Drainage for slab on grade construction

by Ashby

This question is regarding the proper way to create drainage for a slab on grade construction.

We live in a condo, which is a slab on grade and wood frame construction with cement composite siding.
My questions are, how far should the yard grade (sod) be below the bottom of the siding and what should be the minimum percent of slope from that point for proper drainage?


Hi Ashby,
It might be different in different states. I am not sure. In New Jersey, 8 inches of the foundation should be exposed. This means the height of sod or a patio should be low enough to leave a minimum of 8 inches between the ground and where your siding starts.

As far as the slope for proper drainage goes, that should be a minimum of 2%, or 1/4 inch per foot. This will allow for the proper pitch to lead the water away and not create any standing water or even water getting into the house.

Visit these pages for more detailed information.
Landscape Grading
Lawn Drainage
Perforated Pipe

Condo properties are usually not that large, so there are not a lot of options as to where the water (which now follows the correct slope away from the house)can be led too. Ideally the area off your property is low enough and a drainage system has been well thought out for the community.

A last solution is always a drainage pit. This is a very large hole which is dug in the ground. The water is led to it and either enters at the top or via a pipe lower down. The pit has holes in it which allows the water to percolate out naturally. It must be sized correctly for the amount of water that is estimated that will be entering it.

Sometimes due to the property's existing conditions a 2% is not possible, or the water even pitches towards the house. Perhaps there is no room for a swale in the yard (another draiange solution). In that case you can always install a french drain along the perimeter of the house. This would be a perforated pipe laid beneath the ground with gravel on top. The concept is for the water to enter the pipe. The pipe must be pitched and you need an exit area. Again, a drainage pit can be utilized.

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