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allenjacque 04-16-2013 04:39 PM

Exterior slate installation over concrete backer board
 
I am planning to build a slate patio that will be approximately 3 inches above the level of the ground. Rather than pour a concrete surface, I would like to place composite lumber sleepers 85% into the ground and install the cement board so it is just above the level of the ground.

I will then install the slate using an appropriate type of thinset mortar.

I have not yet determined how to stabilize the grid of sleepers but am confident they will be solidly in place. I am more concerned with the ability of the backer board to withstand moisture and the freeze-thaw cycles of northern Illinois.

brockmiera 04-16-2013 05:06 PM

I think you are asking for trouble doing it that way. Do you have any pictures? those sleepers will degrade over time plus they dont have enough bearing surface to stay put under the weight of people walking above. Why not just build up your grade ?

allenjacque 04-16-2013 05:12 PM

No pictures as it only exists in my head.

I do not want to commit to the expense and permanent nature of a poured slab.

I was not aware that the composite lumber would fail if it were exposed to moisture.

brockmiera 04-16-2013 05:20 PM

Quote:

Originally Posted by allenjacque (Post 1160655)
No pictures as it only exists in my head.

I do not want to commit to the expense and permanent nature of a poured slab.

I was not aware that the composite lumber would fail if it were exposed to moisture.

Yeah i dont think the composite lumber would fail but how far would you place between them? I think you'd need something stronger than concrete board to span that distance. I think natural stone needs like L/720 deflection in order to assure it wont fail.

I dont think you need to do a concrete slab but why no just build it up 3" with sand or compacted fill?

A picture of the area in question was what i was looking for.

allenjacque 04-16-2013 05:22 PM

I could probably put the sleepers at 12" O.C. As far as fill, the area is not level so building a surface is preferable.

brockmiera 04-16-2013 05:29 PM

Quote:

Originally Posted by allenjacque (Post 1160662)
I could probably put the sleepers at 12" O.C. As far as fill, the area is not level so building a surface is preferable.

You are going to end up spending more on the composite boards than you would just doing slate on a sand bed. If the ground isn't level then a small retaining wall would work to keep the soil and patio in place. But you are saying that its only 3"????

I guess I dont see the issue.

If you are dead set on doing this not only would you have to put your sleepers in 12" O.C. but you'd have to perpendicularly blocks 12" O.C the other direction. You'd also need to ensure that you will have enough bearing surface on the board so when you couple the dead load (weight of framing) and live load (people walking around) your boards dont just sink into the ground.

allenjacque 04-16-2013 05:54 PM

The highest point of ground would be 3" but the lowest point would have a gap of about 12" between the ground and the backer surface. You are probably right about building the sand bed. I will figure out my needed volume. Thank you for your help.

joecaption 04-16-2013 07:25 PM

Your trying to use one of the most brittle, poris, slippery natural stones on top of two products there were never intended to be used that way.
Cement board has 0 top load strenght when over any span and was never made to be inconstant contact with moisture.

allenjacque 04-16-2013 07:32 PM

Well that pretty well sums it up. Thank you for your input.

JazMan 04-16-2013 08:04 PM

Allen,

Yea, that was a bad plan. The job would have been a mess by January. Either build a patio or a deck that is at least 18" above the ground. In either case you'll have to take special precautions is tiles are to be used.

Jaz


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