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
| Material | Standard / requirement | Minimum material specification (where applicable) | Minimum material thickness (where applicable) |
|---|---|---|---|
| Solid timber tongue and grooved or square edged board | BS 1297 | 16 mm | |
| Plywood | BS EN 636 | BS EN 636 - 2S(2) | 9mm(1) |
| Chipboard | BS EN 312 | Type P5 | 12mm(1) |
| OSB | BS EN 300 | OSB/3 | 9mm(1) |
| Type HR (>0.25MNs/g) and Type LR (<0.25MNs/g) underlays | BS EN 13859-1 BS 8747 | Class W1 1F/5U | |
| Type LR - Low water vapour resistance (<0.25MNs/g) and air permeable underlay (min. air permeability of 34m3/m2.h at 50 Pa) | Technical Requirements R3(3) | Class W1 | |
| Proprietary products | Technical Requirements R3(3) |
Notes
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 approval’s 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 cut neatly to fit tightly around service penetrations and not be torn, ie, where pipes project through the underlay
- 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 manufacturer’s 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
- be draped to allow water to drain behind the tiling battens.
Table 10: Horizontal laps for unsupported and supported underlays
| Pitch | Minimum horizontal laps - unsupported underlay | Minimum horizontal laps - supported underlay |
|---|---|---|
| Less than 15° | 225mm | 150mm |
| 15° and above | 150mm | 100mm |
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 2024