Seams, Joints, and Weathertightness
This domain tests the details that keep water out of the panel field: side laps, endlaps, seam type, sealant, and allowance for thermal movement.
Side laps and seams
- Lap seam: 1½ in. minimum overlap with a continuous bead of butyl sealant and stitch screws.
- Hydrostatic seam: formed with sealant inside the seam and mechanically closed.
- Snap-lock: relies on the profile and factory sealant; more slope-sensitive.
- Double-lock seams are the most weathertight and require a seaming tool.
End laps
Minimum 6 in. at 3:12 and steeper; 12 in. below 3:12. Sealant goes in the lap, and fasteners go in slotted holes so the panel can slide.
Sealant
Use butyl or non-skinning sealant, 100% solids, applied continuously and not stretched. Keep sealant out of the fastener line so it does not extrude.
Movement
Long runs need an expansion joint or a step detail; the maximum run length comes from the manufacturer.
Worked example
An 80 ft steel run with a 100°F swing moves 80 × 12 × 6.5×10⁻⁶ × 100 ≈ 0.62 in. At the endlap, fix one side and use slotted holes plus sealant on the other so the lap slides rather than tears. A rigidly fastened lap in the same run would buckle the seam.
Traps
Skipping endlap sealant; over-tightening the seaming tool and squeezing sealant out; and fastening both ends of a long panel so movement has nowhere to go.