Metal roofing is more expensive that asphalt shingles but it lasts a lifetime, can be installed over existing asphalt shingles, doesn't hold the snow as easily, is lighter and comes in a lot of colours.
Step 1: Strapping the Roof
Measure from the peak to the eave and the width of your existing roof. Take these measurements to your metal roof supplier and they will order the correct amount of roofing for you cut to the right length.
Use 1 x 3 strapping and strap six inches down from the peak of the roof and then 16 inch centers down to the eave. If your existing roof is uneven, shim up the low spots. Strap around any chimneys and stink stacks.
Step 2: Drip Cap and Metal Roof
Chose a quiet day to install metal roofing. Pieces of metal roofing can be easily caught by the wind like a sail and pull you off the roof or damage something.
Nail drip cap along the eaves and gable ends.
Each piece of metal roofing has a starter ridge and an overlapping ridge. Start with the overlapping ridge at one edge of the roof hanging over the roof a bit. Make sure that the first piece is square with the rest of the roof. Screw down this first piece of roofing using the screw bolts. Your supplier will sell you these. They are the same colour as the roof and each one has a rubber washer. Screw the bolts down so the the washers are tight but not completely squished out. It is important to get each screw down snug. Otherwise you could get rain dripping down through the screw hole. You may need to drill pilot holes for each screw if your metal roofing is thick. Don't put any screws in the starter ridge yet.
Put the second piece of roofing so that the overlapping ridge sits right on top of the starter ridge. Make sure that they are flush at the eave and put screw bolts right along the ridge, going through both the starter ridge and the overlapping ridge. Continue until you have one side of the roof done. Put the special gasket in for any chimneys or stink stacks.
Step 3: End Cap and Top Cap
When you have finished both sides, screw on the gable end pieces. Put the ridge caps in place and partially screw them down. There is a special gasket that fits snugly underneath the ridge cap to keep bugs and rain out. Squeeze that underneath the ridge cap and tighten the screw bolts down. There is also a special gasket for the same purpose for the eave end. Insert that gasket. You may have to loosen the bolts to get it in. Tighten them afterwards.
Step 4: Metal Roofing on a New Roof
If you are putting metal roofing on a new roof, simply put the strapping right on the rafters. You don't need roof sheeting.