ANNUAL DRIVING RAIN INDEX
UNITED KINGDOM
In general, any area within 30 miles of a west facing coastline is considered to be in a severe exposure area. However, localised effects on any site must also be taken into consideration. Further reference to BS 8104 is advisable to ascertain localised effects. Map based on BS 5534.
MINIMUM RECOMMENDED HEADLAPS
The recommendations for minimum pitches and laps for slate apply to normal situations. In general, the recommendations apply to rafter lengths of not more than 9m in moderate driving rain exposures and 6m in severe driving rain exposures. Specifiers should take account of any abnormal condition that might apply and may need to specify greater values than the recommended minima. If it is necessary to use pitches lower than the recommended minima please contact Welsh Slate technical department. This table gives a range of sizes for your information Traditional sizes that are not listed below and random diminishing slates can be made to special order, please contact us for more information.
WIND LOAD AND WEATHER RESISTANCE
Slates fixed in accordance with the details given in this guide will have adequate resistance to wind loads, wind uplift and rain penetration under most conditions. The tables below give minimum recommended laps according to exposure, roof pitch and slate size. Detailed guidance on wind load calculations is given in BS 5534 and BS EN 1991.
COVERAGE
COVERAGE AND COST
The actual cost of a roof per unit floor area of the building is determined by a number of factors, including roof pitch, slate size and head lap.
ROOF PITCH AND LAP
Project specific advice is available from the Welsh Slate Technical Department. This table gives a range of sizes for your information. Other sizes may be available, please contact us for further information.
For Slate Sizes or Headlaps not listed on page 15 the following formulae can be used:-
BATTEN GAUGE (Slate Length minus headlap) divided 2 Example 500×300 mm @ 120mm headlap:- (500 – 120) / 2 = 190mm Batten Gauge
LIN M BATTEN PER M2 1 divided Batten Gauge in metres Example 500×300 mm @ 120mm headlap:- 1 / 0.190m = 5.26 lin m
HOLING GAUGE Batten Gauge + Headlap + 10mm Example 500×300 mm @ 120mm headlap:- 190mm + 120mm + 10mm = 320mm Holing Gauge
BATTENING AND HOLING GAUGES
The table below gives a range of sizes for the correct Batten Gauge (distance between battens) for your chosen slate length and lap. It also allows you to determine the holing gauge (distance from hole to tail of slate) and the number of linear metres of batten per m2 of roof.
Other sizes may be available, please contact us for further information.
