Minimise height of flat roof design - Help!

[ex_directory]

We are adding a single storey 3m x 8m extension with flat (warm) roof to the back of our house. 7m of the 8m will be bi-fold doors. We are also adding 2 x large rooflights.

We want to minimise the internal wall height of the rooflight upstand, at the moment this is 610 mm (12mm plasterboard, 150mm joist, 135mm furring, 18mm deck, 140mm insulation, 18mm deck, VCL + membrane ?mm, 150mm kerb for rooflight).

We fear the rooflight will be lost by high sided internal walls.

Does anyone have any suggestions, we are consdering a switch to a cold roof to save 158mm off the roof height. But it seems this is no longer the recommended roofing solution.

We have also been asked for SAP calcs by the BCO as there is so much glass. Will this badly affect our SAP calc?

 

[freddiemercurystwin]

There is nothing wrong with a cold roof, it just means you need ventilation at the eaves and ridge. It is really only comparatively recently that warm roofs have become popular. Think about it, a standard pitched roof with insulation at ceiling level and ventilation at the eaves is exactly the same set-up. If you were going for lead or zinc as a finish these ought to be ventilated anyway.

Why are the firings 135mm? Why not just run the joists pitched to achieve the fall?

You can splay the rooflight cheeks if you wish that will help. If not already you could also have the ceiling pitched rather than horizontal to roof depth.

The result of the SAP calcs will likely just call for an slight increase in the wall and/or roof insulation to compensate not difficult.

Really though, with a 7m Bi-Fold do you really need rooflights?

 

[ex_directory]

Thanks for your reply. To answer some points...

There is nothing wrong with a cold roof, it just means you need ventilation at the eaves and ridge.

Actually it is a parapet wall similar to below image. So thinking some air bricks on the sides with different height battons on top of and opposite to joist direction (the joists runs the 3m length) to create the fall and also create an air gap above the joists.

I guess my questions is are we making a mistake by choosing a cold roof or if we ventilate as described will this be fine?

If you were going for lead or zinc as a finish these ought to be ventilated anyway.

We are going for a single ply membrane.

Why are the firings 135mm?

Using the pic above as an example, the gutter is along the existing back wall, 60mm to nothing, the fall from right to left is 75mm to nothing. Total at highest is 135mm.

Why not just run the joists pitched to achieve the fall?

Joists are already in.

You can splay the rooflight cheeks if you wish that will help.

Nice idea but it is the height not the light that we are more concerned about. Also joists and openings are already in place (but that is all).

The result of the SAP calcs will likely just call for an slight increase in the wall and/or roof insulation to compensate not difficult.

Then I need SAP calcs asap!!

Really though, with a 7m Bi-Fold do you really need rooflights?

See pic, I guess we are creating something similar.

Thanks again fmt, would be interested to see what you think of the idea of cross beams for ventilation over the insulation and air bricks. Could I improve upon this design now the joists are in if we go for a cold roof?

 

[ex_directory]

Now thinking about a warm roof but with 40mm insulation in the joists (at the top) and 80mm on top of the ply deck. I assume this is still called a warm roof?

Anyone had any experiences with this design?