Building Construction Handbook, Eighth Edition

Roof Underlays ~ sometimes called sarking or roofing felt provides

the barrier to the entry of snow, wind and rain blown between the

tiles or slates. It also prevents the entry of water from capillary

action.

Suitable Materials ~

Bitumen fibre based felts † supplied in rolls 1 m wide and up to 25 m

long. Traditionally used in house construction with a cold

ventilated roof.

Breather or vapour permeable underlay † typically produced from

HDPE fibre or extruded polypropylene fibre, bonded by heat and

pressure. Materials permeable to water vapour are preferred as

these do not need to be perforated to ventilate the roof space.

Also, subject to manufacturer's guidelines, traditional eaves

ventilation may not be necessary. Underlay of this type should be

installed taut across the rafters with counter battens support to

the tile battens. Where counter battens are not used, underlay

should sag slightly between rafters to allow rain penetration to

flow under tile battens.

Underlays are fixed initially with galvanised clout nails or st/st staples but

are finally secured with the tiling or slating batten fixings

tile batten

rain blown

under tiles

deflected

to gutter

rafter

100 mm min. head

and side laps to

be adhesive or

tape sealed

vapour permeable

underlay

counter batten

tiling

Roof Underlays