Horizontal Laying

Horizontally laid underlay must be supported if used under long-run metal roofing, unless both edges are supported by purlins. Under metal tiles, self-supporting underlay can be laid over the roof trusses at spans up to 1.2 m.

Underlay laid horizontally must be laid starting at the lowest point of the roof, running over the bottom purlin and must overlap into the gutter by a maximum of 20 mm to prevent wicking. When an eaves flashing is used the underlay should terminate on the downslope of the flashing.

To lay roof underlay horizontally, more than one roll can be progressively unrolled, one roofing sheet width at a time. Running multiple rolls straight can, however, be difficult in windy conditions.

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