Interior basement waterproofing: drains and wall liners

An interior system does not stop water reaching the foundation. It catches the water at the edge of the floor and sends it to a pump before it can spread across the basement.

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You will be put through to an independent local waterproofing company. Calls may be recorded.

How the drain works

A strip of the concrete floor, about a foot wide, is broken out along the wall. The soil under it is dug down to the top of the footing. A perforated pipe is laid in the trench on a bed of clean stone, with a slight fall toward a sump pit. More stone goes over the pipe and new concrete closes the floor.

From then on, water that comes through the wall or rises beside the footing drops into the stone and the pipe instead of running out over the slab.

A basement wall and floor seen from the side. Groundwater (blue) presses on the wall from outside and gets in where the wall sits on its footing. It falls into the stone-filled trench and the ringed green pipe, which runs under the floor to the pit on the right. The green line on the face of the wall is the liner. The grey pipe takes pumped water up and out of the house.

The liner on the wall

Hollow block walls fill with water and weep through the mortar joints at any height. On those walls a plastic liner or a dimpled sheet is usually fixed to the inside face and tucked into the trench. Water runs down behind it, out of sight, into the drain.

Ask whether the liner is closed at the top. An open top lets damp air from behind the sheet into the room. Ask too whether small holes will be drilled in the bottom course of block so the cores can empty into the trench.

What it does to the house

Everything happens indoors. The yard, the driveway and the neighbour's side are left alone, which is why this is the method most often proposed on tight lots.

Indoors it is rough work: a jackhammer, concrete dust, and broken floor carried up the stairs in buckets. Whatever is stored along the walls has to move first. Finished walls over the trench are cut away at the bottom or taken down. A furnace, water heater or oil tank standing against the wall has to be worked around, so ask how the drain gets past it.

Questions for the company

Will the drain follow every wall or only the wet ones, and what is the reasoning?
Where does the sump pit go, and where will the pump send the water once it is outside?
Are cleanout ports included so the pipe can be flushed in years to come?
How thick is the new concrete, and will it sit level with the old floor?
What happens at the foot of the stairs, at the chimney base and at the outside cellar door?
Does the job need a permit from the Clifton Building Department, and who applies for it?

What usually goes with it

A drain has to empty somewhere, so it comes with a sump pump, and a pump in a storm-prone area wants a battery behind it. The basement air will still be damp for a while, so a dehumidifier is often part of the same proposal.

Deal with roof water first. If a downspout empties beside the wall, an interior drain will spend its life pumping out your own gutter water. Yard drainage covers that.

Interior drain questions

Will the walls still look damp afterwards?

Bare walls will. The drain manages the water; it does not dry the masonry. That is what the liner is for: it hides and drains the wall face. Without one, expect the same marks on the block, just no water on the floor.

Can it go into a finished basement?

Yes, with losses. The bottom of the wall finish comes off along every drained wall and is rebuilt afterwards. Agree in writing who does the rebuilding, because many waterproofing companies stop at the concrete.

Does an interior drain help a wall that is leaning in?

No. It relieves water, not soil pressure. Mention a leaning or stepped wall when you call so the company knows a structural opinion is needed first.