Design Considerations

Lightweight single-ply roofs usually consist of profiled steel decking fixed to steel purlins and overlaid with a vapour control layer. Rigid insulation board is fixed through the crowns of the decking and then overlaid with a light coloured, single-ply, polymeric waterproofing membrane which is usually mechanically fixed through the decking (figure 01).

The STYROFOAM™ Solution for insulating single-ply roofs is ROOFMATE™ RL-X.

ROOFMATE RL-X is designed to give the maximum benefit in single-ply roof construction:

• high compressive strength helps reduces the risk of mechanical fixings working loose after installation.
• enhanced dimensional stability minimises the number of fixings required and reduces board movement in service.
• supports pedestrian traffic during installation.
• minimal water pick-up enables insulation to be laid in any weather.
• easy to install.
• ROOFMATE RL-X is suitable for use under all single-ply polymeric waterproofing membranes.

Choice of membrane

ROOFMATE RL-X insulation is designed for use under light coloured (grey through white) single-ply polymeric membranes. ROOFMATE RL-X insulation is not suitable for use under dark coloured waterproof coverings such as bituminous felt or black EPDM. To prevent plasticiser migration PVC membranes should be isolated from ROOFMATE RL-X boards by a suitable separating layer.

Loading

The roof structure must be strong enough to withstand the maximum predicted loads with a suitable factor of safety. ROOFMATE RL-X boards are suitable for occasional pedestrian maintenance traffic but if more frequent traffic is expected, additional protection of the membrane and boards is required, eg. walkway layers or concrete paving slabs on spacers.

Securement

Fixings for the roof covering and insulation must able to withstand predicted wind-uplift forces; refer to the membrane supplier’s and fixing manufacturer’s data for loading limits and calculation methods. Each ROOFMATE RL-X board should be secured to the deck with three fixings, each having a washer of at least 20cm² area: these are in addition to any used to secure the waterproof membrane to the deck. Half boards or less may be secured with two fixings.

Drainage

Good drainage is vital to the long term performance of a flat roof. To ensure the minimum finished falls of 1:80 recommended in BS 6229 falls should be designed at 1:40.

Falls can be constructed by:

• designing the roof structure to falls.
• applying a screed to falls, or
• adding tapered insulation.*

When assessing the requirements of gutters and outlets designers should refer to BS EN 12056-3.

Fire

Building Regulation B4 requires roofs to resist the spread of fire over the roof and from one building to another. Roof covering classified according to BS 476: Part 3: 1958 as AA, AB or AC may be used without restrictions (see Approved Document B, Technical Standard D in Scotland). Experience suggests a roof construction which has a protective layer of 50mm of gravel above the waterproofing layer will achieve an AA rating. Non-ballasted, mechanically fixed, plasticised PVC roofing membranes will normally achieve an AB rating.

BRE document ‘Thermal insulation: avoiding risks’ indicates cellular plastic insulants laid directly on a metal deck may melt when there is a fire source within the building. Improved fire protection may be provided by the use of:

• sprinkler systems.
• ire rated suspended ceilings.
• fire barriers which form a break in the insulation layer, giving effective fire separation.

Consult a fire Safety Engineer where any doubt exists on the need for fire protection.

Thermal performance

Table 01 shows the amount of ROOFMATE RL-X required to meet a range of U-values.

Table 01 ROOFMATE RL-X thickness (mm) to achieve U-values (W/m²K)

U-value	0.16	0.18	0.20	0.25	0.35
ROOFMATE RL-X	200	180	160	120	80

Condensation

Condensation can cause rot, decay and the corrosion of metal components, leading to roofing failure. Whilst the risk of condensation is low for a metal deck roof featuring ROOFMATE RL-X insulation under normal environmental conditions, condensation risk analysis should still be performed, using the method given in BS 5250 Appendix D.

Specification

Use the following NBS clause to specify ROOFMATE RL-X insulation in a single-ply roof:

J42 452		Single layer polymeric roof covering Warm deck insulation
		Insulation: extruded polystyrene board to BS EN 13164 Manufacturer and reference: Dow Chemical Co. Ltd, Building Solutions, 2 Heathrow Boulevard, 284 Bath Road, West Drayton, Middlesex, UB7 0DQ. Tel: 020 8917 5050 - Fax: 020 8917 5413 ROOFMATE RL-X Thickness: 50/60/80/120 mm Board size: 2500mm x 600mm Edge profile: tongue and groove Compressive strength: 300 kN/m2 Design loading: 110 kN/m2 Fire classification: EUROCLASS E