Written by Tim Gaughan, Urban Edge. Published September 19, 2026.
Before finishing a Portland basement, fix where the water comes from, mostly outside the house: gutters, downspouts and grading. Then test the walls and slab through a wet season, and build with materials and details that handle a little moisture. Finishing a basement that leaks traps the water behind new walls, where it grows mold you can't see.
Step 1: find the water
Watch the basement through a few heavy rains, ideally a full winter. Note:
- Wet spots on the floor, and where.
- Damp or white, powdery stains (efflorescence) on the walls.
- Water at the joint where the wall meets the floor.
- A musty smell.
Step 2: fix the outside first
Most basement water in Portland comes from the roof.
- Gutters that overflow and dump water at the foundation.
- Downspouts emptying next to the wall, or into old buried pipes that have cracked. Our Gutters & Downspouts site explains where they should drain.
- Grading: soil should slope away from the house, not toward it.
- Window wells without drainage.
Step 3: test
Tape a square of plastic to the slab and to the walls for a couple of days. Moisture under the plastic means water vapor coming through the concrete. A moisture meter gives numbers.
Step 4: bigger fixes, if needed
If water still gets in after the outside is fixed, the next steps are interior drainage, a sump, or exterior waterproofing. We fix gutters, downspouts, grading and window wells. For interior drainage, sumps and exterior waterproofing, we bring in a waterproofing specialist.
Step 5: build for a basement
- Insulation that doesn't hold water against the concrete.
- Bottom plates that won't rot if they get damp.
- A floor that handles moisture. Our Flooring site compares basement floors.
- A dehumidifier for the first year or two.
How we start every basement
We look at the outside of the house and the basement walls before we draw anything, and the written proposal says what needs to happen first.
Our work is covered by a 2-year workmanship warranty, in writing.
Book a free design consultation or call Tim, (503) 597-8520.