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Waterproof Concrete Block from inside?

Waterproof Concrete Block from inside?

Waterproof Concrete Block from inside?

I want to know how I can waterproof concrete block from the inside.
I have a storm cellar that's about 10' x 15' underneath my house below ground level. The room has a poured concrete floor surrounded by concrete blocks. When the floor was poured, I placed a bucket into the floor, for a sump pump in case one was ever needed. Turns out I do. I installed a sump pump and it keeps the floor dry, however I would like to seal the blocks to prevent the ground water from entering. I can put sealant on the inside of the wall but I would also like to seal the blocks from the inside. I would like to drill some holes in the block wall above the area where I think the water is coming in, and pump some type of sealant into the blocks, and seal them from the inside. I am hoping someone has tried this before and could give me some suggestions.

RE: Waterproof Concrete Block from inside?

"and pump some type of sealant into the blocks, and seal them from the inside."

That's not going to be of much use if the blocks are porous. Moreover, I can't imagine that continuously being immersed in water is good for the blocks.

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RE: Waterproof Concrete Block from inside?

My Dad's house had a damp cellar.
I was a kid at the time, so I didn't notice how many coats of water-resistant magic paints he applied. Maybe it was the last one, maybe it was the combination.

Or maybe it was because he had a guy come in with a jackhammer, destroy and remove about a foot of floor adjacent the walls, install French Drains leading to a sump pit big enough for 3 pumps, and restore the floor surface with a gap adjacent to the walls.

The house is near the top of a hill, but there are Artesian springs in or near the basements of all the houses near the top of the hill. Houses farther down the slope have dry basements.

The house is now 70-ish years old. The basement walls are dry, but if the sump pump stops, it gets humid in a few days.
The next house up the hill, slightly older, fairly often has six inches of standing water in the basement.

Mike Halloran
Pembroke Pines, FL, USA

RE: Waterproof Concrete Block from inside?

Hello Littledon,
I don't have a sure-fire solution for you, but I do have a few concerns. My sister's house has this problem with its poured concrete foundation. At one point she had the entire foundation excavated and a water-barrier was applied. The contractor did not leave the walls exposed to the air long enough to dry, so the barrier didn't stick, and the seal doesn't work completely. She went from liquid water on the floor to damp concrete (partial victory).
The solution that MikeH. offers sounds like it can keep the floor dry, but it will not necessarily keep the walls dry, if they are wet, too. The addition of a membrane still sounds like a solution, but it has to be installed correctly. I do not believe injecting anything into the wall will help. Are you talking about common "masonry blocks"? They are very porous.
Is your storm cellar in any way structurally supporting the house? To my ears the meaning of "storm cellar" is that it does not support the house, but knowing for sure allows the rest of us to avoid making incorrect assumptions.


RE: Waterproof Concrete Block from inside?

Obviously the walls should have been waterproofed from the outside. It is awfully difficult to waterproof from the inside. There are some magic paints as Mike pointed out. The better ones have reactive constituents that expand with moisture contact. Look for floor coatings (to use on the wall) that contain calcium silicate or sodium silicate.

RE: Waterproof Concrete Block from inside?

The cellar does not support the house.

They are not continuously in water.

My problem is similar to the one described above with underground artesian water movement in the area.

Yes, the builder should have waterproofed the block but didn't. I don't think he was familiar with waterproofing, waterproblems, etc.. I was the one that suggested making a place to install a sump pump if it was ever needed. Turns out I was right.

I had a company come in and install a french drain along the inside of the cellar walls so that any water would drain to the sump pump.

My main worry is if the sump pump fails, the electricity goes out during a sustained rain...

I really don't like relying on the sump pump and am trying to figure out a way to solve it permanently.

RE: Waterproof Concrete Block from inside?

I doubt you can solve that permanently without a daylight drain or figuring out a way to have a massive battery backup for your sump pump.

RE: Waterproof Concrete Block from inside?

Check out Xypex. I have used it as a waterproofing admixture in concrete with great success. Although I have not used it in your particular application, I believe it works equally well (based on my recollection from a previous webinar they put on).

RE: Waterproof Concrete Block from inside?

I used DryLok masonry paint on my CMU walls and it's worked well so far but it's basically a temporary band-aid until I can install permanent perimeter drainage. The interior surface of the walls are dry but the blocks are still probably absorbing moisture on the positive side.

RE: Waterproof Concrete Block from inside?

Thank you all for your input.

I agree with XR250, and don't believe there is a permanent dry solution. If I am able to seal the block on the inside of the cellar, the water will go to the outside of the cellar, which would be underneath the house. If I am able to seal the inside of the cellar, and the inside of the block, the water may move to a more undesirable area. One more difficult to deal with. The crawl space underneath the house currently stays dry and I would prefer it to stay that way.

