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
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
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
Sealing hips. For sealing hip ridges, most
manufacturers provide metal J-trim, which is capped by a metal
ridge cap (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
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'
Sources of Supply
Major distributors of screws, closures,
flashing, and sealants for metal roofing
Atlas Bolt and Screw
Co. 1628 Troy Rd.
Ashland, OH 44805
9911 E. 53rd. St.
Kansas City, MO 64133
East Coast Fasteners and
Closures P.O. Box Q
Elkhart Lake, WI 53020