Help designing insulated metal box section clad roof

3 Jun 2017
Reaction score
United Kingdom
I am looking for advice for a roof to an outbuilding that I am renovating. The building is 4.8m wide with a duo pitched roof at around 18deg. The span tables and building inspector have agreed that 6x2 rafters are adequate for the span. I plan on putting 100mm PIR insulation between the rafters with a vapour barrier below the rafters and then a plasterboard ceiling. The metal profile manufacturer recommends purlins every metre and then fixing the sheets to those purlins. Questions:
Can I place additional insulation between the purlins, ie above the rafters? Does this prevent the thermal bridging? BC wanted another layer below the rafters.
I plan on using a breather membrane between purlins and metal roofing. Would I then need to batten the roof parallel with the rafters to allow any moisture to run down the pitch of the roof?
Am I essentially making a pitched warm roof and therefore don’t need inter rafter ventilation or is it still a cold roof set up?
Could the metal roof just be fastened to the purlins therefore there will not be any moisture condensing on the underside of the roof?
If anyone could advise me on the best construction for this roof I would appreciate it.
Sponsored Links
Have you thought of using insulated composite Plastisol coated corrugated metal roof sheeting ?
I did yes but the cost and handling of the sheets made it not really feasible
Why don't you put osb over the rafters and then your insulation and then your metal roofing sheets.
Sponsored Links
Yes this could work. Does this method mean I don’t need purlins?

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local