Direction of Lay

Horizontally laid underlay must be supported, while vertically laid underlay can be self-supported if purlin spacings allow.  Roof and wall underlay can generally be laid either horizontally or vertically.  However, on pitches of less than 10°, E2/AS1 requires underlay to be laid horizontally on support. Horizontal underlay must be laid starting at the lowest point of the roof, running over the bottom purlin. Subsequent layers must be lapped over the lower layer to ensure water is shed to the outside face of the underlay.

Underlay for roof cladding must have a minimum side and end lap of 150 mm.

The lower end of underlay may terminate on the outside of the fascia line, or overshoot it by a small margin. Underlay should not overhang the fascia to the extent that it can wick up moisture from the spouting. In most situations, the overhang should not exceed 20 mm.

Flue penetrations must have a minimum distance of 50 mm from the outer liner to any underlay or flammable material.

Rips smaller than 75 mm on walls or roofs can be repaired by using a compatible flashing tape, but roof underlay damage greater than this requires a new piece of underlay captured by the cladding fastenings.