A compact building has a large roof relative to its volume, so the roof carries more of the thermal and water performance than it would on a house.
Pitch is decided by snow and by transport height
A steeper roof sheds snow and rain and adds height, which is exactly the dimension a road transport cannot spare. Most transportable designs land between five and fifteen degrees, and the ones that go higher pay for it in permits.
Where snow load is genuine — alpine sites, Scandinavian inland — the structure is sized for it, not the pitch. A shallow roof with a correctly calculated frame is safer than a steep roof with an optimistic one.
Standing seam is the durable covering
A folded metal seam has no fixings through the waterproof plane, which is why it outlasts every other option on a building that flexes in transit. It also drums in rain, which is a separate problem solved by mass and a resilient layer beneath.
Membrane roofs are cheaper and perfectly serviceable, with a shorter life and more sensitivity to the detail at every upstand. Ask what the warranty covers and for how long.
Overhangs are worth their width
An overhang keeps rain off the cladding, shades the glass in summer and protects the head of every window. It is the cheapest longevity measure available and it is routinely trimmed to save transport width.
Where an overhang would breach the 2.55 metre transport limit, it can be delivered separately and fixed on site. Ask for that option rather than accepting a flush eaves.
Penetrations are where roofs fail
Flue, ventilation terminals, solar mounts and the aerial all pierce the waterproof plane. Each one needs a proprietary detail, not a bead of sealant, and each one should be listed on the drawing before production.
Add any future penetration now. Fitting a flue through a finished roof means opening the build-up and hoping the vapour layer is repaired as well as it was made.
Gutters and where the water goes
A small roof produces a surprising volume in a downpour and it lands exactly where the house meets the ground. Without gutters, that water splashes back onto the lowest board of the cladding — the place rot starts.
Take the downpipe away from the house and into a soakaway or a butt. A pipe that discharges at the base of the wall solves nothing.