Just wondering whether anybody can help me with a problem we have in a barn conversion we are doing.
We have to lay a concrete slab of 150mm deep, 300 at edges to support a 2 storey timber kit internally. The roof will be supported by the original 560mm thick sandstone walls. The foundations of these walls are very old and are very shallow. They run in parallel to the ground with the building built plumb from them up. So at one end of the building the foundations are below the new slab, all ok. However at the top end the floor slab ends up - at or below the bottom of the foundations of the walls. I am concerned about water ingress from the clay / sand below the walls and how i seal this up, also where does the dpm go in this area. Please see the attached diagrams (4) which should make things a bit clearer.
Many Thanks in anticipation for any help or even ideas ? or even where I can go on the web so see any such pictures / resolutions, etc.
Number 1 shows our basic problem, 2 - a possible solution, i believe a clubfoot foundation ? Number 3 - another possible solution, under pinning so some degree ? NUmber 4 - DPM ? most likley solution ?
The outside ground level here is about 700 mm higher than inside and we do intend to render and tank the outside and install a french drain system down the wall.
The DPM must go under the floor slab and up the walls.
The less you disturb the sub soil in the area of the foundations the better, I would not recommend undercutting the foundations as in 3.
Is it possible to form a step in the floor level so that you avoid this problem all together?
Yes, its one of the options i guess, but i will then have a headroom issue. The building was always designed to be two storeys but the top storey as a granery so it was much lower in headroom. I have calculated that we will have about 2.2 metres headroom both floors if we can keep the slab on one level. The problem is even if we do step the slab, we will still have an issue. To step the slab up so we dont have an issue would be reducing the headroom by as much as a foot and a half.
There must be some other solution.
Should I tank the inside of the sandstone aswell, and make a cement sealed channel in the cavity with drain holes leading down through the wall to a french drain idea on the outside or is this just inviting water into the cavity ?
Also I was thinking about building a few courses of dwarf wall from blocks onto which the timber kit sections could be built. The cavity would then be surrounded by stone and not the more vulnerable wood.
My main concern is that you will be excavating directly adjacent to and below the footings of the existing wall. Is the barn large enough so that you could build the internal walls inside the existing footings.
This would seem like a good idea "Also I was thinking about building a few courses of dwarf wall from blocks onto which the timber kit sections could be built."
"Should I tank the inside of the sandstone aswell" the vertical DPM should suffice.