External sill sizing

[djcdan]

Before potentially ordering the incorrect external sill size, I'd like to check I'm ordering the correct size. 

 

Below shows three concrete options available to us. Our timber frame will be blocked in a 100mm skin with a 65mm cavity. The window would be pertruding out from the frame by 50mm. The aluminium sill leading to the concrete sill is to scale and taken directly from the manufacturers drawings. We'll have a 19mm render and in each of the drawings shown, the overhang is 40mm as per building regs.

 

My questions mainly relate to installation positioning and the amount of 'overhang' required between the aluminium sill and the concrete 'lip' on the main sill. Personally the third options seems best, but I am unsure as to whether the sill should be installed this far in to the cavity position? If the inside edge of the sill should be installed near-enough flush to the inside edge of the 100mm block, then the first option seems best.

 

The shape and installation of the middle option doesn't seem to suit, but could be wrong on that.

 

 

[tonyshouse]

Dont bridge the cavity with the sill, I would slide the window outwards into the cavity as much as you dare, mine are in my cavity/in the insulation layer 

 

for be no 3 but cut the back off the sill keeping some f the upstand 

[djcdan]

I am also leaning towards option 3.

 

One of the windows we have is 3.6m wide, but the largest sill is 1.8m. I presume the best solution here is to install two against each other and paint over the crack/use some form of filler to smooth the crack?

 

 

[keith65]

I think the first one is best  unless you move the sill on the third out so back is flush with blocks and give more overhang