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Question DetailsAsked on 7/17/2017

Interior Basement Water Drainage vs French Drain?

I currently have an Interior Basement Water Drainage system. Do I still need a French Drain system? I have dirt around the foundation of my colonial 2 story 1500sqft house. Everytime it rains, the dirt around my foundation gets pretty wet and there is water build-up. I have a few questions:

1/ Do I need an exterior French Drain system in addition to the basement drain system?
2/ If no, what type of material (i.e. gravel) do I need on top of the existing dirt that would help with the water issue?

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If the basement underdrain system is keeping the basement slab and walls dry (no water coming out of wall or at the slab perimeter joint and slab is not damp) and the sump pump is keeping up easily in the heaviest rains, then a perimeter french drain would not be considered "necessary". Basically, a french drain, if set low enough and free-draining to lower ground, is more reliable than underdrainage under the slab, both because it intercepts the water BEFORE it gets to the slab and also because it is not susceptible to power outages (though sump pumps can be rigged with battery backup which will last for some hours to maybe a day or two depending on inflow, though always a concern during workdays or when away from the house overnight).


In some high groundwater cases, especially in river bottomlands or swampy areas, you need both because the groundwater can sometimes well up under the slab even with the perimeter drain functioning - or the perimeter drain needs to be set a lot lower to permanently draw down the water table.


But basically, if it is working well now, probably fine to go as it is, and certainly start with limiting water access to the foundation perimeter at ground level before looking at several thousand $ for a french drain system (which if done should also include foundation waterproofing and commonly insulation board as well).


As for the water around the foundation - while bearing in mind the dirt level should be at least 4 inches below the top of the foundation AND also 4 inches below the bottom of the siding (whichever is lower) and preferably more like 8-12 inches in invasive insect (termite and carpenter ant and such) territory, the soil should slope away from the house (preferably at least at a 2% and preferably at a 10% slope) for at least 3 feet all around the house and more like 6 feet in more open or free-draining materials. Paved areas should also be drained away from the house with a slight tilt (typically 1-2% so they are not dangerous when wet or icy).


To stop infiltration of water around the foundation you want to either put down a long-lived pond liner plastic (typically 20 or more mils thick) with bark or gravel or such free-draining decorative cover on top to protect from the sunlight, or use 2" or more of soil with a lot of fines (clayey material) sloped and decently compacted (dampen and walk it well in with hard sole shoes, completely packing the surface down at least - tamping with a compactor is better. Of course, it has to be soil the rain will not wash away. Or you can use concrete or asphalt walk around the house for that perimeter protection. In amny cases, just controlling surface runoff near the house and roof runoff will eliminate the need for any other work around the foundation at the ground surface.


The key is to slope away from the house to drain water getting near the foundation - and the prime solution is usually to use gutters and downspouts to get the large amount of water generated as roof runoff away from the foundation and draining to a point where it will stay away from the house, and if necessary using swales and/or french drains and/or berms in the yard (preferably 6 or more feet from the house) to keep surface runoff away from the house. Of course, if using berms, then the drainage from the house roof and immediate area has to be able to make it over the berm to a point away from the house - generally to the low side of the yard, sometimes having to run on the house side of the berm until it gets to the low side where a berm is not needed to keep outside water from getting to the house.


You can find a LOT more detailed previous questions with more detailed answers, and checklists of things to do to keep the water away from the foundation in the first place, in the Home > Basement Waterproofing link under Browse Projects, at lower left.

Answered 1 year ago by LCD




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