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foothill
10-18-2009, 12:43 AM
I'm installing a vanity top in preparation for tile. It will be 3/4 inch plywood sheathing topped with a 1/4 inch of Hardibacker and bonded together with thinset. I think I'll fasten the Hardibacker to the plywood with 3/4 inch galvanized nails or screws every 6 inches. Is the resulting 1 inch thickness enough for a tile substrate?

My second questions is about Red Guard. Do I apply the Red Guard to the plywood or to the Hardibacker?

Thanks in advance.

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Kman
10-18-2009, 04:17 AM
You should add another layer of plywood to the 3/4". While you won't be walking on it (probably), it's always possible someone could jump up there to sit on your countertop. 3/4" ply by itself might not hold up, since it will span 24" over your dishwasher, and probably even further in other places. You could easily add a layer of 3/8" ply to stiffen it up.

Attach the hardibacker to the top layer of ply only with 3/4" screws and thinset underneath. While Redgard is not necessary, if you want to use it, you'll need to put it on the Hardibacker.

For the same price or maybe less, you could forego the Hardibacker and Redgard and use Ditra instead. It would serve the purpose of both of those products, and be easier to install.

Edit: I just noticed you said you were using 'plywood sheathing'. You should be using plywood with a face grade no lower than 'C'. The sheathing has a lower rating, and has voids between the plys.

MNTileGuy
10-18-2009, 09:06 PM
I agree with Kevin in that you might want to consider Ditra and save a step if waterproofing is a concern. You also won't need to worry about using any screws as the ditra is just adhered with modified thinset.

foothill
10-19-2009, 05:59 PM
Thanks for the advice guys. I went ahead and added a layer of 1/2 inch plywood. The plywood is class 1 for highest water resistance but was not marked with a letter designation. I'm going ahead with the Redguard mainly because I had already bought and cut the Hardibacker but the Ditra sounds like it would have been a time saver. When I move ahead to working on the kitchen I'll remember that.