Interior drainage and basement French drains

How an interior drain works

The contractor breaks out a strip of the basement floor, usually about a foot wide, along the walls that leak. They dig a trench down beside the footing, lay perforated pipe in clean stone, and slope it toward a sump pit. A dimpled plastic membrane is often placed against the wall so water running down it drops into the drain instead of onto the floor. Then the trench is covered with fresh concrete.

The system does not stop water from reaching the wall. It gives that water an easy path away, which relieves the pressure that pushes it through the cove joint and up through the slab.

• Concrete cut along the perimeter
• Trench beside the footing
• Perforated pipe in washed stone
• Wall membrane or drainage board
• Sump pit and pump with discharge outside
• New concrete over the trench

When an interior drain makes sense

Interior drains are common when water comes in at the cove joint or up through the floor, when exterior access is blocked by driveways, patios or neighboring houses, or when excavating would be far more expensive. They work for poured, block and many stone foundations. They are less useful when the only problem is one crack or poor gutters, which can be fixed for much less.

What to ask

Ask how many linear feet they plan to cover and why, whether they will drill weep holes in block walls, what size and type of pipe they use, whether there will be cleanouts to flush the line, and how the new concrete will be finished. Ask whether the price includes a new sump pump and a battery backup. If your basement is finished, ask how walls and flooring will be removed and replaced, since the drain runs right along the wall line.

Living through the install

Jackhammering is loud and dusty. Expect the crew to seal off the work area with plastic, but plan to move stored items away from the walls beforehand. Most homes need one to three days for the work, depending on the length of wall and how much concrete has to come out. New concrete over the trench needs time to cure before you put heavy items back.

Keeping it working

An interior drain needs very little care, but the sump pump at the end of it does. Test the pump a few times a year, keep the pit lid closed and clear of debris, and make sure the outside discharge line is not buried, frozen or pointed back toward the house. If the drain has cleanout ports, ask the contractor how and when to flush them.

Typical interior drain costs

National figures from Fixr. Not quotes.

National figures, rounded, not quotes. Your price depends on linear footage, slab thickness, finished space and pump choice.

Sources: Fixr, French drain; Fixr, basement waterproofing