I have been approached to dig out and turn a crawl space into a full basement. The lower floor currently has an office and washroom (approx 200 sqft). The sfr (square footage ratio) has been increased in the area so the client wants to dig out the reamining portion of the crawl space and make it a full basement (add another 800 sq ft). The home is about 10 years old. I am very sure that the footings would be around the entire section of the home. So I am assuming that to perform this task I could dig out down to the footings, lay in 4″ of gravel, a bed of extruded stryrofoam, wire mesh and then lay pvc for the radiant floor heating and cover with concrete. Any references to pictures, documents, or other sites would be welcome.
Any suggestions and/or comments?
Replies
Crawl Footings
I'm not sure if I undertand you correctly but I think you are saying the crawl space footings are at the same elevation as the basement footings. As a builder, I would never install a full foundation wall for a crawl space...so I am questioning why they would have done so on this home. The whole point of creating a crawl is to save money.
I think the project is going to cost too much for what the customer will be getting and I would personally advise them to add space to the house in other, more economical, ways.
Good luck,
Michael
Dig Out the Dirt
Let's say hypothetically that they are the same height. Is there any risk at digging the earth out?
Hypothetically
If the crawl footers are at the same elevation as the basement footers...there is no reason you cannot excavate down to them. Be sure not to excavate under the footer...otherwise you will be ok.
I still doubt the wall is a full wall under the crawl...why wouldn't they just make that a full basement? The cost of excavating that area is almost nothing with a full size track hoe already on site. The only additional cost for the builder would have been the difference between the crawl concrete (usually a slush coat) and the full 4" thick floor...
Good luck to you.
hypothetically
I'm with armchairbuilder here.
However for your hypothetical case, no. Now you have other issues to address. The first being how are you going to dig all that dirt out? If the area to be excavated is bounded by an existing foundation wall and footings on four sides you have no way to get the spoil out except through the access scuttle or excavating equipment into the space. That means hand digging and some kind of sled rig to emove the spoil. Labor intensive and way to expensive per square foot of usuable space gained.
Chance are you are looking the deeper foundation wall in the area of the office and assuming the same depth of excavation was carried around the crawl space area that shares a common wall. A simmple post hole dig outside the crawl space foundation wall will reveal the acctual depth of the footing there.
Take you hypothetical to a reality check, then come back and tell us what you found.
Well, the footing wall would not be designed to withstand soil pressure.
But you're dreaming anyway. No way the footing is deep enough.
There are two ways to handle this:
Jack up the house and put a new foundation (with basement) under it.
Excavate to the footing but don't undercut it. Then dig a deeper footing about a foot inside and install a new, reenforced wall. Grout well between the two walls. You lose about a foot on all sides.
I'd be quite surprised if the footings are deep enough to permit you to excavate a basement to the footing walls without reenforcing them somehow. You'd need to excavate 6-7 feet at least, and nowhere in the US is the frost depth that low.
Site Visit
Hello Guys,
I met with the home owner and got the real story. Apparently the previous owner of the home was a builder who eventually planned to expand to a complete basement (with or without permit). In this area the sfr (square footage ratio has gone from .60 to .75) hence the desire to expand. I guess they had hired a contractor but due to his health he cannot continue on with the job. They have done an exploratory hole and the footings do go all the way to the bottom (so even with the office at the rear/entrance in the attached picture). The height of the current crawlspace is about 3.5' so there would be a lot of earth and a skim coat of 2" of concrete currently convering the earth.
The only economical way to do it is to cut out a section of the current crawl foundation so you ca run a skidstreer down there to excavate it. Did the original builder think this far ahead and put in a header somewhere. No way I would ever do it manually...but you could. If manual is your only option, I'd go looking for 10+ guys who speak funny and work for cash.
Depending on the soil
a Vactor truck might be more cost effective. Particularly if you can dispose of the soil somewhere close.
There was an article in JLC about 2-3 years ago where they did exactly that, you can download the full article for a fee.
Basicly, they excavated short sections then pour the wall under it - they joined the old footings with new wall with nonshrinking grout.
If full height walls are already there - HOORAY! Now you just have to keep the bottoms from collapsing in when you dig the dirt away. The slab you pour at the end will permanently keep the walls from crushing in.
I've done it.
I've dug out my crawlspace and continue to expand my basement. So far, I've created about 800 square ft. of basement space. Some pictures and comments about my project can be found in this thread
http://forums.finehomebuilding.com/breaktime/construction-techniques/make-crawl-space-basement:
Working in clay, I can dig and remove about a cubic yard of packed soil in an average day using hand tools and buckets.