# How to install 3/8 underlayment plywood?



## Termite (Apr 13, 2008)

It would be best to not install the tiles directly to plywood. They should be installed to a 1/4" cementitious backerboard (hardie, wonderboard, durock) that is adhered to the subfloor with thinset and screws or roofing nails. The tile is then set over the backerboard, also in thinset.


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## dangdang (Sep 24, 2009)

Due to the 3 1/2 feet crawl space and 16" OC joist and 15 feet span, the floor was somewhat shaky. I put solid blocking in joist, screwed down the subfloor plywood to joist. 

Somebody suggested to use plywood as underlayment to increase the stability, thats why I got it. Is there solution for underlayment that could increase the stability, good for tiles and at the same time stay flush with 3/4 hardwood that I am going to install in family room.

I appreciate your help.

Arookie


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## JazMan (Feb 17, 2007)

Rookie,

If your tiles were installed direct over a single layer subfloor, that was wrong. Installing tiles direct on to plywood is doable, but an underlayment is required over the subfloor, so you need two layers as you are now contemplating. I do not recommend tiles on plywood though. Very high failure rate. 

You say your floor is shaky and have added blocking, that's good, lets see what else the floor needs. The 3 1/2 ft crawl is not a factor. Joists spaced 16" o.c. is normal, so that is not a negative either. You say the span is 15 ft, but you didn't mention the size of the joists or the species or grade. We at least need to know their size to take a guess if the joists need attention. It would also help to know where this house is located. Adding plywood is not going to reduce the joists' deflection, plywood will only stiffen the space between the joists. 

So get that other info and report back. Once the joists are determined to be OK for tile, we recommend either a 1/4" tile backer or Ditra membrane. Either can be installed over your 1" plywood subfloor, no need to add more ply unless you want to, the more plywood the better.

Another solution so that the tiles area will be flush with the 3/4" hardwood is to use Ditra XL instead of regular Ditra. Check it out, but lets see about those joists first.

Jaz


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## dangdang (Sep 24, 2009)

Thanks for informative reply.

I live in Vancouver,bc,Canada. The joist is 2X10, don't know what wood it is. 

THanks.


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## JazMan (Feb 17, 2007)

I would say your floor is over spanned for ceramic tiles, doesn't mean you'll have a total failure but some hairline cracks in the grout are possible down the road.

It would help to know the species and grade of the joists, but I don't see any species that can span 15' and still meet L360 at 50/20 live/dead load. The standard minimum for residential using standard floor covering is L360 at 40/10. That is the worst the builder could build the floor and that was his goal. 

You can strengthen the framing by either sistering each joist with 2x6 or 2x8, or install a supporting beam across the joists to shorten the span by half.

Jaz


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## dangdang (Sep 24, 2009)

Thanks for the reply. 

I have done solid blocking and looking at installing a solid beam by gluing two 2X4 with 4X4 posts but I have to dig up in the 3 1/2 feet crawl space which is not much fun. 

Thanks again


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## Bud Cline (Mar 12, 2006)

Haven't looked it up yet but typically a 2X10 spanning more than eleven feet is an issue. If you could install a beam at 7.5 feet reducing the span by 1/2 things would fair a little better for you.

If you are intending to make a beam of 2- 2X4's I'm not thinking that will accomplish much. A better beam would be made of sistered 2X8's on 4X4 posts. A pair of concrete footings is also necessary - preferably spread-footings not just post-footings.


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## Bud Cline (Mar 12, 2006)

Any second layer of plywood should not be glued to the first layer. It would be best to screw the second layer only to the first layer delibertly missing all the previous seams and missing all the floor joists. A seam overlap of a minimum of 25% is the way to go. Fasteners every 6-8 inches in all directions. The plywood should be a minimum of 3/8" exterior grade exposure1 underlayment plugged and snaded on one side.


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