Foundation crack repair

Which cracks leak, and why

Poured concrete shrinks as it cures, and most foundations crack within the first years — typically a vertical or diagonal line running from a window corner or the top of the wall toward the floor. The crack goes through the wall from the soil side to the room side, so when the ground outside is wet, water follows it in.

How injection works

1. The crack is cleaned and small plastic ports are fixed along it every 15–20 cm.
2. The surface between the ports is sealed with epoxy paste so the resin cannot run back out.
3. Polyurethane resin is injected from the lowest port upward. It reacts with moisture, expands and fills the crack all the way to the outside face of the wall.
4. The ports are removed once the resin has cured, and the wall can be painted or covered.

Polyurethane stays flexible after curing, so it keeps its seal when the wall moves slightly with the seasons. Rigid epoxy is used instead when the purpose is to restore strength to a structural crack.

Cracks that need more than injection

• A horizontal crack along a block wall, or a wall that bows inward, points to soil pressure. It needs a structural assessment, not a sealant.
• A crack wider than about 6 mm, or one where the two sides are no longer level with each other, shows the foundation has moved.
• Stair-step cracks in a block wall follow the mortar joints. Block walls are hollow, so injection does not work; they are sealed from the outside.

Questions about foundation crack repair