Sheds and garages rarely get pampered. They sit at the end of the garden or beside the house, taking rain on a shallow pitch, baking in July, freezing in January, and collecting leaves in the gutter until somebody finally notices. The roof cladding is the part that has to cope with all of it, and it is not a job you want to repeat in three years' time. Choosing well comes down to four things: what the frame underneath can carry, what the roof actually has to do, how it will look from the kitchen window, and what the paperwork allows.
It is tempting to pick a material first and worry about the frame later. Do it the other way round. Walk outside with a tape measure and note the pitch, the spacing between purlins, the span of the roof, and the condition of the timbers. A twenty-year-old shed with 600mm purlin centres will happily take a lightweight metal sheet. It will not take concrete tiles without reinforcement.
Pitch matters more than most people expect. Every sheet comes with a minimum pitch, and if your roof is flatter than that, water will creep back under the laps in a driving wind. Shallow garage roofs are common, so check the manufacturer's figure before you buy rather than after.
Also look at what is already there. If the old felt is sagging, trapped moisture underneath has probably rotted the boards or the purlins. Recovering a failing roof with new cladding just hides the problem.
A deeper profile spans further between supports, which means you can sometimes clad straight over existing purlins without adding timber. A shallow profile needs closer support but looks neater on small roofs. Box profile, sometimes called trapezoidal, is the standard for a reason: it drains well, stiffens the sheet and hides fixings reasonably tidily.
Ask about side laps and end laps when ordering. Sheets should overlap by at least one corrugation at the side and by a full profile depth plus a sensible margin at the ends on shallow pitches. The accessories matter as much as the sheets: ridge pieces, barge boards, foam closure strips that stop wind-driven rain and insects getting in at the eaves, and flashings where the roof meets a wall or an upstand.
Buy all the sheets, fixings and flashings from the same supplier in one order. Colour batches vary, and a slightly different shade of green between the roof and the ridge looks like an error for the next twenty years.
This is the part most people regret skipping. A steel sheet is cold on the underside, warm damp air rises, and the water has to go somewhere. In an unheated shed that shows up as drips on whatever you have stored underneath.
There are three straightforward defences. Anti-condensation fleece bonded to the back of the sheet absorbs minor moisture and releases it later. A vapour barrier on the warm side, fitted properly with taped joints, keeps damp air out of the roof build-up. And ventilation at the eaves and ridge lets any remaining moisture escape.
If you are turning the garage into a workshop and want it usable in winter, insulation between the purlins with rigid boards, foil face down, is the sensible upgrade. If you plan to use the space as a habitable room, separate building regulations apply, so speak to your local authority building control before you start.
Many outbuildings fall under permitted development, which usually comes with conditions on overall height, eaves height, how close the building sits to a boundary, and how much of the garden it covers. Those limits vary between England, Scotland, Wales and Northern Ireland, and they are tighter inside conservation areas, for listed buildings, and where the local authority has removed permitted development rights.
Re-roofing where the size and shape stay the same is normally treated differently from rebuilding or enlarging. If in doubt, ring the planning department and describe what you intend to do. It costs nothing and takes ten minutes. This is general guidance only, not legal advice, so confirm the position for your own property.
Use the screws supplied for the sheet, with the correct sealing washers, and drive them on the crest rather than the trough. Tighten until the washer just compresses, then stop. An over-driven screw crushes the seal and becomes a leak. Cut sheets with a nibbler or tin snips, not an angle grinder, because the heat strips the coating and rust starts at the cut edge.
Finally, treat the roof as a working-at-height job. Plan how you will get sheets up, avoid working in wind, and use edge protection or a tower if the roof is more than a couple of metres off the ground.
Work in this order: check the frame, confirm the pitch, decide whether the roof needs to be insulated, then choose the material. Order ten per cent more than you measure, keep the offcuts, and ask about lead times before you book a weekend. If the garage is attached to the house, or the roof is flat and you are unsure about drainage falls, it is worth paying a roofer for an hour of advice. That hour is cheaper than a second roof.
Photo: crowlite / Pixabay
