There are many advantages to using metal roofing on a building, particularly on buildings such as garages and workshops. The days of rusting, unsightly and inefficient corrugated iron roofs is over. Modern galvanised steel roofing sheets now come in a variety of finishes and colours. They come as plastic coated or polyester painted sheets, can be either corrugated or, more usually now, box profiled, are pleasing to the eye, require no maintenance, and have a lifetime as long, if not longer, than most traditional roofing materials. They can also, if you really want this, pretend to be something else. They can look like pantiles or slates. The method of fixing them, however, remains basically the same and the instructions given below for box profiles sheets can be easily adapted to any other designs. Just make sure that you remember to have plenty of overlap between the sheets!
The sheets usually have a standard width of 36 inches and a standard length of 6, 8 or 12 feet. The standard length can vary from one manufacturer to another so make sure you know what you are buying! The cost is likely to be around $10 (Â£6.00) for a square yard, which makes it initially a relatively expensive material for an outhouse. This is more than compensated for by its long life. Ideally, buy a length that will cover the roof vertically from the eave to the ridge so that you do not need to have a join half way up the roof.
One of the advantages of metal over roofing felt or shingles is that you do not need a solid base underneath. It is only necessary to have battens, 4 inches by 2 inches, nailed 2 ft apart to the rafters. This is strong enough to support your weight as you fix the roof (unless you are an elephant, of course).
Estimate the amount of material you will need; each sheet needs to overlap the one next to it by fitting the profiles (the raised bits) over each other (so that at the join you will be putting a screw through 2 thicknesses of raised area). The sheets are light and thin and cut easily with an angle-grinder. You will also need strips of flashing, pre-creased, in the same material to fold over the gable ends and the ridge.
You also need dedicated roofing screws, which have a sharp cutting point on them and are tightened with a socket, not a screwdriver. They must also either be plastic coated or come with plastic cover caps, which should, ideally, match the colour of your roof. On the roof itself, these are put in through the top of the profile, not through the flat areas of the roof, and then need to screw at least a third of their total length into the batten below. The length, therefore, depends on the depth of the profile. In general, the screws will be at least four inches long. The gable end screws need only be 2 inches long.
Start by fixing the flashing over the gable end of your building, so that the flashing is fixed along the battens, folds over the fascia boards and is fixed to them. Drill through the metal with a metal drill bit every 2 feet put in a roofing screw.
A word of warning â€“ the metal roofing sheets, unlike shingles or roofing felt, is treacherously slippy when wet. Do this in dry weather.
Start at the bottom corner of your roof. Lay the sheet across the battens with the profiles running parallel to the rafters (obviously, so that the water can run down between the profiles). The metal sheet must overhang the fascia board by not less than 2 inches, both on the side of the building and on the gable end. Use a metal drill bit to drill the fixing holes. The holes are made through the profile, so that they are less exposed to running water. Holes should be drilled up the profile 2 feet apart so that they meet the batten underneath. Use a power driver with a socket to drive the screws into the battens below. Fix all the sheets along the bottom of the roof, overlapping each sheet as described above.
If you need a second row of metal sheets, this should overlap the first row by at least 6 inches, more if the roof has a low pitch.
When you have completed both sides of the roof, lay the ridge flashing along the ridge and fix it down to the ridge rafter, again putting in screws at 2 ft intervals.