Do you need a drain under a new slab foundation?

Answered by Jeffrey ~ January 24, 2012 ~ No Comments

I'm building a new home which has both a crawlspace and some slab areas (garage and patio). Do I need to put some sort of drain under the slab portions of the foundation? There is a French drain system around the exterior perimeter. Thanks

Bill W ~ Hickory, North Carolina

Jeffrey Anderson

Hi Bill. The drain system around the foundation is a good idea, but in most cases outdoor patios don't receive any type of drainage under their concrete slab. Of course, that doesn't include the four inches of gravel that should be installed before the concrete for the patio of poured. I'm not sure what common construction practices are in North Carolina, but I would also suggest using wire mesh in the patio slab pour. Many concrete contractors don't bother with the wire when doing a patio and several years later the concrete is breaking apart -- especially if the soil is questionable or hasn't been compacted properly.

My opinion is that you should have a drain system under your garage slab although it often isn't required by code or put in by contractors. A garage isn't considered part of the living area so moisture weeping through the concrete isn't going to damage any floor covering such as wood or carpet that might be inside the house. However, it costs very little to install the drain system under the concrete and might help on a winter day when you're laying on your back while working on a car.

I assume your garage slab will have a footer and a block foundation to contain the concrete. If your contractor is pouring a turn-down slab where the footer and slab concrete is combined into one pour, then I would definitely add drainage under the slab.

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