Drainage and Waterproofing

Water comes from above first

Most wet basements are a roof problem or a grading problem, not a groundwater problem. Water dumped at the foundation by a short downspout or a flat yard soaks the backfill, which is the loosest, most permeable soil on the lot, and finds every crack.

Shed and slope

• Gutters and downspouts carry roof water well away from the foundation. An inch of rain on a 1,500-square-foot roof is about 900 gallons.
• Grade: the finished soil must slope away from the house at least 1/2 inch per foot for at least 10 feet ¹, so 5 inches of fall in the first 10 feet. Paved surfaces next to the house must drain equivalently, and where a lot line or wall prevents 10 feet, a swale has to carry the water away ¹.

Backfill settles for years. A lot graded correctly at move-in often slopes toward the house a few years later; re-grade it.

Coat the wall

Masonry block foundation walls must be parged with at least 3/8 inch of type M or S portland cement mortar from footing to grade, then dampproofed with a bituminous coating, acrylic-modified cement, surface-bonding material, or a waterproofing treatment; flexible coatings listed for direct application can skip the parging ¹.

Dampproofing resists moisture moving through soil. It doesn’t resist water under pressure. Waterproofing, with membranes or sheet systems, does, and it’s worth the cost where groundwater rises high. Either way, the drain tile’s job is to keep water pressure from building against the wall at all.

Drain tile

Wisconsin requires a complete drain tile system where groundwater occurs above the bottom of the footing, and a municipality may set the soils and groundwater levels that require one ¹. The foundation excavation inspection is where that call gets made ¹. A complete system has ¹:

• Tile or perforated pipe inside and outside the footing, at least 3 inches in diameter, with its top at or below the top of the footing (in a walkout, where the footing is more than 4 inches below the slab, interior tile only, right under the floor).
• Bleeders, 3-inch pipes through or under the footing every 8 feet or less, connecting outside tile to inside tile.
• Clean coarse stone: at least 2 inches under the pipe and 12 inches over it and on the side away from the house, washed stone that’s nearly all between about 3/8 and 1 inch (ASTM #6 or #67).
• A slope of at least 1/8 inch per foot in the line leading to the sump; the rest level or sloping toward it.
• A sump pit, discharge piping, and a pump or a gravity outlet to grade.

The basement slab over an interior system sits on at least 4 inches of clean sand, gravel, or crushed stone ¹.

Pump it out, not into the sewer

The sump discharges to natural grade or through a pump ¹, out to the yard and away from the house, never into the sanitary sewer, where clear groundwater overloads treatment plants. Put the discharge where it won’t run back toward the foundation or freeze across a sidewalk, and consider a battery backup pump: storms that fill the sump are also the storms that knock out power.

What to take from this

Keep water off the foundation in layers: gutters and downspouts out, grade falling 1/2 inch per foot for 10 feet, masonry walls parged and dampproofed, and where groundwater rises above the footing, a complete drain tile system of 3-inch tile at the footing in 12 inches of clean stone, inside and out, bleeders every 8 feet, sloped to a sump that pumps to the yard. Dampproofing handles moisture; drainage is what keeps water pressure off the wall.