Sagging floors in an old house: what is normal, what is not, and how I fix them

Sagging floors in an old house are almost a rite of passage in Somerville and Cambridge. Many of our triple-deckers and two-families were built between 1900 and 1930, and a century of settling shows up as slopes, humps and doors that drift open. Most of it is harmless. Some of it is not, and the trick is telling the difference.

Below is how I diagnose a sagging floor, the fixes I use, and what national cost guides say it runs.

Why old Greater Boston houses have sagging floors

When I go down to the basement of a sloping house, I usually find one or more of these:

• A settled center girder. Most of these houses have a beam down the middle of the basement on a few posts. If a post sinks or its footing was never more than a flat stone, the whole middle of the house drops.
• Undersized joists. Framing that was fine for 1910 loads now carries stone counters, tile baths and big appliances.
• Notched and drilled joists. Generations of plumbers and electricians cut through joists. Today the 2021 IRC (R502.8.1) limits notches to one-sixth of the joist depth and bans them in the middle third of the span. Old work often ignores both rules.
• Rotted sills. Water at the bottom of the walls softens the sill and the joist ends that rest on it. My wet basement guide covers the usual sources.
• Removed posts or walls. A past owner took out a basement post or a wall upstairs and never added a beam. The code (R502.4) says a bearing wall running across the joists should not be offset from its support by more than the joist depth unless the joists are sized for it.

Normal settling or a real problem?

A gentle, even slope that has not changed in years is usually just the house's age. These signs tell me to dig deeper:

1. The slope is getting worse season to season.
2. The floor feels bouncy or a spot dips when you walk over it.
3. Plaster cracks keep reopening after they are patched.
4. Wood under the floor is soft when you push a screwdriver into it.
5. Post tops are crushed, tilted or sitting on crumbling brick.

For context, the IRC (Table R301.7) limits floor deflection in new construction to L/360 under live load. By my math, that is about half an inch on a 15-foot span. Old floors rarely meet it, which is fine, but it shows how small the numbers are.

Sagging floor repair options

• Sistering joists. We fasten a new, full-length joist alongside the weak one. The new joist needs real bearing at its ends: the IRC (R502.6) calls for at least 1 1/2 inches on wood and 3 inches on masonry or concrete.
• New posts and footings. Adding a post under a sagging girder shortens its span. Under IRC R407.3, wood columns must be at least 4x4 and steel columns at least 3-inch Schedule 40 pipe, restrained at the bottom. I put each one on a proper footing, not on the old slab.
• Jacking slowly. We lift the floor in small steps over days or weeks. Going fast cracks plaster, tile and pipes upstairs. Sometimes the right answer is to stop the sag where it is and not chase perfectly level.
• Girder replacement. If the center girder is rotted or cracked, we shore the floor and replace it, often with an LVL or steel beam.

Sagging floor repair cost

HomeGuide's 2026 national figures:

Greater Boston usually runs higher. What moves the number most: how many joists need work, whether pipes and wires are in the way, a finished basement ceiling, the girder and footings, and lead-safe work if old painted surfaces get disturbed. Every house is different, so I price it after seeing the framing.

Permits and when to call an engineer

• Somerville's ISD requires a building permit for all interior work except painting, tiling and flooring, so structural floor repairs need one.
• Owner-occupants can pull permits on one- and two-family homes. On a three-family or larger, a licensed contractor must apply.
• The building official can require plans from a registered design professional where special conditions exist (780 CMR R107.1).

I bring in a structural engineer when a girder or footing is failing, when the sag is active, or when we are adding posts that change how loads reach the foundation. My Somerville permit guide explains the process.

Need a hand?

I fix sagging floors in triple-deckers and two-families across Somerville, Cambridge and Greater Boston. Book a free walkthrough and I will check the basement framing and send a written line-item budget, usually within 48 hours.

Planning work on your home? See everything I do, how I price work and the towns I work in.

About the author

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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