Search the forums

Loading

Increasing the height of a basement b...

Nick_Roberts's picture

*
I was thinking of increasing the headroom in my basement by raising the house and increasing the foundation height. The one thing that concerns me is the external pressure on the foundation. Could the existing foundation be topped with a two foot pressure treated knee wall, and then tie this to the floor joists with 45 degree bracing. This would create an insulatable two foot space at the top of the basement wall (just where you most need it) and give your basement a coffered cieling effect as well.

(post #171792, reply #1 of 9)

*
I was thinking of increasing the headroom in my basement by raising the house and increasing the foundation height. The one thing that concerns me is the external pressure on the foundation. Could the existing foundation be topped with a two foot pressure treated knee wall, and then tie this to the floor joists with 45 degree bracing. This would create an insulatable two foot space at the top of the basement wall (just where you most need it) and give your basement a coffered cieling effect as well.

(post #171792, reply #5 of 9)

*
We have just been asked to do the similar job for another client. He is in a flood plain and wants to raise up off his exsisting foundation 2 feet.To do so we have to lift his house about 2-1/2 feet. dowel the stem wall on 4 foot centers and 1 foot from each corner and then go up with 8 inch block install j-bolts or straps and fill all cells.Lower the house back down. If his stem walls had not been 8 inch thick we would have to re do the stem walls. His house is only a 1 story. He has never been flooded just gets lots of standing water under house in winter. We advised running some drain lines, a sump pump and a french drain, but he still wants to raise the house.
As far as cracking goes a good EVEN lift on a solid house will produce few cracks. We have lifted a few single stories and a 1890's two story with no breaks....oh oh shouldnt have said that I might have jinked us, kind of like talking about good weather....dont do it till the day is over. Our biggest problems is what RJT wrote of. Putting a house down on a level foundation when the house was built out of level to begin with. We always check before hand and advise clients. We use Lasers.
Josh at Silver Hammer Construction (Oregon)

(post #171792, reply #7 of 9)

*
Great feedback. Thanks to all for your insightful help. Now to get a more focused answer, pose a more focused question. The house I'm thinking of raising is one that dates to the 1880's, but has been extensively added to by me. I failed to mention that I'm a contractor, so for me any woodworking can be done by yours truly, but concrete or block work are going to have to be sourced out. 1/3rd of the house still has the original stone foundation under it and has only 5.5' of headroom. When I put on the addition I matched the floor levels because no one lives in a basement right. Since then I've had kids and I need a playroom badly. Before I finish the basement for short kids I thought I might as well raise up the house, replace the stone foundation with a new poured one with weeping tile and get enough headroom that one day it can be an adult playroom, Yeah pool! As far as the existing house having settled, forget it! the footing is right on bed rock took a backhoe with hammer a week to get what I've got now and it broke down twice and ended up catching fire! This is why I can't go down. The house perdates all its neighbours, who have all built higher to avoid going into the bedrock, so the other advantage to going up is that I will be able to back fill to a similar level.Even if the house has settled ever so slightly, it is my intention just to add the two foot band around the top of the existing foundation and as B.E.D. says it will be of 2x6 pt on 16" centers on a pt sill plate with 1/2"pt ply sheeting. It is the desire to add more backfill that made me pose the original qusetion about the idea of bracing the bottom of this new knee wall to the floor joists at 45 degrees.

(post #171792, reply #8 of 9)

*
I was thinking of increasing the headroom in my basement by raising the house and increasing the foundation height. The one thing that concerns me is the external pressure on the foundation. Could the existing foundation be topped with a two foot pressure treated knee wall, and then tie this to the floor joists with 45 degree bracing. This would create an insulatable two foot space at the top of the basement wall (just where you most need it) and give your basement a coffered cieling effect as well.

(post #171792, reply #2 of 9)

*
You can raise your house any amount that you like. Two feet sounds like a challenge. The angle brace that you are contemplating sounds excessive.

I'm assuming that the entire kneewall will be above grade. The size of the house above could alter my opinion. Bearing points must be considered.

I'd probably be thinking 2x6 studs minimum, with a treated bottom plate if it will contact the concrete foundation.

Sheath the kneewall with 3/4" ply. I'd also sheath the interior with ply after insulation and inspection.

Blue

(post #171792, reply #3 of 9)

*
Nick, tring to raise a house just to get two feet is a lot work and not some thing for a home owner to try. We have had to jack a few and some of the problems that you encounter are drywall or plaster cracks,plumbing leaks,roof leaks window cracks,ect ect ect..... It is usally cheaper and quicker to go down in your case, but some thing that a pro needs to involed in Good luck with your job

(post #171792, reply #4 of 9)

*
Bill; we jacked our house up 3' to put a basement in. Beforehand it was sitting on field stone. Once it came down on a level foundation we found out that the once level house was no more as it was built 'level' in relationship to an unlevel foundation. Due to the masterful carpentry performed decades ago, the house could have split apart where they added on with little more than lathe, plaster and exterior paint holding the two together. I'd nix the idea if I were you.

(post #171792, reply #6 of 9)

*
RJT and Josh, know the feeling about raising housing off foundations. Had a client who bought a old not maintaned farm house and wanted to rehab the whole thing. I tried to talk him into tearing down and starting again. Won't take the job as a set price and finally got the job as time and materials. One year later house finished owner moved in at a parity there he addmitted that I was right and they should have build new and used what was in the old house.

(post #171792, reply #9 of 9)

*
Nick:

You have reason to be concerned about your kneewall. Gabe, how do you account for wind loading against the side of the house? Soil and hydrostatic pressure? And how do you justify removing one of the foundation wall supports? The loading is not just vertical. Your kneewall to concrete anchors need to be capable of resisting the horizontal loading load. Can you use concrete block and dowels instead of a wood kneewall? Also, your concrete basement foundation wall is designed as a beam supported at the top and bottom; top by the floor joists, bottom by the floor slab. Since Im sure your wall is not-reinforced (no, one bar at the top and bottom does not constitute reinforcing) it is unstable without these supports in place since it cannot act as cantilever. You need to think this through a little better and get an engineer involved if you are unsure of how to calculate the loading and do the anchorage design

Dave