Basement floor has sunk

By Peter Hotton
Globe Correspondent / March 25, 2010

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Q. We own a split-entry house which is about 40 years old; we’ve lived there for 35 years. The lower part of the house is finished, including a bedroom and large family room. The flooring is poured concrete. We are having the carpeting replaced, and when our carpenter was pulling back our old carpeting, he noticed that a certain section of the concrete basement floor had sunk in one corner; about 3/4 to 1 inch along a 17-foot span on one side of the corner and 10-foot span on the other. It separated from the foundation and there are some hairline cracks in various areas of the floor we assume are related to it. Along the walls the studs are securely in place but you can run your fingers underneath this area in question and feel the foundation walls.

We’ve had water damage in this area several times when our water heater leaked and a drain clogged. We have no idea how long this problem has been there. The carpenter told my wife that we need to get an excavator to fix the problem which will cost us a lot of money, he then said we might need to get a structural engineer to address this.

There is a rumor that 40 years ago the builder of the hundreds of houses in our development poured concrete foundations, dumped trees between the foundations that he cut down, and covered this with concrete for the basement floor.

Is this sinking a major issue, how is it corrected, what are the options — disruptive and less disruptive — and who should correct it? How do we find a person to fix this? I’m a retired paper pusher and have no clue about houses and my wife is expecting the worst out of this situation.

RICK A. That is quite a large area for the floor to have dropped, but if the drop is 3/4 to 1 inch deep, I don’t think it is a big problem, and you and your wife can breathe easy. The flooding of the floor may have caused the water to erode the earth under the slab, and it may not get any worse. So far, I don’t think you need an excavator or any expensive work. Remember, for the slab, as with most concrete floors in basements and on grade (as in a garage), the concrete is poured inside the foundation walls, and is not structural. You mentioned the floor separated from the foundation, but it was never attached anyway. That rumor of 40 years ago is probably true, because in those days contractors often put tree trunks and other waste wood under slabs. It was cheaper than carting it away. Then of course it rotted, leaving gaps that allowed slabs to drop.

The gap near the stud wall is not important, I think, because the stud wall is on top of the foundation, not the floor slab. You could have a contractor add concrete to bring the floor back to level, but that won’t stop the floor from dropping further. The only thing that will stop any future dropping of the floor is to dig up the concrete, check the earth underneath for any rotting wood, and lay a new floor in that area. All expensive. I don’t know if your insurance policy covers things like this. A structural engineer can give you more information, perhaps. Finding a contractor to do something is a hit or miss thing, but you can do this: Join a club called Angie’s List, and for $45 a year that firm will find competent professionals.

An inch drop in a floor can be lived with. Many old houses have such a drop, for a different reason, and people live with it. Tell your wife that her house is not falling down.

Q. During a recent renovation, I noticed that when the men worked on the gas service that the old pipe in my moist basement seems like it was rusting. I wanted to throw a coat of paint on it to try to slow down the rusting which I think could cause an issue over time. What do you think?

JOHN, in Hotton’s chat room

A. First, check with the gas people to see if it is allowed. I have never seen an unrusty or painted gas pipe, and I think that when such a pipe rusts, the rust only goes so far, then rusts no further. Be sure to check first.

Globe Handyman on Call also appears in the Sunday Real Estate section. He is available 1-6 p.m. Tuesdays to answer questions on house repair. Call 617-929-2930. Hotton ( also chats online about house matters 2-3 p.m. Thursdays. Go to