Your thoughts on my basement plans

EddieWalker · Oct 25, 2015

My goal for some time next year is to build an addition onto my house that will include a 16x16 basement that will be used for storage and when needed, a storm shelter from tornadoes. I don't feel it is practical or even possible to install a drainage pipe under the basement because of the gradual slope of the land and everything that I have built downhill from it's location. While not impossible, it's a bigger job then any other part off the addition with a ton of clean up, replanting and some rebuilding involved.

My thought is to dig the hole with my backhoe, then line it with a rubber, water proof membrane similar to what is used in koi ponds. Then pour the floor right on top of the rubber membrane. Then build up the inner walls with 3/4 plywood and brace it all up against each wall without any forms on the outside, just the same rubber membrane up against the dirt walls of the hole. Then pour concrete into the wall cavities and when dry, remove the bracing, but leave the plywood for interior walls.

What is wrong with this idea? By not having any sort of drainage, I'm thinking that I'm sort of doing the exact opposite of a swimming pool. Whatever moisture that does work it's way down into the soil next to the basement will just remain there and not have anywhere to go, which in my thinking means more runoff. Gravel is good if I have drainage to carry the water away, but if I don't have a way to get rid of the water, then all I am doing with gravel is creating a storage place for water.

I thought about a sump pump, but we lose power all the time during severe weather, and I hate the idea of running a generator if I don't have to, especially in a small space that will require outside venting.

TCJatko · Oct 25, 2015

How about some kind of drywell to channel the water away and let it percolate into the soil at its own pace?

SirReal63 · Oct 25, 2015

What is the life expectancy of the membrane?

EddieWalker · Oct 25, 2015

In my experience, any sized area under the slab that I put gravel will fill up. I received 6 1/2 inches of rain two days ago, and inch and a half yesterday and another inch today. This is extreme, but it's not uncommon to get 3 inches out of a heavy storm here.

I'm searching online for different types of pond liners and roofing membranes. Some have 30 year warranties, and they are designed for exposure to the sun every day. I'm wondering if it would every break down buried in the ground?

GPintheMitten · Oct 25, 2015

I would put in a subpump. Run a generator if necessary but leave it outside with an extension cord to the inside.

My subpump has an auxiliary 12 volt pump with battery and trickle charger.

bigtiller · Oct 25, 2015

My 2¢ worth says to make a plan for removing water. With my luck, some time, some how, it will spring a leak and there will be water down there. Maybe a dehumidifier will be enough to collect it before it becomes a problem.

I am sure you have thought of this, but why not build the storage/storm shelter above ground, close too but separate from the house?

EddieWalker · Oct 25, 2015

Yes, we did consider other locations, but when two tornadoes hit less then ten miles away last year, nothing above ground is going to be safe. The question becomes how far do you want to have to go to get to it during a heavy storm at night? My thinking is that if we are going to build one, it should be close enough to not hesitate getting into it. Then there is the added advantage of using it for storage.

There are some 45mm EPDM pond liners from Firestone that are claiming a lifetime warranty and I've found some 60 ml roofing EPDM liners with 20 year warranties that should be stronger, and even some 90 ml sizes.

If I overlap my seams by 2 feet and glue them together using the best glue I can find, I don't see why this wouldn't work.

In every example of water proofing a basement that I can find, they are all about shedding water down the walls to a collection area under the basement, then removing the water by either gravity drainage or electric pumps. Why doesn't anybody just make the entire hole in the ground 100% waterproof?

GPintheMitten · Oct 25, 2015

EddieWalker said:
Yes, we did consider other locations, but when two tornadoes hit less then ten miles away last year, nothing above ground is going to be safe. The question becomes how far do you want to have to go to get to it during a heavy storm at night? My thinking is that if we are going to build one, it should be close enough to not hesitate getting into it. Then there is the added advantage of using it for storage.

There are some 45mm EPDM pond liners from Firestone that are claiming a lifetime warranty and I've found some 60 ml roofing EPDM liners with 20 year warranties that should be stronger, and even some 90 ml sizes.

If I overlap my seams by 2 feet and glue them together using the best glue I can find, I don't see why this wouldn't work.

In every example of water proofing a basement that I can find, they are all about shedding water down the walls to a collection area under the basement, then removing the water by either gravity drainage or electric pumps. Why doesn't anybody just make the entire hole in the ground 100% waterproof?

Maybe because no one has found a 100% effective cost efficient way to do it. Too much potential in all the seams for water to enter.

EddieWalker · Oct 25, 2015

Good point. I'm also worried about wear, or friction over time. Actually more worried about that then I am about not getting good seal, I think that's pretty easy to do, it's what happens long term that has me nervous.

Looking at the different websites, 24ml EPDM seems pretty standard for the walls of basements. I wonder if that ever wears through from movement in the soil?