Head lap (roof tiles)

The head lap in a tiled roof is the amount by which each row of tiles overlaps the row immediately below it, measured up the slope from the lower row’s nose to the upper row’s nose. Head lap is the primary mechanism by which a tiled roof sheds water from the upper surface of one tile down across the surface of the lower tile, rather than letting water track up under the upper row. Head lap requirement is pitch-dependent: the lower the roof pitch, the larger the head lap must be to maintain weathertightness. The lap is specified by the tile manufacturer in conjunction with AS 2050:2018 (verified 2026-05-16).

Typical head lap requirements by pitch:

Roof pitch	Typical minimum head lap	Comment
15° (minimum for tiles)	100 mm	At absolute minimum tiled-roof pitch; sarking mandatory; reduced rainfall threshold
17.5°-20°	90 mm	Standard for low-pitch tiled roofs
22.5°-25°	75 mm	Common Australian residential pitch
30°-35°	65 mm	Higher pitch, less head lap needed
40°+	55 mm	High-pitch traditional roofs

(Indicative ranges; consult the specific tile manufacturer’s data sheet and AS 2050:2018 for the actual minimums for the chosen tile profile, pitch, and exposure category.)

Why head lap matters:

Water shedding: water on the upper surface of a tile runs off and lands on the surface of the tile below (within the head lap zone), not into the gap between tiles.
Capillary resistance: a sufficient head lap means the water doesn’t reach the unprotected lower edge of the upper tile’s underside.
Wind-driven rain: in high-wind events, water can be blown horizontally and even upward. Larger head lap absorbs this without leak.
Sarking redundancy: head lap is the primary water defence; sarking is backup. A tiled roof with inadequate head lap leaks even with good sarking, because water tracks under the tiles in volumes the sarking cannot manage.

The head lap and batten gauge relationship:

The batten gauge (the horizontal centre-to-centre spacing of roof battens up the slope) is calculated from:

Variable	Effect
Tile length (specific to product)	Fixed
Head lap (set by pitch and manufacturer spec)	Fixed once pitch is set
Batten gauge	Calculated as: Tile length − Head lap (approximately)

Example: a 415 mm tile at 22.5° pitch with 75 mm head lap gives a batten gauge of approximately 340 mm. Battens are spaced 340 mm centre-to-centre up the slope.

Setting out the tiles:

Measure the roof slope length from eave to ridge.
Determine the head lap required for the pitch and product.
Calculate the batten gauge.
Mark the first batten close to the eave (per manufacturer’s eave detail).
Mark the last batten close to the ridge.
Distribute the intermediate battens evenly so the lap maintains throughout.
If the total slope doesn’t divide evenly, increase the head lap on the top course to absorb the variance (never reduce below the manufacturer’s minimum).

Common defects:

Head lap too small because the batten gauge was set from a wrong tile-length assumption. Visual signs: water entry at gable ends in heavy rain.
Inconsistent head lap up the slope: some rows tight, some wide. Looks ugly and may leak.
Top row head lap less than manufacturer’s minimum: setter-out left the variance on the top course and went short. Replace top row.
Reduction in head lap to “use up” tiles: never. Cut tile length, never reduce lap.
Side lap correct, head lap missed: easy to do; head lap is the across-slope inspection.

Inspection (at frame and weatherboard stage):

Pull up a single tile and measure the head lap directly with a tape from upper tile nose to lower tile nose.
Sample randomly across the slope; consistent head lap is the goal.
Photograph as part of the as-built record.

Also known as: roof tile lap; tile head lap; vertical lap; up-slope lap.

Category: Roof.

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