self build waterproof basement formwork membrane concrete corbels
 
self build basement corbel  

Planning and building corbels.

Or, planning not to need a corbel.

A corbel is a widening of a wall to create something wider or a shelf to support something.
  self build basement


In the graphic at the top of every page, I avoided a corbel by making the basement retaining wall thicker all the way down, and by not using beam and block or precast concrete floor.

In these next two photos you can see that we had to form a corbel then form an upstand on top.

Both were necessary to support the beam and block floor inside and to stop water getting in under the brickwork built off the corbel.

Corbels are a lot of work compared to simple walls.

Corbels also tend to make the basement smaller inside. Or, perhaps it is the beam and block floor that makes the basement smaller inside because it needs to be supported. The corbel becomes necessary because there needs to be an upstand outside the beam and block then cavity insulation and brickwork outside that. So the overall width of construction might be
125mm bearing for beam and block
100mm upstand to take blockwork inner skin above
75mm insulation
25mm cavity
110mm brickwork
= 435mm
far thicker than a basement retaining wall ever needs to be for a house.

If the corbel was inside to support the beam and block floor it would allow the basement to be bigger but you would see the corbel inside your finished basement, which would be odd.

So my advice would be to try not to need a corbel.

Another way of avoiding casting a corbel in your waterproof, basement concrete would be to build blockwork outside off your basement slab and make the wall wider at the top that way instead.

waterproof basement corbel construction waterproof basement corbel construction

screws in a board to start a corbel with    
This customer needs a corbel for his brickwork plinth detail.

He put some screws through the 7th board so that when he strikes the formwork that board can remain.

The timber that will form the underside of his corbel need only sit on top of this and be screwed to it.

So simple.

Then he can lever the whole lot off snapping these screws after the corbel concrete has hardened.


Looking back, I must have convinced people not to have a corbel about the same time I convinced them not to use ICF. I don't have many photos of corbels made with timber formwork, but you can imagine how much easier it would be being able to screw timber together compared to trying to adapt polystyrene.

Remember. ICFs come from North America where they use a lot less steel and poorer concrete and put up with leaks. ICFs have corbel blocks, but your UK structural engineer is likely to want 5 times as much concrete and steel as the off the shelf solution from over the water.



When planning your corbel construction, bear in mind you need to stop water getting in over your basement wall plus you need to accommodate all your above ground construction: floor, structural wall, facing brickwork.

There can be a lot to accommodate.

Obviously, choosing a floor that needs to sit on your wall means that the top of your wall needs to be wider.

Choosing traditional cavity walls but with 100mm of insulation to meet modern standards is wider than modern methods of construction as well. The project with the curved walls had both.

So did this:

waterproof basement corbel construction

This one is to have an engineered timber floor, you can see the floor ledge ready to be cast in, then a blockwork inner wall and stone outer, which needed quite a large corbel.

waterproof basement corbel construction

This basement in Somerset had a timber frame on top, about 140mm I think, then a cavity and brickwork. The top of the wall just needed a little extra so we put ply at the top and omitted the polystyrene. The same ifdea will be much easier with scaffold board formwork because screwing into it will be so much stronger.

waterproof basement corbel construction

A similar idea here though we relied on the Polarwall ties less.

waterproof basement corbel construction

The corbel concrete will need supporting underneath if it is any more than 50mm extra.

If you can backfill first you will need a lot less supporting material.

waterproof basement corbel construction

This was the steel for the corbel.

waterproof basement corbel construction



waterproof basement corbel construction    
Anther example of corbel steel.





Corbels are not difficult but they introduce a whole extra task compared to straight sided walls, so they cost money in labour and materials.

You or your designer needs to start with the finished appearance you want then work back. If you need a corbel you need it.



But most projects with an engineered timber floor do not need a corbel, just the upstand, so they are cheaper to build.

waterproof basement corbel construction



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