Warm deck roof insulation and position of joists

[Porthole]

Hello - I am hoping someone can help me with this one. We are embarking building a flat roof extension but different builders, architect and structural engineer all seem to have different views on the detail of the build up on a warm deck flat roof and how the joists should sit. It is very confusing and we seem to be going round in circles alot.

 

We wish to build a 6m deep and 9m wide extension at the back of our house. There will be an opening of 7.5m for 3 sliding doors to the garden and a large steel beam sitting above the doors. Joists in the extension running perpendicular to the sliding doors will be attached to the large steel beam. Does the top of these joists need to be flush with the top of the steel beam and the OSB deck sit on top of both the top of the steel beam? Alternatively, could the bottom of the steel beam sit flush with the bottom of the joists and the OSB deck just sit on top of the joists, but the steel beam would still be insulated in some way. The steel beam is likely to be around 250mm and the perpendicular joists will be smaller than this, at around 150mm.

 

The structural engineer is insistent that the top of the joists should be flush with the top of the steel beam so that the deck can sit nicely on top of both to reduce the risk of issues in the future. This seems sensible but our architect didn't mentioned this before and it could impact on the height of the extension which is a problem as we are building under permitted development where the eaves height is critical (needs to be 3 metres).

 

Also, we have a much simpler question, but does anyone know whether it should be 100mm or 120mm celotex on a warm deck flat roof - or is that just our choice?

 

Thank you - hoping we can get some clarity for those who have a lot more experience than us and put us out of our misery!!!!

 

 

 

[Big Jimbo]

If you are putting a lot of glass in your extension, you might need to have a sap calc done. If the glazed area of your extension is more than 25% of your floor area - windows and doors that you are taking out of the existing rear external wall, (putting them inside the new external envelope) then you will need a sap calc. The level of PIR in the roof will have to meet building regs for an extension. 100mm won't do it 150 or 175 might. 200mm defo should. There is no way i would put the steel within the roof structure. That is a poor idea.

[Olf]

10 hours ago, Porthole said:

Does the top of these joists need to be flush with the top of the steel beam and the OSB deck sit on top of both the top of the steel beam? Alternatively, could the bottom of the steel beam sit flush with the bottom of the joists and the OSB deck just sit on top of the joists, but the steel beam would still be insulated in some way. The steel beam is likely to be around 250mm and the perpendicular joists will be smaller than this, at around 150mm.

 

I think neither - the joists need to be somehow restrained. My SE stipulated tight fit ledgers and joist hangers off them.

 

10 hours ago, Porthole said:

The structural engineer is insistent that the top of the joists should be flush with the top of the steel beam so that the deck can sit nicely on top of both to reduce the risk of issues in the future

Mine wanted 10mm of top of the joist over top of the beam - but that was assuming using C24 timber and chance for shrinkage, engineered joists should be more stable.

Anyway, the reason you're paying the SE is so he/she comes with a solution that YOU like and it is physically possible - supported with calculations so they can ultimately bear the responsibility for your safety and still sleep well.

 

10 hours ago, Porthole said:

whether it should be 100mm or 120mm celotex on a warm deck flat roof - or is that just our choice?

You need to meet minimal required U value for the whole roof structure, use https://www.changeplan.co.uk/u_value_calculator.php

for your specific case. That is something your architect should know and should have checked designed structure for meeting Building Regulations.

[redtop]

Our house is all flat roof. Joists level to top of steel, osb across both steel and joists and fastened to both. Vapour barrier, 120mm PIR for flat roofs (high compression and paper backed) and epdm on top. Passed building regs / sap no problem. Then we battoned underneath joists to create a level platform for the plasterboard

[Porthole]

Sorry for the delayed response - been busy period. Many thanks for these responses - really interesting and helpful. Much appreciated!!!