Tiling directly to Exterior Glue Plywood (EGP) - Tiling Tips Pros and Cons

[JohnfrWhipple]

As a quest for speed and production the appeal of tiling directly to exterior glue plywood is very appealing to many. Is this practice even approved? Can you tile directly to plywood? The answer might shock you but the simple answer is "Yes" - you can tile directly to plywood. The more important question is "Should you?"

What are the Pro's and Con's of tiling to EGP - Exterior Glue Plywood?

In the Pro list right at the start has to be "Cost Savings" "Speed" and "low build up height"

On the Cons you would have "Deflection" "low build up height" and I think "Cracking grout" as the top three reasons not to.

In the past 12 plus years we have not done any tiling over basic plywood but I know many people do.

Not every renovation has the luxury of a "Slush Fund" for added things like cement board, Ditra, NobleSeal TS and the like. If you are going to skip the added protection of these items what can be done to ensure your installation is as strong as possible?

I'd love some pointers from anyone that is familiar with this style of tile setting and will try and track down the recommendations from the pros on this subject of tiling over engineered plywood.

 

[JohnfrWhipple]

Laticrete's PDF Files - Tiling over Exterior Glue Plywood (EGP)

So first off studying Laticrete's PDF files is that tiling over a single layer of plywood is a No-No.

The total plywood thickness must be a minimum of 1 1/4" (32mm) thick. To achieve this a double layer of 5/8" or some 3/4" topped with 1/2" is a start.

We never like using plywood thinner than 1/2" and in order for these floors to be acceptable the bottom layer needs to be tounge and grove. The TTMAC requires that all plywood be at least 5/8" thick so 1/2" plywood is out of the question.

Here is a link to Laticrete's recommendations for tiling directly over EGP - http://www.laticrete.com/distributors/product_support/faq/planning_a_job/surface_prep.aspx

They have four drawings at the above link showing some great cross sections;

Bonding Ceramic Tile, Stone or Brick Over Wood Floors TDS 152
LATICRETE Drawing ES-F143
LATICRETE Drawing ES-F150
LATICRETE 254 Platinum DS 677.0


Never use a non-modified thin-set to bond to plywood. The stickier the better and Laticrete 254 is one of our all time favourite thin-sets.

 

[Sponsor]

 

[Jadnashua]

TCNA outlines a method of tiling directly on ply, and it dictates two layers, properly installed. The issue is generally jacking of the ends of the ply at the joist interface. Think of the end of the ply as a lever that pivots on the edge of the joist when there's deflection between the joists. That jacks the end edge up which will telegraph into the tile. By adding a second layer, offset properly (1/4-span of the joist spacing), you reinforce that joist top junction and put the second sheet's end point at the point of least reaction to any between joist deflection forces. The TCNA approval is ONLY for (light?) residential. I do not know of any approved method for use in commercial applications directly over plywood. I haven't priced a second layer of ply with the fasteners verses something like Ditra or CBU. Laborwise, properly putting down a second layer of plywood with all of the fasteners required (ideally, screws, not into the joists) and I'm not sure there's a cost savings.

 

[JohnfrWhipple]

TTMAC Guidelines - For tile applied over wood subfloor in a dry areas thin-set method

Specification # 313F-2012-2014 in the TTMAC Specifications has four options for tiling over plywood (dry areas)

Detail A - Thin-set on Plywood

* This specification specs two layers of plywood staggered with a 50% overlap of sheets. This plywood to be installed with it's grain at 90 degree angle to the floor joists.

* The screws or fasteners should penetrate both layers of plywood but not the floor joists or cross blocking.

* The specs include a gap between sheets on the top layer of plywood of at least 6mm (0.23622")

* Top sheet of plywood has to be 16mm (0.629921") or thicker.

* Screws to be 30mm (1.1811") and placed on 150mm (5.90551") centers around the edges and 200mm (7.8740157") centers in the field.

* Floor joists need to be spaced on 406mm (15.984252") centers

* Other Considerations include the use of expanded metal lath with a double layer of plywood and this is acceptable if metal lath is fully filled with the bond coat.





Detail B - Thin-set on Backer Unit/Board

Detail C - Mortar Bed with Cleavage Membrane

Detail D - Thin-set on Uncoupling System

.... By adding a second layer, offset properly (1/4-span of the joist spacing), you reinforce that joist top junction and put the second sheet's end point at the point of least reaction to any between joist deflection forces. ...