I think my approach will be to install a large battery with rectifier, in parallel with the AC to the pump. If the power goes out, the battery will keep the pump operating for some time. I will also create a drain hole inside the cellar, in the concrete block, a couple of inches above the top of the sump pump, and pipe it away from underneath the house. If the pump fails or if both power sources fail, gravity will keep it from filling up the cellar.

Thank you again.

RE: Waterproof Concrete Block from inside?

Maybe find a leftover Prius battery

RE: Waterproof Concrete Block from inside?

"...a large battery with rectifier, in parallel with the AC to the pump..."

There's a bit more to providing back-up power than that (ref as quoted).

Plus, the duration capabilities of any feasible battery capacity (running a sump pump) is unlikely to accomplish much more than delaying the inevitable.

RE: Waterproof Concrete Block from inside?

My son has a DC sump pump, purchased as a package/system with battery and charger.
As you might expect, it worked fine for a few years, until the battery died.
The battery looks like a fairly ordinary 12V boat battery, with wingnut/stud top terminals, but is of course labeled as extra special, so he bought a replacement from the system supplier.
Based on component sizes, I wouldn't expect the pump to run more than an hour from the battery alone, but if power outages are generally not long, it might be a satisfactory solution.
The obvious next step toward a more robust system is to substitute a truck battery, but a sufficiently smart charger might detect the difference in capacity and refuse to help, or otherwise annoy you.
The next obvious next step is a backup generator system for at least the critical circuits.
... and if you have that, an AC pump makes sense.

Mike Halloran
Pembroke Pines, FL, USA

RE: Waterproof Concrete Block from inside?

I can't offer much since I don't know yet what your house foundation is - peir, slab, concrete, stone, etc or how old the house is... I am a little concerned about the battery-backup plan for the sump pump. If you force yourself to keep a faithful daily charging, monthly monitoring and yearly replacement schedule then you can be reasonably confident that it will work when the power goes out. Otherwise, as Mike said, the battery could be dead on the day you need it.


RE: Waterproof Concrete Block from inside?

The battery backup for my electric sump pump is a deep draw marine battery. It looks like a regular automobile battery but supposedly is not. I have no clue if it is still good but I'll test it in the spring when there is no snow or ice out there.

RE: Waterproof Concrete Block from inside?


RE: Waterproof Concrete Block from inside?

If you have a public water supply you could install a water powered sump pump. They use a lot of water when operating, but no battery to worry about.

RE: Waterproof Concrete Block from inside?

I bought a home that was 75' long (50' house and 25' of garage footings 4' deep). It intersected a natural drainage pattern adjacent to golf course that was under construction. I had landscaping, AC, deck and a patio around the basement, so no work could be done on the exterior.

The next summer, I discovered that a lot of the drainage was intercepted by my house (and some neighbor's homes). About 15 minutes a good rain started, I had water running across the basement to a floor drain that was tied into the city drainage system.

I had a friend that built 200 to 400 homes a year and borrowed a little on his system because he never wanted a wet basement complaint. Because it was so successful, I had to adapt it to a newly built home. He used a special block for the first course that that had a filled top block on the first course that had 1" holes on the bottom and he installed 1" plastic pipe from the block cores into a pipe inside the footings and under the slab that lead to a waste-basket sized sump (16" diameter x 24" deep) with a pedestal sump pump.

I took his system and had a man use a concrete saw to cut into the floor slab about 2" deep, 12"to 16" along the exterior wall(cost of $150). I broke out the concrete and me and my son's "buddies" lugged the pieces out for disposal.

Since a typical basement wall is built on a strip footing and the slab is poured to restrain the walls from movement (9' clear ceiling height), I left a 6" wide sections of slab to continue the structural restraint.

The basement did not leak and has not leaked in the 20 years since I sold the home.

I think the contractor never sealed the exterior judging by the appearance of the walls when I moved in. I did put Thoroseal on the block walls to hide the joints/tooling and the applied Thorosheen(sp?) for a finished interior. I did use 2 coats of Thoroseal - one to seal and hide the mortar joints and the second to brush for texture.

I ended up using the block cores to collect any failure in the original basement exterior wall waterproofing. Any water leaking into the block cores was drained away into the interior plastic pipe into the sump with a pedestal pump. We did have 2 fatalities when my daughter's 2 gerbils got loose and drowned in the sump. I did installed a plywood cover over the sump with holes for cord and and pedestal.


Engineer and international traveler interested in construction techniques, problems and proper design.

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