Flagstone Damage

by Chris
(Alexandria, VA)

Damage to edges of stone

Damage to edges of stone

Damage to edges of stone
Test cut showing wet setting bed


Background info:
I’m working on a project dealing with 1” Pennsylvania flagstone that has been installed on a second level outdoor screen porch over a finished sunroom. The stone is currently sitting on a sloped (1/8”/foot) cement mortar setting bed on top of a waterproofing membrane on a flat plywood substrate.

The stone is beginning to flake and deteriorate at locations near the center of the porch (see attached picture). The damage looks to be near edges of the stones near the mortar joints. A test cut was performed and the setting bed was very wet (see additional picture).

Possible problems:
1. whatever water from wind driven rain that lands on the porch makes its way to the setting bed and because it’s on a flat plywood deck it is not draining on the waterproofing as intended. The structural joists supporting the stone/setting bed may even be deflecting enough at the center of the span to the point where the water is actually draining to the center of the room (hence the damage being observed near the center and not the edges).

2. incorrect pressure washing/sealing. I've read a couple posts with similar damage, and people suggested it might be this.. lyme residue from the mortat just sitting on the stone. I have contacted the owner and they will get back to me on any details about pressure washing/sealing that has been done.

Comments for Flagstone Damage

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AS A PA flagstone dealer and installer i can tell you,
one--i advise people to never pressure wash! It forces water into any crack and will work stone loose over time! 2nd--the stone should have been sealed with several coats of stone sealer which would have helped seal the grout joints.
if there is flexability in the floor you will have problems continue with the stone..the joints will never be water tight..
unfortunatly there is now real fix to the problem without fixing the supporting floor first.

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