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.
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 the first is usually right; on a habitable room the second usually is.
What has to be under a metal roof
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.
What a metal roof price includes
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.
Questions people ask about metal roof
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.