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Discussion Starter · #1 ·
Here's the deal, there are two schools of thought on this. We have a parking deck roof, which also serves as an exterior walkway corridor between buildings. The slab is 6" thick of 6000 p.s.i. pea gravel. we have a pitched flagstone cover we are placing on a mortar bed to drains approx 18' apart. The slab will drain any standing water to the drains, however over a prolonged period of rain, ( which does not occur that often in Raleigh,N.C.) water will absorb and migrate to the base of the mortar and flagstone and dampen the slab.

The question is should we waterproof the 6" slab and trap that moisture in the upper flagstone and mortar layer allowing it to only dry out from the top, or not seal it and allow the 6' slab to act as a sponge reservoir so it can dry out from both sides of the slab. The two concerns are :
1) In not sealing it we risk lime leaching in the parking deck ceiling," efflorvescence" and a possible damp ceiling during prolonged rains.

2) By sealing it, we trap water in the inch and a half stone and mortar layer , risking the occasional freeze / thaw that would pop loose the flagstone overlay and be a maintainance headache.

Opinions and suggestions welcome
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