Durock screw penetration depth

[cahermit]

I'm building a hearth and surround for a free standing wood stove. I went with .25" durock on studs 16" O.C. and plan on cutting durock furring stacked and fastened on the studs as spacers for the 1" air gap, then topping with .5" durock, and finally 1-2" thick natural slate on the face (we live near a slate quarry).
The issue I am having is that the total width of the .5" durock and furring (providing the 1" air gap), and the 1/4" durock under that is 1.75". With 2.25" screws I'm only going to have .5" of penetration. Manufacturer's specifications call for minimum .75" penetration at 8" spacing for walls (6" for ceilings). I suspect those specifications are also intended for a tile face and not a heavier stone.

My plan is to space the 2.25" screws at 4", and I'm considering adding in some 3" deck screws every 12".

Does anyone see any issues with this or have a better solution?
Will the 4" spacing be sufficient or should I include the deck screws?

 

[rox3406]

Do the entire install with the 3 inch screws.

 

[cahermit]

6" spacing?
Seems like I read on here somewhere that it's highly recommended to stay away from deck screws and anything other than the durock screws.

 

[rox3406]

The only difference is the durarock screws have small ridges on the back of the head to help cut the durarock to make room for the head to countersink into it. Deck screws go in just fine and are stronger. You are going to be putting a lot of weight on it. I’ve laid miles of durarock laying tile. Never used the special screws. Go 6 inches apart. If you over fasten it will crack the board. More isn’t always better

 

[cahermit]

Fantastic. Thanks

 

[BlankBlankBlank]

When I installed four layers of Durock on my hearth floor, I ran into the issue of the special screws being too short. I used longer galvanized Spax screws in #14 size. The larger head secured the Durock very well.


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