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Should we glue & screw plywood or only screw hardi backer

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Hey All,
Question for the real experts.

I made a kitchen counter top which includes a large island, rolling cart, half wall and the actual kitchen using HDF as it was going to be laminated.

Now they want to tile it.

My feeling is that I can glue and screw down 3/8" ply and tile over that.
Another friend said its better to screw down 1/4" cement / hardi backer board and tile over that.

At first I was against that since it would only be screwed down, but then I got to thinking. We do ONLY screw down that board frequently in kitchens and bathrooms even though ideally you want to mud it and screw it.

So we've decided to see what the community says here.
 

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I'd screw down the hardi, first running some beads of whatever unused stuff in tubes i had - old sikaflex, silicone, floor adhesive, whatever, then run a v-notch trowel.
 

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A countertop is usually flatter than a floor, thinset is placed between for "air" gaps, when a heavy object, say a fridge wheel with extreme psi due to weight and tiny contact to floor, tile may crack.
A countertop receives no where near the weight, so me, I would screw down making sure screws don't push Hardi up, I usually screw down thru the Hardi, then reverse the drill and ream out the Hardi then screw it in so the Hardi isn't threaded and pulls tight against the HDF.
My main consideration would be waterproofing it and using an epoxy grout.
 

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I would screw some type of tile backer board, not plywood. You could coat it with Red Guard, or something similar to increase waterproofing.
 

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1/4" cement board with thinset and screwed down. I don't why so many want to skip thinset between backer board and substrate. I'd probably prime the HDF.
It's nearly pointless on a counter top because no one will be walking on it.

Thin-set fills any gaps that could cause movement when someone walks on the floor, I don't think anyone will be walking on the counter top.

I'd use the plywood and paint with a coating of liquid water proofing. That you don't risk water infiltrating the substrate around the sink.
 

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I make my tiled countertops from 11/2" ext. plywood with 2x coats of Red Guard or Aqua D.

In this case, since the existing countertop is already flat I wouldn't bother with thinset. If you want to glue your substrate, use a contact cement then waterproof.
 

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It's nearly pointless on a counter top because no one will be walking on it.

Thin-set fills any gaps that could cause movement when someone walks on the floor, I don't think anyone will be walking on the counter top.

I'd use the plywood and paint with a coating of liquid water proofing. That you don't risk water infiltrating the substrate around the sink.
I wouldn't say it's pointless. I would never use CBU on a horizontal surface without some sort of cement. I too use plywood with a membrane, but since he's working with HDF he's has to use a different approach. Either way weather it's thinset or some sort of glue he still should be cementing the CBU. Thinset will give him a nice solid void free base while also cementing. If he wants even more protection he can membrane over the CBU. Just seems pretty simple to me.
 

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Paul
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If you use two layers of the appropriate plywood, ran perpendicular to one another, glued and screwed - cbu is not necessary. A typical counter top is a small enough area that using good plywood won't add that much to the cost anyway, and I hate cbu. Membrane it with noble or a liquid and it's bullet proof. You guys over think ****e sometimes lol.
 
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If you use two layers of the appropriate plywood, ran perpendicular to one another, glued and screwed - cbu is not necessary. A typical counter top is a small enough area that using good plywood won't add that much to the cost anyway, and I hate cbu. Membrane it with noble or a liquid and it's bullet proof. You guys over think ****e sometimes lol.
I think some people have it programmed in their brain that if they try to set a tile onto plywood it will jump off and run away.
 

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If you use two layers of the appropriate plywood, ran perpendicular to one another, glued and screwed - cbu is not necessary. A typical counter top is a small enough area that using good plywood won't add that much to the cost anyway, and I hate cbu. Membrane it with noble or a liquid and it's bullet proof. You guys over think ****e sometimes lol.
Why use two layers of ply when one backer board would suffice? You don't have to use cement, he could use fiber too.
 

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My cabinets have 3/4" ply tops. I go over the top of this with 3/4" ply, then 1/2" densshield.

This has worked well for me. Over the dishwasher I only have one piece of plywood. The worst part is over the sink because there isn't much in the back and front of the sink.

Make sure you look over your sink closely. Sometimes the fasteners are difficult to work with. You may have to have the sink base cut back more than the second layer of plywood to make the fasteners work. If you get into trouble, I think you can drill out a piece of plywood with a forstner bit in order to make room for a fastener.

What type of edging will you use?
 

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Paul
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Why use two layers of ply when one backer board would suffice? You don't have to use cement, he could use fiber too.
Why add another material into the assembly when it's unnecessary?
 

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You still haven't said what edging, profile your going to use, that may determine your thickness.
I've seen premade granite profile to be used with granite tile, haven't used them though.
 

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I think a wood edge in combination with the tile looks nice. I have used a tile edge and didn't care for it.

I used a metal edge once in a truckstop.

I would explore the metal edging a bit more. Problem is the expense.

I'm all for a combination of masonry, wood, and metal in everything.
 
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