Wet basement repair in Greater Boston: causes and fixes, from cheap to expensive
Wet basement repair in Greater Boston: why old Somerville homes leak, fixes from gutters to waterproofing, national costs and engineer red flags.
Mateus MagnoLicensed Construction Supervisor · October 26, 2026 · 3 min read
Most wet basements in Greater Boston are fixed from the outside in: clean gutters and extend downspouts, regrade so soil slopes away, then add an interior drain and sump pump if water still gets in. National cost guides put interior waterproofing at $6,000 to $12,000 and exterior waterproofing at $10,000 to $18,000; Greater Boston usually runs higher.
Wet basement repair in Greater Boston should start with the cheapest fix that could work, not the biggest contract a salesperson can write. In old Somerville and Cambridge houses, a lot of water problems come from the roof and the yard, not the foundation itself.
Here is how I diagnose a wet basement, the fixes in order of cost, and when you need an engineer instead of a contractor.
Wet basement repair starts with the cause
- Roof water. Clogged gutters and short downspouts dump water right at the foundation.
- Grading. Over a century, yards settle and patios tilt back toward the house.
- Old foundations. In my experience, many triple-deckers and two-families sit on fieldstone or brick foundations with mortar that has washed out over the years.
- High groundwater. A Cambridge DPW stormwater study of the Concord-Alewife area found groundwater generally within about 3 feet of the surface there, with poorly draining soils.
- Sewer backups and flooding. Somerville's Flood Ready page warns that street flooding can push sewage back up into basements, and that the city also faces Mystic River flooding during large coastal storms.
Step 1: gutters and downspouts (cheapest)
The EPA lists cleaning and repairing gutters as a basic moisture control step. I clean them, check the pitch and extend every downspout well away from the house. It takes an afternoon and, surprisingly often, solves the problem on its own. HomeGuide's national cost guide puts replacing downspouts at $220 to $625 as a national average.
Step 2: fix the grading
The Massachusetts version of the code, in its Chapter 4 amendments, says lots must drain water away from foundation walls, with ground falling at least 6 inches within the first 10 feet. It also says grading must not flood or damage a neighbor's property. HomeGuide puts regrading at $1,000 to $5,000 as a national average.
On tight Somerville lots, I often re-pitch a walkway or patio rather than regrade a yard that does not exist.
Step 3: interior drain and sump pump
If water still comes through the wall-floor joint, an interior perimeter drain feeding a sump pump is usually the next step. HomeGuide lists these national averages:
| Fix | National average |
|---|---|
| Sump pump | $600 to $2,500+ |
| Interior French drain | $4,000 to $17,000 |
| Interior waterproofing, total | $6,000 to $12,000 |
| Exterior waterproofing, total | $10,000 to $18,000 |
Greater Boston usually runs higher. In Somerville, the Engineering Division may review any new or existing sump pump or foundation drain, per the City's Engineering permits brochure. Where the pump discharges matters, so plan it with the permit. I also like a dedicated circuit and a battery backup, because the storms that flood basements often knock out power too.
If your basement has a floor drain, toilet or shower, the City says you may need a professional plumber to install a backflow preventer.
Step 4: exterior waterproofing (most expensive)
Digging down to the footing, waterproofing the wall and installing a footing drain is the most complete fix and the most disruptive. Where a high water table is known, the 2021 IRC requires foundation walls around below-grade space to be waterproofed, not just dampproofed, and it calls for foundation drains around masonry foundations that enclose habitable space. On a triple-decker lot with a driveway on one side and a neighbor three feet away on the other, access alone can drive this price.
When you need a structural engineer
- Walls that bow inward, or horizontal cracks in block or poured walls.
- Stones loose or falling out of a fieldstone wall.
- Doors and floors upstairs going out of level quickly.
- Plans to lower the basement floor or dig near the footings.
These are structure questions first and water questions second. I bring an engineer in before quoting, and the engineer's repair plan becomes the scope we price.
Wet basement now, finished basement later
A dry basement is step one of any basement project. Once it stays dry through a spring, see my guides to egress windows and basement apartment rules. Every house is different, so I give a written line-item budget after a walkthrough.
Need a hand?
Tired of a damp basement in Somerville, Cambridge or Medford? Book a free walkthrough. I will trace where the water comes from and send a written line-item budget, usually within 48 hours.
Frequently asked questions
How much does wet basement repair cost?
National cost guides put interior waterproofing at $6,000 to $12,000 and exterior waterproofing at $10,000 to $18,000. Simple fixes like downspout extensions or regrading cost far less, and Greater Boston prices usually run higher than national averages.
What is the most common cause of a wet basement?
In my experience in old Greater Boston houses, roof water and grading cause most problems: clogged gutters, short downspouts and soil or patios sloping toward the house. Fix those before paying for drains or waterproofing.
Do I need a permit for a sump pump in Somerville?
Somerville's Engineering Division may review any new or existing sump pump or foundation drain as part of a building permit, so check with the City before installing one.
When should I call a structural engineer about my basement?
Call an engineer if walls bow, you see horizontal cracks, stones are falling out of a fieldstone wall, or you plan to lower the floor or dig near the footings.
Planning work on your home? See everything I do, how I price work and the towns I work in.
Mateus Magno founded Magno & Co in 2020. He is a licensed Massachusetts Construction Supervisor (CS-123658), registered Home Improvement Contractor (HIC #220484) and certified Lead-Safe Renovator Supervisor who has renovated and managed over $5 million in projects across Greater Boston.
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