Metal Shingle Layout, Alignment, and Fastening
Layout and fastening are where appearance and wind performance are won or lost. The exam tests control lines, course offset, exposure control, interlock engagement, fastener placement, and torque control on thin metal.
Layout rules
- Establish square: a control line at the eave and a perpendicular rake line; check diagonals before the first course.
- Start from a chalk line, not from the rake edge, and keep exposure constant with a story pole. Check every few courses with a string line.
- Offset courses per the manual, usually half a shingle or a fixed bond, so vertical joints do not align. Adjust the offset at hips and rakes so cut units are not slivers.
- Engage the interlock fully. A partly engaged sidelap is both a leak path and a wind failure.
Fastening rules
- Use the specified fastener, length, and number per unit. Fasten at the high point of the overlap or the hidden flange, not in the drainage path.
- Drive snug, not dimpled. Overdriving cuts the washer and fatigues the metal; underdriving leaves the head proud.
- Use a torque-clutch screw gun and check the first units. Keep fasteners perpendicular to the deck.
- Leave thermal movement free at hips, ridges, and terminations unless the manual specifies a fixed point.
Worked example
A 40-ft rake with 12-in.-wide shingles needs about 40 units per course. If the layout leaves a 3-in. sliver at the rake, do not force it. Shift the starting line by half a shingle, recheck the bond, and cut equal units at both rakes. The reasoning: slivers are hard to fasten, look poor, and concentrate wind load.
Traps
Starting at both rakes and meeting in the middle; stretching a tape along the roof; fastening low in the exposure; mixing screw types; overdriving with an impact driver.