Problems With Large Metal Roofs: Drainage Issues

Metal Roofing Types

• Water-shedding metal roofing, similar to slate, shingles and tile, is usually installed on an underlayment of asphalt-saturated felt on slopes greater than 3:12. A designation 3:12 means a ratio of 3 inches of rise to 12 horizontal inches of roof or 3 feet of rise to 12 horizontal feet of roof. Water-shedding metal roofs are usually made in short panels, and their seams are not watertight. They rely on the underlayment and drainage from a steep pitch to remove water. Wavy panels of corrugated metal roofing, a type of water-shedding roofing, are usually applied on steeply pitched roofs in the subtropics and tropics. They sometimes overlap vertically, presenting special problems. Moisture-barrier metal roofs are structural panels complete with vapor barriers and insulation that are applied directly onto purlins or beams that support the roof. You can install them on flat or nearly flat roofs of 1:12 to 4:12. They rely on fastening clips and splices on the seams to make them waterproof and able to expand and contract.

Water-Shedding Drainage

• If slow-moving water on water-shedding metal roofs freezes in gutters and downspouts, water can back up the roof, a phenomenon called damming. Dammed water can penetrate the asphalt underlayment causing these metal roofs to leak. Water-shedding metal roofs are commonly applied on roofs of a 4:12 pitch or greater to encourage drainage and avoid damming.

Corrugated Roof Drainage

• Rain carries minute particles of dirt and pollutants down roofs. If a corrugated metal roof with vertical seams drains slowly, these particles can accumulate along the overlaps, creating a seal. This seal of debris is usually thicker near the purlins. If water enters a crack in the accumulated seal of debris and backs up, it can leak through the screw holes on the purlins.

Moisture Barrier Drainage

• Moisture barrier roofs have both vertical and horizontal seams between the panels as well as panel fasteners and perimeter flashings. As cold and heat contract and expand the panels, the sealants in their seams become brittle. If winter ice creates dams in gutters and downspouts, it can back up, penetrating cracks in the brittle sealant. Steel panels can also rust and are subject to puncturing and kinking over time. If the roof drains slowly, water can leak through the deteriorating panel.