Weather Resistance: Rain and Snow

Exposure to Driving Rain

The UK has a high risk of severe driving rain, and even in some sheltered locations, may be subject to high levels of deluge rainfall.

BS 5534 defines two categories of exposure, based on the driving rain data given in BS 8104 and BR 262 and should be used for buildings up to 12m in height (see map opposite). For buildings over 12m in height, the influence of increased wind speeds should be taken into account using BS EN 1991-1-4. Guidance on the use of Marley tiles and slates on roofs greater than 12m in height should be obtained from the Technical Advisory Service.

Performance of Tiles

Rain penetration of the roof covering is dependent on a combination of the rainfall rate, wind speed, and the ability of the roof tile to resist the ingress of snow and rainwater. The designer should therefore be aware of the various means by which rain and snow can, under certain conditions, penetrate the roof covering. These include:

  • Capillary action and rainwater creep
  • Raindrop bounce and negative pressure rain suction
  • Driving rain, deluge rain and flooding
  • Surcharging of rainwater at overlaps of tiles on long rafter length roofs
  • Wind-driven snow

Roof Pitch

When determining the pitch, headlap and/or sidelap of a tile, the roof pitch is taken to be equal to the rafter pitch. Hence, all references to pitch refer to the rafter pitch, with the laid angle of the roof tile always being less than roof pitch.

The actual pitch of a tile should be determined in accordance with the following guidelines:

Tile/slate to rafter pitch angles

  • Plain tiles: 7° less than rafter pitch
  • Interlocking single lap tiles: 5° less than rafter pitch

Marley plain tiles, and interlocking tiles are designed and tested to minimum rafter pitches. If the design rafter pitch is less than the minimum recommended rafter pitch for the particular tile, a build up of surface water may cause the product to leak., and it can only then be considered to have an aesthetic function. In such cases, the true weatherproofing of the roof system must rely on a fully supported waterproof membrane with an uninterrupted drainage path between counterbattens to the eaves gutter. Details of the full specification for such a roof construction can be obtained from the Technical Advisory Service.

Head and Side Laps

All products are subject to the following recommended minimum laps:


Double lap tiles: The headlap for double lap products is taken as the distance by which the upper course of the tile provides a lap with the next but one course below.

Plain tiles: For Marley double lap concrete and clay plain tiles, the headlap should be not less than 65mm and should not exceed one-third of the length of the tile.

Double lap concrete plain tiles have a negative longitudinal camber, which prevents capillary action and can be used satisfactorily at a minimum roof pitch of 35°.

Double lap clay plain tiles are produced in a range of both machine and handmade designs, and can be used at roof pitches from 30° (Acme Single Camber) to 40° (Canterbury Hand Made).

When determining the head lap of feature and ornamental double-lap tiles, the shape of the tail should be taken into account.

Single lap tiles or slates: For single lap tiles, the headlap is the distance by which a course of tiles provides an overlap with the next course below. In the case of Marley interlocking single lap tiles, a minimum overlap of 75mm is sufficient for most roof pitches of 30° and above. Below 30°, the overlap may be increased to 100mm to cater for different surface finishes (smooth, granular or sanded).


Double lap tiles: For plain tiles, the notional sidelap is the side distance by which the tile overlaps the tile in the next course below.

Single lap tiles: For interlocking tiles, the sidelap is accommodated by an interlocking channel design feature incorporated into the side edges of the tile.

Plain tiles: For plain tiles, the side lap should be not less than one-third the width of the tile and using Marley plain tiles at 100mm gauge, the side lap should be not less than 55mm for clay plain tiles and 56mm for concrete plain tiles.

Single lap tiles: The side lap for Marley single lap products is the amount by which one tile overlaps the adjacent tile in the same course by way of a side interlock or upstand feature which forms part of a proprietary side-lock design.

Performance Testing

All Marley tiles, fittings and accessories are designed and tested in a purpose-built wind and rain tunnel, using a test method in accordance with the requirements of PD CEN /TR 15601: Hygrothermal performance of buildings – Resistance to wind-driven rain of roof coverings with discontinuously laid small elements - Test methods.

Further guidance on categories of exposure to driving rain can be obtained from BS 8104 ‘Code of practice for assessing exposure of walls to wind-driven rain’ and BRE Report 262 ‘Thermal insulation: avoiding risks’.

The effectiveness of the tile or slate to operate at the minimum recommended pitch and lap may be influenced by special circumstances. Guidance on pitch and lap should be obtained from the Technical Advisory Service for the following:

  • Interlocking tiles and slates where the roof slope exceeds 6 metres in length and/or the site is rated to be in a severe exposure category.