Hip and Valley Flashing

Cutting a panel diagonally across a rib produces a large irregularly shaped void that is difficult to flash and close off. Unfortunately, hip and valley details are usually left for the installers to figure out in the field. A contractor can save a crew a lot of grief, however, by selecting a panel profile with the narrowest possible rib. Similarly, it is best to select a panel that is as flat as possible between ribs - one without minor stiffening ribs or flutes between the major ribs. Common practice calls for installing a bitumen or modified bitumen underlayment under valleys and over hips to protect the roof sheathing from damage and leaks. However, relying on this added protection may prove to be a mistake, especially in cold climates. Once water gets under the roof panels, it can corrode the underside of the metal panels or lead to freeze damage of the panels and fasteners. Such problems are impossible to remedy, short of a roof tearoff. Instead, hips and valleys must be made absolutely watertight with closures and tape sealants. Because closures are designed for use at the end of a square cut, however, fitting the closures can be difficult. For standard hips and valleys, some panel manufacturers might provide special skewed closures. Since the closures are made by a separate company, you may have to press the panel manufacturer to find out if these are available. However, skewed closures are expensive. In most cases, you will have to customize the standard closures. Sealing valleys. For valleys, cut off the rib portion of a standard male closure, so you are left with several "plugs." Position these plugs square to the panel's length. Install tape sealant on the top and bottom of the plugs, and use a close fastener spacing in the pattern shown in Figure 7.

Figure 7.

Seal the diagonal valley cuts on the roof panel with plugs cut from a closure strip. The plugs must be positioned square to the panel's length and sealed top and bottom with butyl tape. When nailing in valley flashing prior to installing roof panels, be sure the nails are positioned upslope of the sealant. As always, be sure the screws pierce the sealant, or are placed on the dry side of the joint. Use a valley flashing made from prefinished flat roofing stock, bent to the roof slopes, with an inverted "V" in the center. Install the flashing prior to installing the roof panel by temporarily nailing along the upslope edge of the flashing. Be sure that the nail locations will be upslope of the sealant in the finished assembly. Keep the roof panel edge at least 3 to 4 inches upslope of the valley centerline, and make sure the flashing extends at least 4 inches past the panel edge so the closures seal to the flashing. On long valleys, use several valley sections to avoid problems with thermal expansion. Overlap the sections at least 3 inches and use a double row of tape sealant. If the valley sections nest well, don't fasten the sections together with screws. If you need to snug up the connection, place the screws in the upper section only, just upslope of the lower flashing section. This will hold the lower section in place with friction and allow for differential movement between the sections. Sealing hips. For sealing hip ridges, most manufacturers provide metal J-trim, which is capped by a metal ridge cap (Figure 8).

Figure 8.

To seal hip ridges, use metal J-trim and closure plugs set square to the panel's length. On simple hips in steep roofs, this assembly can be installed without tape sealant, but on complex or low-slope roofs, use plug closures and butyl tape. On simple hips in steep roofs, this assembly can be installed without tape sealant. However, on complex roofs where snowmelt and ice damming patterns are difficult to predict, use plug closures and tape sealant to render the assembly watertight.

Cutting Panels

Be sure to use the proper tools whenever you are cutting panels in the field. Cutting metal panels with abrasive cutting blades will overheat the panel at the cut, burning the paint and permanently damaging the metallic coating. It also leaves a ragged edge. Use electric nibblers for cutting across the panel and electric shears for cutting parallel to the length of the panel. Be sure to sweep the roof clean of all cutting and drilling debris daily, or you will end up with unsightly rust stains on the panel surface. Rob Haddock is director of the Metal Roof Advisory Group, an independent consulting firm with international clientele, and a former roofing contractor with more than 20 years' experience.