Garden Room Roof Design

I’m looking for a detailed roof design for my new 8.4 m × 4.8 m garden room. The roof will sit at a 20° pitch with 300 mm overhangs all around. I want the rafters left exposed on the underside, with rigid foam boards fitted neatly between them so the finished ceiling stays clean and truss-free.

The finished build will be clad in corrugated metal sheets, so the specification needs to cover correct batten spacing, suitable fixings for that profile, airtight yet ventilated membrane layers, and edge treatments that suit the 20° slope. Please size the rafters, ridge beam and any perimeter timbers so that local snow and wind loads are fully met; include simple span calculations or reference tables to back up your choices.

Deliverables
• Scaled plan, section and detail drawings in PDF and DWG
• Material schedule (timber sizes, rigid foam board thickness, fasteners, membranes)
• Brief structural notes or calcs that a building inspector can follow

I’ll handle the rest of the structure once your roof package is complete, so clarity and buildability are key. Let me know if you need site photos or further load data.