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Laps

By definition, a curved roof is flat at the crest of a curve, and because it is below the specified minimum roof pitch required by the NZBC for unsealed laps, side laps should be sealed over the crest of the arch until the minimum pitch is reached.

All vertical laps should be sealed if the pitch is less than the allowable minimum as tabulated below:

Curved Roof: Sealed Lap Pitch

Minimum pitch below which vertical laps should be sealed
ProfilePitch
Corrugate
Symmetrical Low Trapezoidal
Asymmetrical Low Trapezoidal
Secret-fix Tray
(optional caption)

 

When the pitch of the roof is below the minimum, the side lap is required to be sealed over the crown, and lap tape or silicone sealant should be placed on top of the rib and firmly held down while fixing takes place. Intermediate side stitching is required at the midpoint of all side laps using self-sealing rivets or stitching screws.
The side lap of profiled sheeting is designed with anti-capillary provisions to be self-draining.
Before the continuous manufacture of corrugate from coil, symmetrical corrugate sheets were often laid with two nesting laps, which commonly corroded due to condensation, even when the laps were primed. All metal profiles now produced in NZ have capillary grooves. Trapezoidal profiles are designed for one lap only and corrugate used for roofing is designed for 11/2 laps with an under and an over.
Avoid double lapping because condensation can become trapped in the lap, which can cause accelerated corrosion with all steel products, including pre-coated steel. Lap priming should not be used as the permeable paint surface can retain moisture and accelerate corrosion.
 
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