building concrete basement floor

[Wendi]

Hey group! my current basement floor is about 5'5" from the top of the gravel to the bottom of the 2x8" wood plankings (from 2" side). i'd like to put down a concrete floor but dig out a little first. the gravel takes about 2" and then i hit dirt. i'd like to add about 6 inches of head room WITH the addition of the cement floor. could anyone enlighten me on the repercussions of digging 6 inches out of the dirt floor? like footing badness? and such. fyi-house was built in 1849 in Boston area. thanks for any info! W-

[raven]

Excessive footing badness. And an 1849 house besides? You want to get an engineer in on this, becuase you're going to be altering the stability of the house in a fundamental way.

John

[default]

You could probably dig out 4" of dirt and replace it with plastic sheet and 4" of concrete, without any great risk. But:

You cannot dig below the level of the footings without risking a cave-in. (how much of a risk that is depends on the soil, water levels, and type of foundation). If you're in Boston, you cannot turn the basement into "habitable space" unless you can get to 7'3" of headroom, after the slab is poured. you should check to see how close to the floor the water table is. You probably have at least one masonry chimney stack, near which you should not dig. If you aren't an inveterate DIYer, you should certainly have a contracter out to look the place over for specific problems.

What are you planning on doing with the space (why are you looking at this?) How big is the basement, and is it clear from wall-to-wall, or are there posts down the middle? What type of building, and is it freestanding, or attached?

--Goedjn

[Tom Baker]

Agreed. Foundations that old are often marginal when new. Age makes them less likely to be sound.

I managed to do something like that with my 1840 foundation. It took time and caution.

Tom Baker

[Wendi]

Even if I build a 2 foot high by 6inch deep supporting cement wall? w-

[barry martin]

Hi Wendi!

W > my current basement floor is about 5'5" from the top of the gravel to W > the bottom of the 2x8" wood plankings (from 2" side). i'd like to put W > down a concrete floor but dig out a little first. the gravel takes W > about 2" and then i hit dirt. i'd like to add about 6 inches of head W > room WITH the addition of the cement floor. W > could anyone enlighten me on the repercussions of digging 6 inches out W > of the dirt floor? like footing badness? and such. W > fyi-house was built in 1849 in Boston area. W > thanks for any info!

As the others indicated (and you probably know!) undermining the footings would not be a Good Thing. Dig down, but keep several inches from directly down from the footing. Dig about 3-4 feet wide at a time, pour cement retaining wall, continue with the next section. My parents did this sort of thing at their place except had to dig down several feet. No problems.

BTW, I would put some drainage tubing and a layer of gravel under the new flooring to allow an escape of any possible ground water.

- ¯ barry.martinþATþthesafebbs.zeppole.com ®

• Lobsters report that scientists dumped in boiling water feel no pain.

[raven]

As others have said: foundations that old are often marginal. The only reason why I'd have faith in the foundation is that it's stood the test of time. But that does not mean that it could withstand any mucking about with it. It was made to whatever passed for good practice in the day, out of who knows what materials. It's very possible that sections of the foundation are depending on the soil inside the basement for support and will collapse as soon as you dig: or it's possible that the things as solid as if were hewn from a single chunk of granite. That's why you've got to get an engineer on site to assess the thing: otherwise you're asking for big trouble.

I wouldn't do it.

John