Pitched roofs

7.2.14Underlay and sarking

Underlay and sarking shall be provided to resist the passage of moisture.

Underlay and sarking should:

  • be in accordance with the manufacturer’s recommendations
  • take account of the type and fixing of the roof covering
  • be used in accordance with relevant assessments.

In areas of severe exposure, a rigid sarking with underlay is recommended.

Table 6: Acceptable materials for use as underlay and sarking

Tongued and grooved or square-edged boardingBS 1297
Exterior grade plywoodBS EN 636 service class 3
Chipboard (type P5)BS EN 312
OSB (type OSB3)BS EN 300
FeltBS EN 13707
Proprietary productsTechnical Requirement R3

Underlay should:

  • be provided to all tiled roofs
  • where it is above rigid sarking (fully supported), be of low vapour resistance, i.e. less than 0.25MNs/g (where the underlay is highly vapour-resistant, increased ventilation to the roof space or between the underlay and sarking should be provided as necessary)
  • where exposed at eaves level, be UV resistant or of type 5U felt or a proprietary eaves guard used (type 1F may be used for the remainder of the roof)
  • be supported by a continuous fillet or proprietary eaves support tray to prevent sagging (which can form a water trap)
  • be securely fixed
  • at vertical laps, be fixed only over rafters, and at horizontal laps, be held in place by battens
  • be cut neatly, fit tightly and not be torn, i.e. where pipes project through the underlay
  • be dressed into the gutter and cut neatly to fit tightly around service penetrations
  • where traditional mortar pointing is used to bed ridge tiles, extend over the ridge
  • continue over hips to form a 150mm minimum lap parallel with the hip rafter
  • at abutments, be supported and turned up by a minimum of 100mm
  • be draped to allow water to drain behind the tiling battens.

Table 7: Horizontal laps for unsupported underlay

PitchMinimum horizontal laps
Less than 15°225mm
35° and above100mm

At valleys:

  • the main roof underlay should be cut to the valley batten line
  • a strip of underlay should be laid under the main roof underlay and held down by the valley battens (where used).