Allex

I have been given a measurement of 800mm to cut an opening in the outside leaf of the wall. What should the measurement be for cutting the inside leaf of the wall? PS I think it should be 820mm (10mmeach side over). Thanks.

I am looking for a section (upvc, Alu, etc) to go over the side and rear edges (currently lapped felt on an angled fillet) to lay on the top surface by say 100mm and down next to existing upvc facia. This will effectively cover the old felt (may need to remove the lap). Then I can lay new felt under it and stick it down or even pin it into the roof. Any suggestions as to any available product appreciated.

I understand these follow the line of the rafters and are nailed on top of the underfelt to allow any rain run-off getting under the tiles. Are they really necessary if I allow a bit of slack in the underfelt under the tile battens? Thanks. PS I've got to fit a Velux window and am also wondering if counter battens are necessary for one of these.

I have a pitch of 18deg. how do I work out the difference in height between the outer and inner (incl. wall plate)? The cavity is uninsulated (old build) and the rafters are 100x50. Want a minimum overhang. Thanks.

I have found one with the flashing for tiles (up to 13mm) but my tiles are Marley modern (30mm) thick. Will I be able to make the flashing fit the thicker tiles say by lifting the window a bit?

Isn't this an unusual situation to have the wall plate on the outer leaf?

Project this year is to improve a small porch off the back door. Remove pitched roof and make lean-to roof with 2nd hand Velux I've acquired. Move a door from end to side. Remove two old windows and replace one with a DG one I've bought cheep. Take up hard floor and insulate to stop floor condensation. Try and insulate as much of the cavity as possible. At nearly 80 I do wonder sometimes! Will need some help!