How Water Gets Into a Basement: The Drain Tile System, Explained
This article is for general educational purposes only and is not engineering advice for any specific property. Reading it does not create an engineer–client relationship with VM Engineering. Foundation and structural conditions vary. Have a licensed professional engineer inspect your property before making repair, purchase, or legal decisions. Full disclaimer
A wet basement is almost always a water management problem, not a mystery. Water follows a path from the roof and the soil to the wall, and a drain tile system is built to catch it along the way. Here is how that path works, and the common ways water gets past it.
Interactive model
Follow the Water Through a Block Wall
Drag the model to turn it. Scroll or pinch to zoom. Use the buttons to turn the moving water and the labels on or off, and hover over a numbered item in the list to find it on the model.
The system
What a Drain Tile System Is
Drain tile is perforated pipe laid along the bottom of the foundation. It collects groundwater before it builds up against the wall and sends it to a sump crock, where a pump lifts it out and away from the house. The pipe is one part of a system, and every part has to work:
Grade and downspouts
The first line of defense. Soil should slope away from the house about ½ in. per foot for 10 ft, and downspouts should discharge well away from the wall.
Exterior drain tile
Older homes often have clay tile beside or on top of the footing. After decades it is commonly silted in or crushed and cannot be seen without digging.
Interior drain tile
Perforated pipe in clean stone just below the basement floor, along the wall. This is what most drain tile replacements install.
Weep holes and drainage board
Holes drilled into each block core let trapped water out of the wall. A plastic board covers the holes and carries the water down into the tile.
The last part is the sump crock and pump. The tile is only as good as the pump at the end of it. A sealed lid, a check valve, and a discharge that runs well away from the house all matter.
How it gets in
Seven Common Ways Water Enters a Basement
- Surface water at the wall. Short downspouts, flat or reversed grading, and patios that pitch toward the house put roof and rain water right against the foundation. This is the most common cause, and the cheapest to fix.
- Cracks and mortar joints. Water pushes through cracks in the wall and through open mortar joints. In a block wall it then runs down inside the hollow cores, out of sight.
- The floor-to-wall joint. When the drain tile is clogged, water builds up under and beside the floor and pressure pushes it up through the joint where the floor meets the wall.
- Cracks in the floor. A high water table, or tile that is not draining, can push water up through cracks in the basement floor.
- Window wells. A well with no drain, or one filled with leaves and soil, fills like a bucket and leaks through the window or the wall below it.
- Over the top of the foundation. Soil, mulch or a deck built up above the top of the block lets water run in at the sill, above any waterproofing.
- Pipe openings. Gaps around water, gas, sewer or electrical lines through the wall leak if they are not sealed.
Not all water comes from outside. A failed sump pump, a floor drain that backs up, or condensation on a cold wall in summer can look like a leak. Where the water shows up, and when, is the best clue to the source.
Warning signs
Signs the System Is Not Keeping Up
- White, powdery mineral deposits (efflorescence) on the lower block courses
- A damp or wet line where the floor meets the wall
- Water on the floor after heavy rain or a spring thaw
- A sump pump that rarely runs in wet weather, or a crock full of silt
- Peeling paint, a musty smell, or mold at the bottom of the wall
- A wall that is wet and also cracked, bowed or leaning
What to do
Start Outside, Then Work In
- Extend downspouts so water lands several feet from the house and keeps flowing away.
- Rebuild the grade so soil slopes away from the wall, and keep it below the top of the foundation.
- Clean out window wells and add covers.
- Test the sump pump by filling the crock with a bucket of water, and confirm the discharge is clear.
If water still gets in after the outside is fixed, a drain tile test will show whether the tile is draining and which sections are blocked. If the same wall is also cracked or moving, have an engineer look at it before pricing drain tile. Water and wall movement are often one problem, and the repair should address both.
Keep water away from the wall first, and give what gets through a clear path to the sump. A drain tile system moves water. It does not hold up a wall. When the wall is wet and moving, the drainage and the structure need to be looked at together.
VM Engineering LTD. We provide the plan. We do not do the repair. Contact us.
More reading: Foundation Wall Movement and Settlement: What Homeowners Should Know · What Is a Drain Tile Test? The Whole System, Explained. References: WI UBC Chapter VI, 30.50 Foundation Repair & Damp Proofing (WAFRP Best Management Standards).
