7.2.14 Underlay, sarking boards and sheets
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 sarking sheet or board with underlay is recommended.
Table 9: Acceptable materials for use as underlay and sarking
Note(s)
1. Minimum thickness suitable only where roof coverings (eg slates and tiles) are independently supported on battens, secured to counter battens. Minimum thicknesses quoted, actual design requirements may differ.
2. S = Structural.
3. Propriety products should hold a satisfactory assessment by an appropriate independent technical approvals authority acceptable to NHBC.
Underlay should:
- be provided to all tiled and slated roofs
- not be left exposed to sunlight for longer than recommended by the manufacturer
- where fully supported on a sarking sheet which offers a high resistance to the passage of air or water vapour, such as plywood, OSB, chipboard, or tongue and grooved sarking boards, be treated as a Type HR underlay for roof ventilation purposes
- be supported by a continuous fillet or proprietary eaves support tray, laid to inclined falls (see Figure 41), to prevent sagging (which can form a water trap)
- be securely fixed in accordance with manufacturers' requirements
- at vertical laps, be fixed only over rafters, and at horizontal laps be held in place by battens (where no batten is provided over a horizontal lap, the underlay manufacturers’ guidance should be sought)
- be dressed into the gutter where exposed at eaves level, be UV resistant or of type 5U felt or a proprietary eaves guard used
- where traditional mortar pointing is used to bed ridge tiles, extend over the ridge, in accordance with Clause 7.2.19
- be detailed in accordance with manufacturers' requirements where proprietary ventilating ridge tiles or dry ridge systems are used
- 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.
Underlays which are unsupported ie, not supported by sarking boards/sheets, should be laid with a drape between rafters or supports:
- to allow water to drain freely beneath tilling battens
- to ensure that water is deflected away from batten fixings.
Excessive drapes should be avoided to prevent transfer of loads to the outer roof covering and excessive noise nuisance under wind action.
Where no underlay drape is provided, counterbattens or proprietary spacers between the tiling battens and underlay should be installed, to ensure that there is a clear drainage path beneath the tiling battens, and a suitable sealing tape should be used to prevent water ingress via fixings.
Penetrations through the underlay should:
- be cut neatly to fit around service penetrations and not be torn ie, where pipes pass through the underlay
- follow manufacturers’ fitting instructions where proprietary ventilation units and similar products are specified
- where no guidance is provided, be protected from water ingress using tapes or proprietary seals used in accordance with the manufacturer’s instructions.
Table 10: Horizontal laps for unsupported and supported underlays
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).
Last updated: 2nd January 2025