Cold flat roof

[iwanttodiy]

I've challenged my structural engineer and builder about using a warm roof.

My builder - says there is no problem, other than it will cost more.

My SE - has suggested designing the roof using 150mm timbers and 150mm UBs to hold a lantern instead of using 200mm timbers (2x8"). By reducing the joists by 50mm, we can use 50-100mm on top of the roof and if necessary even use some insulation in the joists to do a hybrid setup.

Now for the lantern, my builder typically builds a simple timber upstand for similar roofs. Also looking at the manufacturer installation instructions, it suggests the same thing for an upstand/kerb.

Now my question is; do we not need to insulate the upstand? Looking at other posts on this forum, it seems the preferred approach is to build a stud wall upstand, and fill the void with insulation. Possibly put a thinner - 25mm insulation on the outside layer then plasterboard the inside with a VCL between the stud wall and plasterboard. I personally think we should insulate the upstand? Id rather not get into a situation where once the builder has created the upstand, I would need to put a 100mm board on the inside, which will eat up the space.

 

[iwanttodiy]

So, from my alcohol induced understanding, PD says eaves can be 3m, ridge height 4m. I have a bastardised pitched/flat roof..... the eaves are 3m, the top of the warm deck is well over.

To conclude on this thread. The design we ended up with is a warm flat roof.

Based on the building regs technical guidance documentation on eaves height (https://assets.publishing.service.g...der_Technical_Guidance__-April_2017_FINAL.pdf)

- Eaves height is measured from the ground level at the base of the outside wall to the point where that wall would meet the upper surface of the flat roof - the overhang and the parapet wall should be ignored for the purposes of measurement. So from a planning perspective I am well within permitted development.