This plywood off set is a specification detail of Schluter's and the Ditra install (uncoupling membrane) does not require the installation of the second layer of plywood. The second layer however if added should be install like Jim mentions to conform with Schluter's guidelines. If however you are following Detail A above you should follow the TTMAC Guidelines of the 50% off set.

+++++++++++++++++++​

I'm never surprised to find so much poor information online. Just now while searching for images to use I came across some "How To" advice where another tile pro is quoted. In the quotes the tile pro says he screws off the plywood 8" on the corners/edges and 6" in the field (or the middle of the board). Clearly this is backwards and an oversight by the author of the article. These typos and the like can wreck havoc on any tile installation - everyone should get their own copy of the TCNA or TTMAC Guidelines to ensure that the proper steps are taken. That's only part of it following local code is also a factor. Always double check the info of any advice you find online. What a shame to screw up a job when you have taken the time to research things in depth.

Poor DIY advice here: http://homerenovations.about.com/od/tiling/f/Can-I-Install-Tile-On-Plywood-Subfloor.htm I emailed both the tile pro and the author this oversight. Hopefully the mis-information is changed sooner than later....

 

[JohnfrWhipple]

TTMAC Guidelines - For tile applied over wood subfloor in a dry areas thin-set method

Detail D from the TTMAC Specification 314F-2012-2014 is a specification which includes the use of an uncoupling membrane and not a second layer of plywood for the tile assembly.

Ditra is the clear leader in this specification and has two options in uncoupling membranes. New on the market is a product called Spider Web 2 and another new exciting product is called Strata Mat from Laticrete.

I started testing these products but as yet have not finished all the series of tests. I can say straight off from the little testing I have done that the Custom product (Spider Web 2) is not one I would use since it's design is to inferior to Ditra's by Schluter. You can see some of this testing here.

https://terrylove.com/forums/showthread.php?51383-SpiderWeb-II-Review-of-Custom-Building-Products-new-Uncoupling-Mat/page3

The Strata Mat from Laticrete however looks like a real winner and might even out preform the Ditra product. The proofs in the pudding and I will put these two products head to head very soon.

The real advantage to this type of install is that you have a very low build up in floor preparation. Often a designer or client will want a seamless transition from the hallway or bedrooms hardwood flooring to the tile in the bathroom. This is where these products shine. You get an upgrade over a plywood install and an installation approached approved by both the TTMAC and TCNA.

Looking at this picture you can see how the omission of the second layer of plywood allows for the even transition from 3/4" hardwood to 3/8" tile.

 

[Jadnashua]

If you ever look at the I-beams that have seams on them, those joints are always made 1/4-span past the support beam for a reason...it is the point of least deflection between the support and the middle of the unsupported span. This is the reason to use the 1/4-span point. The goal of a second layer is two-fold: increase between joist strength AND to decouple. Screwing the second sheet into the joists at 1/2-span of the first sheet adds the same strength, but severely restricts the decoupling effect. I suppose if you offset them approximately 1/2 sheet then applied the 1/4-span rule, you'd get the best of both worlds.

 

[JohnfrWhipple]

50% Stagger Plywood Offset with 1/4 span detail factored in.

If you ever look at the I-beams that have seams on them, those joints are always made 1/4-span past the support beam for a reason...it is the point of least deflection between the support and the middle of the unsupported span. This is the reason to use the 1/4-span point. The goal of a second layer is two-fold: increase between joist strength AND to decouple. Screwing the second sheet into the joists at 1/2-span of the first sheet adds the same strength, but severely restricts the decoupling effect. I suppose if you offset them approximately 1/2 sheet then applied the 1/4-span rule, you'd get the best of both worlds.

That is my take on it as well Jim. 50% offset on the top layer but set so that it comes at 1/4" span off the floor joists. This way the screws on top can be driving right through those edges and not hit the floor joist below.



This is my take on this install Jim. We often do not offset the full 50% because we are not worried about any of the detailed specs. We like a good offset but because we cover the plywood with cement board or an uncoupling membrane the point is not valid. I like to ensure that the second layer of plywood is well screwed and before screwing ot off we lay out where the floor joists are. Most times we use 1.5" - 2" screw for the top and both of these would hit the top of the floor joist if we have them in place.

Also we install blocking on most jobs where we can. This needs to be labelled as well.