To avoid a transverse lap, or if the sheet is longer than can be transported or safely handled, a step in the roof structure should be provided. See Step Apron.
At a step or a lap, the end span must be reduced. See
Point Load.
If a transverse lap cannot be avoided, it must be mechanically fastened and sealed and must be made watertight from the inside by lap tape or sealant.
The sealant should ensure that the condensation flows past the joint and either be absorbed by the underlay or drain to the eave.
Severe corrosion problems have been caused on curved roofs by condensation running down the inside of roof cladding and into the laps. This was a common mode of failure when short lengths of galvanized corrugated sheeting were used in the past, but long run roofing without end laps has significantly reduced this type of failure.
Do not assume that the paint coating would provide barrier protection. The manufacturers' and industry requirement, since 1995, is to seal all transverse laps.
The time of wetness, which is a major factor of corrosion, is increased when unsealed metallic coated steel cladding and flashing laps are subjected to a continuously damp environment. This situation is also detrimental to pre-painted metal cladding, which are attacked through the permeable paint coatings and at cut edges.
Where a draped roof is regarded as too long to transport or too difficult to handle as a drape curve in one sheet, the crown sheet should be as long as practical and the transverse lap should be placed as far down the roof as possible to increase the pitch at this point.
At the termination of curved sheets at minimum pitches in exposed areas, additional weathering is required at the turn down. Ventilated filler blocks and/or baffles should be used to prevent blowback, which can cause corrosion because the underside of the sheeting becomes an unwashed area.
Penetrations or end laps must not be placed in the region of the curve where the roof pitch is below the minimum pitch for the profile in
Minimum Recommended Pitch.
Additional timber or steel supporting structure must be installed upside and downside of any penetration hole greater than 300 mm x 300 mm to provide fixing for the sheet and a reduction of the end spans.
Support must be provided to resist the uplift on sprung curved sheets at all penetrations.
All side laps of curved sheets below the minimum pitch for the profile must be mechanically fastened and sealed.
Continuity over a minimum of three purlins is required for successful drape curving and therefore any interruption, such as a penetration or other cutting of the sheet, may require machine curving to ensure the curvature is maintained.
Purlins must be accurately positioned with the top faces tangential to the radius of the arch and should be within a 5 mm tolerance to avoid purlin creasing. Roof traffic should be restricted to avoid damage, particularly in the low pitch region or in highly visible areas. Damage as a result of walking traffic can be seen as creasing at purlin lines or canning in the profile pans.
Some purlin creasing is to be expected with stronger profiles, and at low pitches this can cause corrosion due to ponding. For convex roofs, the minimum radii should be adhered to because the pans are in compression, whereas with concave roofs the pans are in tension and the panning or distortion of these roofs will be less , although it depends on the profile.
Only vented profiled filler blocks should be used at the eave on curved roofs so that some air movement is provided within the ribs. See
Ventilation.Provision for expansion should be provided in the same manner as required for straight lengths, but the configuration of curved roofs means that some expansion will be taken up by a springing of the profile further up, which results in less movement. When the total sheet length is considered for expansion, positive fixing using oversize holes, should be made at the crown.
