Metal roofing covers two quite different products that share a word. One is profiled sheet, screwed through its crowns to purlins, which is what most farm buildings, workshops and garages wear. The other is a seamed or interlocking panel system laid over a deck, which is what appears on a modern house extension. They cost different amounts, last different lengths of time and are fitted by different contractors.
Metal roofs for homes and garages: profiled sheet against panel systems
Profiled sheet, whether box profile or corrugated, is fixed directly through the sheet into a purlin with a washered screw, which is fast and cheap and puts a fastener through the waterproof layer at every fixing point. Panel systems hold the sheet down with concealed clips and fold the joints upward, which costs more in fabrication and labour and leaves nothing penetrating the surface. On an outbuilding, a metal roof garage or a workshop, the first is usually right; on a habitable room the second usually is. So a metal garage roof is normally box profile or corrugated sheet, and a panel system is kept for the house itself; garage metal roofing on a heated garage also needs the lining described below.
What has to be under a metal roofing system
Condensation, not rain, is what ruins metal roofs. Warm moist air rising from inside meets a cold sheet and condenses on its underside, and if there is nowhere for that water to go it drips back into the insulation. The answers are a warm deck build up with the insulation above the structure and a vapour control layer below it, or a ventilated cavity with a real air path, or an anti condensation lining on an unheated building. A metal roof laid straight onto rafters with a bit of insulation between them is the arrangement that fails.
Steel roofs for houses: what domestic metal roofing involves
On a house, a steel roof is almost always a seamed or interlocking panel system on a solid deck rather than screwed sheet, because a habitable room needs the warm build up described above and a surface with no fixings through it. Coated steel is the cheaper of the metals for this; zinc and aluminium cost more and weather rather than rust. Domestic metal roofing is still a specialist trade, so ask a roofer which systems they are trained to fit and whether the manufacturer's guarantee depends on that.
Flat metal roofing on low pitches
Metal can cover a roof that looks flat, but no metal roof is laid dead flat: each seamed system states its own minimum pitch, and the deck has to be built to fall at or above it so water runs off at the joints rather than standing on them. Below a system's stated minimum, a membrane or other flat roof covering is the usual answer, so check the manufacturer's minimum pitch before a metal finish is chosen for a low roof.
What a metal roof replacement price includes
Metal roofing prices are built from the same parts on every quote: the sheet or panel and its coating, the fabrication of any hips, valleys, ridges and flashings, the fixings or clips, the deck or purlins if they are being renewed, the insulation and the vapour control layer where it is a warm roof, and the access. Sheet roofs are quoted close to the price of the material because the labour is quick; seamed roofs are largely labour, and the difference between two quotes is usually how much of the detail is being formed on site by hand. A standing seam roof cost per m2 is rarely published by installers, so compare seamed quotes on the metal, its gauge and the build up rather than a headline rate.
Questions people ask about metal roof
What is the best metal roof for a garage?
For a detached garage or outbuilding, box profile or corrugated steel sheet screwed to purlins is the usual choice because it is quick to fit and cheap. If the garage is heated or used as a room, add an anti condensation lining or a proper insulated build up, or the underside of the sheet will drip.
Is a metal roof cheaper than tiles?
Profiled sheet on an outbuilding is much cheaper than tiling it. A seamed metal panel system on a house is dearer than a concrete tile roof and comparable with or above natural slate, because most of the cost is fabrication and labour.
Can metal go over an existing roof?
On a commercial building an overlay system over an existing metal roof is common and is a recognised way to avoid stripping. On a domestic tiled roof it is not, because the loading, the ventilation and the detail all change.
Do metal roofs rust?
Coated steel corrodes at cut edges and around fixings long before the coated face fails, which is why cut edge treatment exists as a trade of its own. Zinc, aluminium and copper do not rust in that sense; they weather.