How do you waterproof an unfinished basement floor?

You waterproof an unfinished basement floor from the inside, with a full-perimeter interior drain set below the slab at the floor wall joint and tied to a sump pump that carries water out. Coating the slab does not last. That interior drain is the core of our Forever Dry System, and most basement waterproofing runs around $3,000 to $30,000.

Here is what surprises a lot of homeowners. Water rarely comes up through the middle of a bare concrete floor. It comes in at the edges, where the floor meets the wall. That seam is the weak point.

So sealing the slab does almost nothing.

Paint-on coatings and floor sealers feel like a fix because they are cheap and fast. They peel. Water pressure under the slab pushes water right back through. You spent a weekend, and the floor is wet again next spring.

The edges leak because of the dirt around your house. When the foundation went in, the crew dug a hole wider than the walls, then shoved the soil back in. That backfill is loose. It drains slow and holds water against the foundation. Add a New Hampshire winter, frost heave nudges the walls inward, and you get water collecting right where the floor and wall meet.

An interior drain solves that at the source.

What an interior drainage system actually does

We cut a channel in the perimeter of the slab, set a drain pipe in clean stone, and route every drop to a sump pit. A sump pump (we size it at 1/2 hp per 120 feet of drain) lifts the water and sends it outside. Then we patch the slab back over the drain. The floor is solid again, just dry this time.

For a full basement system we also add a 12-mil wall vapor barrier and a dehumidifier so the air stays dry, not only the floor. That stops the damp, musty smell that hangs around a wet basement.

We do not do exterior excavation waterproofing. Digging up the outside of your foundation is a big, expensive job, and in our experience it tends to fail in two to three years as the soil settles and cracks reopen. Interior drainage is the durable fix. We would rather put your money where it lasts.

You may not need the full system. If your floor is bone dry and you only get seepage at one crack, you might just need that crack handled. We will tell you that when we look. Crack injection runs about $1,000 to $3,000, and it carries a 10-year warranty.

What drives the price

A few things move basement waterproofing cost up or down. How many linear feet of drain your perimeter needs. Whether the slab is thick or has obstacles. How many sump pumps the layout calls for. Whether you want a battery backup added.

On the backup: it is battery only, and a charged one keeps the pump running until power comes back. Worth it if your power flickers in storms.

The Forever Dry System is for basements only. If your wet space is a crawl space, that is a different fix called crawl space encapsulation, and it runs around $3,000 to $25,000. If you are seeing wall cracks or bowing, that is foundation work, not waterproofing. We will point you to the right one.

We are based in East Kingston, NH, and we serve New Hampshire, Maine, and Massachusetts. We have done this for more than 5,000 homeowners in New England, we hold a 4.9 rating across 257 Google reviews, and we are BBB A+ accredited. Every basement system is backed by the 603 guarantee: 100% guaranteed to be dry for life, transferable if you sell.

Want it looked at? Call us at 603-610-1770 or book your free inspection. Free estimate, and we get you a quote within 24 hours. If financing helps, we offer it through Hearth (subject to approval). Learn more about basement waterproofing here!

Related questions

Can you just paint or seal a basement floor to waterproof it? No. Floor paint and slab sealers do not hold against the water pressure under the slab, and they do not address the floor wall joint where most water enters. They peel, and the floor gets wet again. The lasting fix is interior drainage to a sump pump.

Where does water come into a basement floor? At the floor wall joint, the seam where the slab meets the foundation wall. Loose backfill soil holds water against the foundation, and pressure pushes it through that seam. The middle of a solid slab rarely leaks.

Does an interior drain ruin my basement floor? No. We cut a narrow channel at the perimeter, install the drain in stone, and patch the slab back. The floor is solid and usable. You can finish over it later.

How much does it cost to waterproof a basement? Most basement waterproofing runs around $3,000 to $30,000, depending on the linear feet of drain, the number of sump pumps, and whether you add a battery backup. We give a free estimate after we see the space.