Pitched roofs

Also see:

7.2.19Fixing tiles and slates

Coverings shall be suitably fixed to protect the building from weather. Issues to be taken into account include:

  1. eaves, ridge and hip tiles
  2. verges
  3. mortar
  4. vertical tiling and slating.

Careful setting out will improve the finished appearance of the roof, help avoid problems such as unequal overhangs, and reduce excessive tile cutting at abutments, chimneys and similar obstructions.

When installing coverings:

  • clay tiles that do not meet the dimensional and geometric requirements given in BS EN 1304 should not be laid at pitches less than 40°
  • joints between tiles and slates should be slightly open, which provides some flexibility in setting out and should help to avoid tile cutting (single lap interlocking tiles have a tolerance of approximately 3mm at the joint)
  • double tiles, tile-and-a-half or half tiles can be used when available from the manufacturer (to avoid the use of small sections of cut tiles). Alternatively, where the tile manufacturer provides guidance, small sections of single lap tile can be bonded to full tiles
  • the bottom edges of double-lapped slate and plain tile roofs should be finished with an under-eaves course.

Table 12: Pitch, gauge and lap

Type or tileGaugeMinimum headlapMinimum permissible pitch (°)
Plain (double lap)Maximum 1/3 length lap65mm generally for clay tiles
75mm in severe exposure conditions
35 (clay)
35 (plain concrete)
Concrete (single lap interlocking)Comply with the manufacturer’s recommendations75mm or to the manufacturer’s recommendations30⁽²⁾
Slates (double lap)Maximum 1/3 length lap54mm⁽¹⁾ minimum, increased with lower pitch and severe exposure conditions20 subject to headlap


1 For pitches greater than 45° in sheltered and moderate exposure zones only.
2 For pitches below 30°, evidence shall be provided as to suitable performance.

When fixing coverings to a pitched roof:

  • the fixing schedule should be produced by the tile manufacturer; fixings for single and double lap tiles should be in accordance with BS 5534 and BS EN 1994-1-4 (evidence of calculations in compliance with Technical Requirements R3 and R5 may be required)
  • coverings should be fixed in accordance with the design and the manufacturer’s recommendations
  • slates and tiles should be fixed using clout or slate nails (these should be either silicon bronze, aluminium to BS 1202-3 or copper to BS 1202-2).
  • galvanized steel nails should not be used for slates and tiles (but are acceptable for fixing battens or underlay)
  • fixings should be a minimum of 38mm long, and penetrate a minimum of 15mm into battens
  • tile clips should be of plastic, aluminium or stainless steel
  • slates should be fully nailed over the whole roof, and nailed twice where centre nailed.

Eaves, ridge and hip tiles

At eaves:

  • tiles should project a minimum of 50mm across the gutter
  • when using slates or plain tiles, an under-eaves course should be used
  • the height of the facia should maintain the tile pitch, in accordance with the tile manufacturer’s recommendations.

Where ridge tiles are mortar bedded:

  • the underlay should extend over the ridge.

At hips:

  • underlay should continue to form a 150mm minimum lap parallel with the hip rafter
  • where wet bedded tiles are used, they should be supported at the base by a galvanized hip iron and project to the centre line of the gutter.

Ridge and hip tiles should be mechanically fixed with self-sealing non-ferrous fixings into timber battens, and have a nominal joint thickness of 10mm where wet bedded. Wet bedded ‘baby’ hip/ridge tiles to low level roofs, such as those over porches and ground floor bay windows, do not require mechanical fixing, unless recommended by the manufacturer.

Proprietary dry fixed systems should be in accordance with BS 8612.


Unless a proprietary dry verge system or cloaked verge is used, tiles should be bedded into a 100mm wide bed of mortar on an undercloak of cement-based board, plain tile or slate. Plain tiles should not be used as an undercloak below 30°pitch or on a bargeboard.

Undercloak should be:

  • fixed in accordance with manufacturer’s recommendations
  • installed to a true line
  • installed at the correct level to ensure that the line of the tiling is maintained where it passes over the wall, and not tilt inwards
  • bedded on roofing mortar and struck off flush with the external surface of the wall (alternatively, a suitable exterior grade bedding sealant should be used in accordance with the manufacturer’s recommendations)
  • securely nailed to a true line where a bargeboard is used.

Where verge tiles and slates are wet bedded, pointing should be completed as soon as possible using the same mix.

Verge clips should be in full contact with the tile to resist uplift, nailed twice to battens and sized to ensure that they are in direct contact with the top surface of the verge tile.

Where plain tiles and slates are used at the verge:

  • they should project 38-50mm beyond the gable wall or bargeboard
  • cut plain tiles are not acceptable, and purpose-made plain tile-and-a-half tiles should be used
  • natural slate verges should be formed with full slates and either slate-and-a-half or half slates that are a minimum of 150mm wide.

Where interlocking tiles are used at the verge:

  • they should project 30-60mm beyond the gable wall or bargeboard
  • small sections (less than a half tile width) of cut interlocking tiles should not be used.


When bedding tiles or slates in mortar:

  • the mortar should be 1:3 cement:sand with plasticiser
  • the mortar should be a mix based on sharp sand with soft sand added to achieve workability; the proportion of sharp sand should not be less than one third of the total sand content (proprietary mixes may be accepted by NHBC where they are shown to have similar strength, durability and workability)
  • pointing should be completed as soon as possible using the same mix.
  • tiles should be wetted on their contact surface, and surface water allowed to drain away before fixing
  • concealed or decorative dentil tiles should be fully bedded into joints in excess of 25mm thick.

Vertical tiling and slating

When fixing vertical tiling and slating:

  • a suitable moisture barrier should be used
  • where the wall structure is solid brickwork or blockwork, the moisture barrier should be underfelt or equivalent
  • where the supporting structure is of timber construction, the moisture barrier should be used with a breather membrane
  • batten sizes should be in accordance with this chapter
  • every tile or slate should be nailed twice and the bottom edges should be finished with an under-course tile
  • at internal or external angles, purpose-made corner tiles or soakers should be used to form a weathertight joint
  • where pitched roofs abut tiled walls, a stepped flashing should be specified and turned in behind the tiles
  • at dormer cheeks, the tiles or slates should be specified to be cut close to the slope of the roof and over a flashing fixed to the side of the dormer.