it's always best to go with a little more.

OK, sounds good...however I am confused how centering an 8ft 2x6 in the centre of the 12' span will do a whole lot...I was under the impresion that it would have to go beam-to-beam. However when I think about it I guess by doing that we are simply adding "beef" to the centre of the span which will stiften in end to end. Are my assumptions correct?

Most of the deflection is in the center of the joist. Running the extra support all the way to the ends would be a marginal increase over the center 2/3.

Every little bit will help and you are correct in your assumptions DSeibel.
There are more factors involved in engineering more structural support for a floor system. Species, grade and size of lumber are all factors in strengthening existing floor joists.

In your case, a 2x6 will have little effect and even less when it does not bear on supporting walls or beams.

This is a basic list of procedures for strengthening existing floor joists.
1- Sister joists with the same size lumber ensuring full bearing support. Fasten with nails 2 per row @ 8" to 12" O.C.
CON- Extremely difficult to achieve with existing mechanicals in place.

2- Install solid or bridge style blocking at 1/2 span. 1/3 spans is even better if possible. Blocking will actually do more for the structure in terms of bounce than sistering.
CON- Existing mechanicals make solid blocking next to impossible.

3- Cover the bottom sides of the joists with 1x3 strapping installed 12 to 16 inches O.C. perpendicular to the joists or full sheets of min. 1/2" drywall or full sheets of plywood.

4- 2 layers of plywood on the top side will help decrease between the joists deflections.

5- Installation of a bearing wall or beam at midspan below the floor system.
CON- Impractical due to existing lower floor plan use.

FYI, without knowing where you are located and the species or grade of your existing joists it's hard to tell whether you make the minimum 1/360 deflection rating required for a ceramic tile installation.
For commonly used floor joists, Kman is correct on the 12' span areas.