Avoiding Concrete Cracks Telegraphing Into Flagstone Patioi

Mike F

Not sure if this should go in the landscaping forum, but...

I will be building a large, multi-level flagstone patio this summer. The flagstone will be set in a mortar bed on top of concrete pads. I'm scratching my head now wondering about the expansion joints in the concrete pads below the flagstone. You put those expansion joints in because concrete will crack, there is no avoiding it, so you want it to crack in the expansion joints, however when you are going to put a mortar bed and flagstone on top of the pads, what do you do so that those cracks in the concrete don't end up cracking the mortar around the flagstone?

The only thing I have even heard about is to lay a 6 inch wide strip of roofing felt over the joints and then the mortar bed. Is this the answer?

hatchet

Look for what's called an anti-fracture mat that goes below the mortar bed that's supposed to diminish the chances of cracking. But I couldn't honestly say if it would work in that situation. It's typically used for interior applications.

__________________
Rich
WIT Construction Co.
Superior MT Contractor

Unregistered

Still having troubles logging in here!! orto here. I have a lot of experience with flagstones and hard surface stones all in the Southwest US...The matting hatchet spoke about is the best solution to your concerns. However,to minimize propogations of cracks not only using a slip sheet over the joints helps, but by adding a fiber to your mortar bedset (Nikon nylon fibers works best for us) increases the stability of your mortar setting bed as well. making joint transfer that much harder. If your patio is rather large, think of your flagstone as a concret pour, and diamond saw in joints (that are not really visibly pleasing) off of major corners and possible stress points and guarantee that the cracks occur in those areas. In commercial construction we always use the simple rule "no area will be larger than 100 sq. ft. without a sawed joint(s) and an expandable colored caulk (they even make them with sand content). This will almost always stop cracking and give the stone laots of room to expand and contract. Hope that helped