I recently got an e-mail from a reader who is considering a dry-laid, bluestone patio rather than a wet-laid one. For clarity I’ll state the obvious difference: wet-laid is where the stones are set on a concrete slab with mortar.
Dry Laid Stonework Is Great If Built Correctly
She already knew quite a bit about the comparative features of each. She liked the fact that dry-laid stonework can “move” with nature making it less susceptible to cracking. She also saw the advantage of being able to lift and reset the dry laid stones or pavers if necessary. This feature in itself is valuable, especially around pools where you never know when you might have to dig to repair an underground water line.
“Her one concern was the issue of the patio stones “heaving.” This is where some of the stones get pushed or lifted out of their level setting and protrude above the rest. As you might suspect this not only looks terrible, but also poses quite a hazard.”
Heaving occurs when moisture is retained by the base material underneath the stonework. When this freezes it expands causing the stones (or bricks, etc.) to lift.
The solution is to construct the base so that it doesn’t hold moisture.
In the pictures below you’ll see 2 types of gravel being used: 3/4″ and 3/8″ crushed, clean gravel. Think of building the base like a layer cake. (Click on the pictures to enlarge them.)
Start at the bottom and compact the sub-grade giving it a slight pitch. Some contractors will then lay filter fabric over the sub-grade to keep the gravel from migrating down. I would say filter fabric becomes most important with areas that will get heavy traffic like driveways.
Over the compacted base (and filter fabric if used) goes a layer of 3/4″ gravel. For a walking surface (no vehicles) the 3/4″ gravel should be no less than 4″ deep. Run the compactor over this layer.
Now on top of the 3/4″ gravel add a layer of 3/8″ gravel. This layer of 3/8″ gravel can be one to two inches deep. This is what the flagstone is then set on.
Realize that there are several variations of this “recipe,” but they should all be based on the principle to not hold moisture.
There are situations where the gravel base of your dry-set patio, walkway, etc. could become a “reservoir” where excess moisture gathers. For example, this could happen because “water run-off” from a nearby area could end up in the area by the patio. Perhaps the patio just happens to be in a lower portion of the property.
If you anticipate this happening, be sure to install a perforated pipe drain within the gravel base to collect and re-direct this excess moisture away from the base.