Expert: Daniel Humphrey - 1/4/2008

Question
QUESTION: I am in need of help in figuring how to safely raise a 2 car garage in the mountains. I want to pour a concrete pad under the garage then set the footprint on top of the concrete. Right now the floor is dirt and it is on 8 6"x6" concrete footers which is raising it 4" from the ground so you are able to look under the wall and see the outside. I want to turn it into a hunting cabin and need to support and level the garage. The footers have sunk into the ground leaving the garage off level.
I would like to see if anyone has any good ideas for me, I am very handy but have not done this before and I am just a bit concerned about doing it right. I want to make sure I reinforce the concrete at the exterior to support the walls so the concrete won't crack.

Thanks

ANSWER: Leigh,

I've done almost exactly what you're describing. Basically, you need to tie the walls together really well so that the structure is maintained as the building is raised. The walls cannot be allowed to bow in or out much. You can achieve this with a combination of wood spreaders and cables. You can actually straighten the walls this way once you start working with them.

I dug beneath the wall and jacked it up to level, then placed the wall on treated 4 x 4 held by adjustable steel screw post bases on concrete pier blocks positioned every 8 feet around the perimeter. I excavated about a foot deep for a footer all around, leveled and compacted the interior, and poured the slab with reinforcement so that the concrete came up three inches and surrounded the pier blocks, locking them in as part of the permanent foundation.

Something along these lines should work for you too. Use your imagination and good sense, and include a friend in the discussion to come up with one better.

Good Luck,

Daniel

---------- FOLLOW-UP ----------

QUESTION: Being a garage/cabin floor how thick should I go with the slab. Being I am deep in the ADK mtns getting a concrete truck up there is expensive, so i would like to do it all in one pour. The building is 20'x 20'. To tie in the walls would you recommend cable or wood and what size, tie into the header or footer or both. Its an old camp garage open framing with horizontal wood exterior that has shrinkage opening between the boards. We use it for our 1947 willys. We sold the main camp and lands last summer after 70 years, this piece is on a town road. The camp was 2 miles in the woods the garage was a stepping stone.

Answer
Leigh,

A 3-1/2" slab is fine. And 8 to 12" is all you need at the perimeter. You need to divide the floor into four squares so that your control breaks create areas no more than 100 square feet. Level and compact under the slab so that you minimize the amount of concrete you use.

You need to cable to the sills if you can, from corner to corner to create an X. Then you need to put in wood spreaders across to hold the dimensions. You may need an additional cable in the middle to keep the walls from bending in or out there. If it doesn't stay put, tie it in.

When you jack up the building, plan it so that you can insert one 8' treated 4x4 at a time, supported on each end with an adjustable post base. Eventually, you'll have the 4x4 reinforced sill all around.

You may need to put some forms on the outside before you pour so you don't waste concrete.