View Full Version : Digging crawlspace deeper
11-08-2008, 06:09 PM
I have half a basement and half crawlspace. Many of the houses around here have basements that were once crawlspaces and they dug them out.
Since things are not going well these days I have more time than money. I have been toying with the idea of digging some of that crawlspace out to enlarge the basement.
The basement and crawlspace are poured concrete 8" thick. The top of the footing in the crawlspace is 41" from the top of the stem wall. I would need to go down about 4' to have the new basement as tall as the existing basement.
The soil here is very fine sand and clay mixed together. Some of the houses I have seen they dug right up to the edge of the footing and left it as is. Others they built a block wall up to the footing and some they build a block wall up to the floor joists.
I am concerned about going that close to the footing. I really don't want the soil to collapse and have the house fall in on me.
Some have said to stay 1' out from the footing per foot dug down.
If I dug all the way to the footing on all sides I would have a 25x30' room. I really don't need all that space so I would rather not go near the footings at all.
I don't know how important it is but I thought I better mention it, the water table is 2'-5' below the basement floor depending on the season. 4' below the basement floor it is solid sand and gravel.
11-08-2008, 06:12 PM
You are right to be concerned about the footers. I would get a general contractor familiar with foundation work and your local soil conditions to take look at it for you.
I had a very similar sized house with a crawl and we had it dug out right to the footing. Then we had a new 10" wall poured that extended from the new depth to even with the old footing ... no issues after 7 years. I would not use block in this application.
Typically you are not supposed to dig closer to the footing than a 45 degree angle, which is 1' away for every foot of depth. A qualified concrete contractor would be able to tell you how to make a safe foundation if you want to go closer than that.
11-09-2008, 09:41 AM
The rule of thumb is to stay out of the 45 degree "splay" below the footing. Based on your description of the soil, however, it will hold a vertical wall for long enough time to violate that rule if you do it the right way.
First make sure you have good grading around the house, generally 6' drop in ten feet and carry the water around and away from the house. Get rid of any overgrown bushes and especially large trees that have branches overhanging the house. There's generally as much of a tree underground as above and all the root mass plus the tree moving around in the wind can exert huge loads on a foundation.
Next I identify any significant point loads on the foundation wall, such as girders, and provide shoring to reduce the load to the wall at those points. I then tell the excavation contractor to cut sections out, right to the edge of the footing and straight down up to 4', as narrow as practical. Since most use a bobcat to move the dirt around we do a bucket width plus one foot. Then build your new sub-wall in sections, which has to be designed as a cantilever retaining wall.
Don't let the soil stand more than a day or so without shoring up the floor above. If the soil starts to fall or crack then the situation should be re-evaluated.
11-09-2008, 10:50 AM
Since money is not somethign I have a lot of right now I was thinking of staying about 6-7' from the footings and making a slope at a 45° angle and of course covering all soil with a vapor barrier. The I would pour a slab at the bottom of the slope and build partition walls.
I have good drainage around the house so water should not be a problem. The first week we were here the irrigation lateral broke open and flooded our pasture and eventually made it to the house flooding the garage then basement.
A buddy of mine brought his tractor over and we graded the pasture so if that ever happens again the water will end up in the ditch by the road and not go near the house.
11-09-2008, 02:27 PM
You shouldn't have any problem doing that, and the slope would eliminate the need for any structural sub-wall. I would recommend that on top of the slab you first run two rows of 4" or 6" wide concrete block to act as the base of the partition wall. That will keep the wood away from the soil even if you have some sloughing. The Code here is 8" min and yours is probably the same.
I'm not sure what you insulation code is but I'm guessing that it's r-19. That means a 2x6 wall and 2"?? of polystyrene foam on the outside of the block.
Fill the block cores with grout and use anchor bolts or Simpson mudsill anchors to fasten a pressure treated 2x4 or 2x6 sill plate with one edge flush with the side of the block course that will be your finished space. Build the 2x6 partition wall on the sill, braced at the top by a single top plate and the first floor joists. Build in one or more access hatches at least 20" x 30" (check your local code) so you can get to the crawl space. Use triple 2x4s with (2) 1/2" plywood spaces for headers on these small openings.
Insulate behind the block and sill with foam and the remainder with kraft faced fiberglass. The foam will be the vapor barrier for the lower section of wall.
Hang your drywall on the studs and hanging down over the block to about 1" from the floor. Use a bead of liquid nails on the lower edge to glue it to the block.