We are tiling a small bathroom, replacing vinyl, and do not want to significantly change the height of the fixtures. Can I successfully lay tile on the subfloor, or do I need a backer?
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What is the thinnest profile backer material available?
We are tiling a small bathroom, replacing vinyl, and do not want to significantly change the height of the fixtures. Can I successfully lay tile on the subfloor, or do I need a backer?
​
What is the thinnest profile backer material available?
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Replies
What is the composition of the subfloor (all layers)?
OSB decking
How thick?
What are the dimensions of the tile?
Standard thinset or something else?
I'm highly skeptical as to whether a single layer of OSB sheathing is sufficient support for ceramic floor tile of larger than about 3x3". In particular, if there are any joints in the OSB, they are suspect. There are "membranes" which can help, but since they add negligible thickness they also add negligible stiffness -- they mainly serve to "decouple" across joints, etc.
Dan,
It doesn't really matter if you're skeptical. The TCNA guidelines allow for single layer substrate under certain conditions which are laid out in F148-07. Also, Jake Bruton, who writes in the FineHomebuilding's Fall/Winter 2015 issue article titled "Build Up, Not Down" is apparently not skeptical. He writes in his article how to determine structural sufficiency:
"Subfloor. The existing subfloor must be rigid enough to properly support tile. Rigidity is measured using the L/360 formula, which means a joist should not deflect more than the length of its span between supports (L) divided by 360 when a 300 lb. per sq. ft. load is placed on the center of the span."
In fact, he goes on to point out that his system generally won't work if an additional subfloor layer is needed for reinforcement in the shower proper. It is interesting to note that in his article he is installing large format tile and no decoupling memberane is actually used in the shower.
not necessarily
1. I believe the only installation approved by the Tile Council of North America over a structurally sound 19/32 T&G subfloor without a wood underlayment or cementious backer board involves a decoupling memberane. In the case of a decoupling membrane (i.e. Schluter's Ditra) the joist spacing is limited to 19.2" O.C.
So....
2. If your wood subfloor is a structurally sound 19/32" T&G material over 16" or 19.2" O.C. joist spacing, then your thinnest subfloor build up would be about 1/4" with the regular Ditra and thinset. I don't believe thinset manufacturers have any issue bonding underlayments or decoupling memberanes to structurally sound OSB or plywood with latex modified thinset. There is no reason for it to break down over time unless your installation does not follow TCNA and material manufacturer guidelines. Of course, with a polyethylene based decoupling material your finish tile (and any possible kerdi taping of Ditra joints) should be bonded with unmodified thinset due to curing issues.
Lastly, keep in mind that Ditra is not considered a "backer" material and imparts no additional structural capacity to your floor diaphragm. As Schluter points out in their installation handbookt: "a single layer of plywood or OSB wil not provide the rigidity necessary to limit deflection and curvature of the sheathing between the joists, which can be problematic. Two examples include installations of natural stone and any installations over joists with spacing in excess of 19.2 inches."
what do you consider a significant change
the thinnest backer commonly avaliable it 1/4. this should not have an affect on your vanity. for your toilet you can use a flange extension (if you dont have the play to raise it).
people have done it, people still do it, but in most cases it is not because they know the subfloor is adequate. many, if not most of the tile floors i have repaired in that past have been applied directly to the subfloor (hence the need to be replaced)
Maybe...
Most thinset manufactuers don't recommend their products for direct application to OSB. the bond can break down over time.
IF your floor is in good shape, one of the lower profile and better performing ways to go might be to use Ditra. Or something similar to Ditra. Ditra is about 1/8" thick. Bond the Ditra to the OSB with a modified thinset, then tile over the Ditra with an unmodified thinset.
EDIT: I realize the first sentence of my reply could be confusing. The "...direct application to OSB" part needs clarification.
Thinset can be applied to OSB. By "direct application" I meant to convey that most thinset manufacturers don't recomend their products for bonding tile in a direct tile-to-OSB application.
Because of the give-and-take structure of a material like Ditra, thinset can be used to bond a transition material like Ditra directly to OSB. But for a rigid material like tile? It's not recommended.
Mongo wrote:
Most thinset manufactuers don't recommend their products for direct application to OSB. the bond can break down over time.
Interesting. Can you reference a couple of manufacturers that do not recommend their product for bonding to OSB substrates...or maybe even link us to where they inform of this in their product literature? As for the bond, everything breaks down over time. What do you feel causes this and what kind of time frame are you talking about here (5, 10, 20, 50, etc.) in years?
differential movement...
Read the edit I made to that sentence. I tried to clarify that thinset manufacturers don't recommend bonding tile directly to OSB using their thinsets. It's a differential movement issue. Wood-chip OSB versus rigid tile. The movement in the OSB can cause the bond to weaken or break over time.
Here are a couple:
For Custom's thinsets (language edited for brevity):
Wood-based panels, like Oriented Strand Board (OSB), expand and contract with changes in moisture content. Therefore they are not recommended as a substrate material for direct bonding of ceramic or natural stone tile with cement mortars. OSB will change dimensionally as the moisture in the environment and board changes. This regular expansion and contraction with changes in the environmental humidity will weaken the bond of a cement mortar to the OSB surface.
The full language is found in their tech bullitens.
For Laticrete (edited):
Exterior glue plywood is the only suitable wood underlayment for installation of ceramic tile by the direct bond method (interior applications only). OSB, or similar reconstituted wood panels are not suitable substrates for the direct adhesion of tile or stone.
It's also more fully adressed in their tech bulletins.
Mapei? Do not use over dimensionally unstable substrates such as ...oriented strand board (OSB)...
On the flip side, I think TEC Sturdi-Flex is rated for use over OSB.
tiling a small bathroom #212132
Do not lay tile directly on OSB! Always use a backer, because you are dealing with two materials (wood and tile) that expand/contract at vastly different rates. The idea is to "isolate" the two systems.
The thinnest tile substrate I can think of is a decoupling mat. You lay it into a bed of thinset mortar. No screws; no nails...just thinset. Sometimes I use staples to create a good bond between the mat and the thinset along the edges of the floor.
A roll of decoupling mat can sometimes be purchased by the linear foot. Otherwise it is only available in rolls of, say, 3x50'. It is relatively expensive...but practical! There are other thin, cement-based products you can use which are layed into thinset and mechanically fastened to the OSB. I like the mat. After the mat is installed and the thinset dried, you can add a coat of thinset to build it up a bit and even out irregularities. Then you lay tile. Keep an expansion joint where tile meets the wall.
Since the bathroom is small, subfloor deflection will probably not be a concern, but you may want to check. You want a minimum 3/4" of OSB subfloor below the tile backer. And you want to make sure it is firmly anchored to floor joists with screws.
Mel Fros
http://www.froscarpentry.com