Flagstone Mats on thin-set concrete.

by John "Red" Burke
(Nevada City, CA)

Should I seal this deck?

We have a 1300 sqft elevated deck with flagstone mats. We did a thinset concrete over an impermeable membrane on 2 layers of 1' plywood. We are at 3000 foot elevation in the Sierra. We get 2 or 3 snow storms a year averaging 1 to 2 feet and about 30 inches of rain. Our property is on a North facing slope in the pine trees.

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Thanks Robin
by: David

Hi Robin,

Thanks for the great comment!! That is something we didn't know about.

We'll definitely advise people of that in the future.

Good Drainage
by: Robin Morris

There are several key components to a long-lasting upper deck installation and they all address the issue of water and thermal expansion.

1) You've addressed one of the more obvious ones here by making sure that your flag mats absorb the least amount of moisture possible. (And Prosoco has an excellent website and product line: http://www.prosoco.com/). Most of them are made of quartzite materials but there are several much softer slates that need particular attention.

2) The next most obvious is positive drainage so that you have no standing water.

But all masonry installations without frequent expansion joints will begin suffering subtle cracks between grout and stone as temperature and moisture cause cycles of expansion and contraction. No amount of sealing or positive drainage will prevent moisture from penetrating your work.

3) This is the part that most masons don't properly plan for. I call this the "2 tier drain system". The top tier is what you see. The second tier is BELOW your work. You MUST drain your work beneath the setting bed on top of the liner if you want a permanent, trouble-free installation. Do your research of available drainage mats by J-Drane, Keene Building Products, etc. Your installation will dry out faster (from underneath!), you won't have efflorescence issues, and your patio can expand and contract until the cows come home.


sealing is always a good idea
by: David

Dear John,

It is always a good idea to seal, especially in your situation. You just want to use a quality sealer.

My dad has been using Prosoco products for over twenty years and has had great luck. You should call them for more info. Their number is: 1-800-255-4255.

Great luck!

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