QUESTION

I had a new flagstone (bluestone) patio installed on a new concrete sub-base just over a year ago. I notice now that many of the slates are loose, move when I walk over them and sound hollow underneath. There is also a significant amount of efflorescence coming from the mortar joints and accumulating at the drains in the deck.

Am trying to determine if any of that is at all normal after only a year or if the installation was incorrectly done. Contractor is ignoring me and saying this is all normal, which I believe is ridiculous. This patio replaced an identical one in the exact same location that lasted some 25 years without any of the efflorescence I am now seeing, and none of the flagstones started to come loose for a decade or more after they were installed.

I recall a vapor barrier was installed under the concrete when the original patio was installed 25 years ago. This time, I was advised a vapor barrier was not required in outdoor locations and would actually trap moisture and hasten the deterioration of the patio. Would appreciate your advice as to if a vapor barrier below an outdoor concrete/flagstone patio is appropriate.

Can you advise how I may secure advice re this locally given you indicate on your website your services are fairly expensive and generally not intended for homeowners like myself.

Location is in Arlington VA. where we had a relatively mild winter this past year.

ANSWER

ANSWER - If the flagstone shaped Bluestone is loose and has excessive efflorescence staining then it wasn't installed correctly.   If the patio had been designed correctly and the stone installed properly it should perform well over many years.

Standards state that any finished product that is moisture sensitive that will be installed over a concrete slab should have a vapor retarder installed under the slab.  The vapor retarder should continue up along the slab footings and sides to prevent moisture from the surrounding solid from migrating into the concrete slab and into the stone.

Chances are the problem you are having with the loose stones with excessive efflorescence is due to a poor design of the patio and perhaps improper stone installation.  Water is probably getting below the stone from both water migrating up from the ground below the slab and also coming in through the grout joints at the stone surface level.  Water then becomes trapped below if there is no type of drainage plane and design to allow the water to evacuate from under the stone.   This could be even more problematic in a freeze thaw environment where the trapped water freezes and expands that can cause problems.  If there are no movement joints installed in the patio to mitigate stress that can be another contributing factor to the problems.  There could also be some issues with how the installer installed the stone.

The water being trapped below is generally the cause of the excessive efflorescence as water is constantly migrating to the tile surface and evaporating that causes the precipitation of the efflorescence minerals.

The concrete slab should be sloped at a 1/4" per foot to a some sort of drainage system so allow water to migrate out from under the stone.  Many times drainage mats are installed to facilitate the evacuation of the water to drain or weep out of the assembly.

The only way to determine what the exact situation is and how to best remediate it is to perform a forensic investigation by a company such as www.CTaSC.com.   We do have a CTaSC Inspector located about 30 miles from Arlington, VA.  Although it isn't always practical to retain a company such as ours as due to the cost.  And there are not many people who have the expertise or who are knowledgeable as CTaSC who can accurately and thoroughly perform this type of work