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Question DetailsAsked on 5/30/2011

Basement waterproofing instructions

How do I waterproof a basement?

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3 Answers


Alot depends on the elevation of the ground around the home. There are two ways to do this, one from insid the basement next to the walls by installing a drain system with a lift pump. I dont like this but in some cases its the only option, because you will always have a moisture problem that will have to be solved by placing a dehumidifier in the basement. The best way if possible is to dig the foundation, install friench drains below slab so that it drains naturaly with t-60 waterprofing membrane to foundation walls with at least 3/8 styrofoam insulation to protect the t-60 membrane, in some cases you will have to install a lift pump as well.


Answered 9 years ago by gary clemons


The BEST way to keep water out of your basement is usually to make sure that the ground around your foundation on the exterior of the home grades AWAY from the home. The ground should slope down about 12 inches over a distance of about ten feet.
If you have a properly sloped grade around the entire foundation, then most of your basement water problems will disappear.
Another easy fix is to make sure that your downspouts empty water so that it will drain AWAY from the home. I can't tell you how many downspouts I've seen that dump water out right next to the house, or very close to it, and then the existing grade forces that water right up against the house. Add elbows and extensions as needed to make sure that the downspouts empty the water beyond any grading crests that would force the water back up against the house.
If you have proper grading and no rogue downspouts and you STILL have a basement water problem, then the fixes get more complicated and more expensive.
One 'cheaper' option is to dig up the entire perimeter of the foundation right down to the footings. Then you have to scrape and brush all of the dirt off of the foundation. Hose it down if necessary. The next step to "damp proofing" a foundation wall is to smear a thick tar sealant over any cracks or any tie-rod holes in the foundation wall. Trades people call this product 'elephant turd'. This should be allowed to set up thoroughly at which time a coating of liquid tar can be applied over the entire foundation wall. Make sure it runs down onto the footing to ensure a seal where the wall meets the footing.
This tar coating should be allowed to dry thoroughly before backfilling. It's best to backfill with sand and the backfill should be compacted thorougly. Without proper compaction, the backfill will just settle over the next couple of years creating a grade that slopes TOWARDS the home. As I said earlier, that negative slope creates more basement water problems than anything else.
If you have a high water table you might have to consider installing a complete sump system. This will cost thousands of dollars! To install a sump system properly your contractor will have to dig up the entire perimeter of the foundation/footings and then lay a perforated PVC drain tile all around the house. The tile should be wrapped in the cloth filter. More than likely, you will need to then core holes through the concrete footing and install T's every 20 feet or so. The T's should snap to your exterior drail tile and then it should all be completely covered with clean pea stone. There should be a mesh filter that runs underneath the pea stone and then wraps over the drain tile and pea stone before the backfill material is placed on top of the whole system.
This is only part of the job!
Then you need to saw cut your basement concrete slab along the entire perimeter of the foundation, remove the concrete and do the exact same thing, again tying your drain tile into the T's that you installed through your footings.
In one corner of your basement, they need to dig a big hole and install a sump crock. Your drain tile will tie into this crock from both directions. Then you put a sump pump in the crock, attach PVC piping with a back check valve and run the pipe up the wall before elbowing it and punching it through the band joists and emptying the water outside. Plug it in and let the pump lower the water table around your home.
Again, you've got to make sure the water runs away from the home and not back towards it!

Source: J. G. Hamm Construction - Ypsilanti, Michigan

Answered 9 years ago by JGHamm


No insult meant to JGHamm, but connecting the exterior foundation french drain through the foundation wall to the interior sump pump is absolutely the WRONG way to do it - you should NEVER connect the outside drainage system to the inside.

He was absoloutely right about keeping the source of water away from the foundation in the first place - with surface slope, impervious layer (asphalt, concrete, underground liner) near the house if you have very permeable soil, controlling roof drainage and getting it away from the house, etc.

The intent is to keep the water outside the foundation area - by waterproofing the foundation walls and if necessary with drainage (french drain) at the base of the foundation, which if a drain is necessary should be free-draining to the open surface at at least two locations (in case one gets blocked or freezes) if feasible, otherwise it may require a wetwell OUTSIDE a bit away from the house, with an outdoor service sump pump discharging at an appropriate location well away from the house.

The LAST thing you want is a connection of a large water source into your basement, which is what you have if you connect to the outside perimeter drain. Your sump pump and interior sub-slab drain system should only be asked to remove the little amount of water that makes it to the bottom of the slab, not the entire perimeter drain flow.

On the backfill around the house - the old school is to use sand against the house to protect the sprayed asphaltic membrane or stick-and-peel bitumastic membrane. Far better is to continue the waterproofing down to the bottom of the footer (I like membrane far more than sprayed - far thicker and more impervious), AND extend a foot or two in a narrow trench BELOW the footing with bitumastic membrane, to cut off water from short-circuiting under the footer into the interior slab area. This has to be applied in short dection to avoid weakening the foundation bearing capacity, and heavily compacted with backfill. Then apply 2 inch or thicker styrofoam insulation board with bitumastic caulk or spray-on tar to protect the membrane, and also provide insulation to the wall to reduce interior sweating. If the styrofoam sheets are caulked at the joints that also provides another layer of moisture barrier. To really reduce sweating onthe interior surface, a double layer of 2" closed-cell styrofoam with all joints offset really makes a tremendous difference. Then backfill with compacted structural fill, topped off with 6-12 inches of highly compacted material with enough fines (or cement) so it becomes essentially impervious after placement, of course sloped away from the house. Using compacted structural backfill limits water infiltration along the foundation, rather than the "pooling" that occurs if you use sand or gravel, which basically puts your house in a swimming pool reservoir of water, which is inviting leakage through the foundation.

In really difficult situations where a house is basically sitting in a swamp or low spot and there is no way to free-surface drain the french drain, to prevent the wet well from having to handle too much water you can put heavy-duty HDPE pond liner suspended on the outer face of the trench as you backfill, so there is a barrier to the water even getting to the french drain in the first place through the surrounding soil, and the drain only handles what falls directly on the few feet of fill around the foundation, and what gets to the french drain at the bottom of the foundation wall through groundwater flow at depth.

Answered 7 years ago by LCD

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