Extending on Top of a Flat Roof - Few Questions Which are Niggling me All manner of questions and solutions to problems and issues surrounding the subject of brickwork, blockwork and stonework

[Steve_Brand]

Hi, new to the site.
I'm in the process of adding a substantial extension to my house. I'm very lucky with most of my family and friends being in different trades. So the work on it is being done by myself and family and friends.

I have a couple of questions which are niggling me.

1)The flat roof walls I'll be building on only have a 50mm cavity. Building inspector wants a 100 mm cavity insulated on top. What's the best way to deal with the 50mm overlap on the inner leaf?

2)joist finding...from the outside? Is there an accurate way of finding out where the joists are without removing floorboards, bricks etc? I'm running steels in from the detached garage via a block column and ideally don't want a step into the extension.

3)using pre stressed concrete lintels as padstones for ub's. I have 2 running into the side of the house. When there all bolted there'll be 300mm, so I was thinking a 600mm lintel to spread the load over the wall. That still gives 150mm bearing either side. SE has put down 3 courses of engineering bricks. I think a lintel would be easier to fit.

Any help would be greatly appreciated

Cheers

Steve

[welsh brickie]

to increase the cavity width, I think piers inside the garage with concrete lintols spanning them but check with building control.
you can check the joist position by removing the carpet upstairs the floorboards will always span the joists and nails will show the position,
the use of lintols for padstones will be fine