Xtra-Gard Flat Roofing Systems

Roof Design, Installation and Site Work

This section is designed to provide general guidance on the design and installation of Anderson Reinforced Bitumen Roofing Membranes, for further information or specific advice contact Icopal Technical Services on 0161 865 4444.

Structural Decks

Concrete, timber boarding, plywood, OSB 3 and profiled metal decking are the most common types of structural deck used for flat roofing. It is important to carefully select the type of structural deck material to suit the use of the roof and its capability to withstand imposed loads and any possible snow loading etc.

Roof Types

There are generally three types of flat roof construction:

1. Warm Roof – thermal insulation is positioned above the roof deck and vapour control layer and immediately below the waterproofing, resulting in the structural deck and ceiling being at a temperature close to that of the interior of the building. Ventilation of the roof void is not required. However due consideration must be given to providing a minimum upstand requirement of 150mm above the finished roof level for all details.

2. Cold Roof – this type of roof must be carefully designed to avoid premature failure and should be avoided wherever possible. Thermal Insulation is positioned immediately above the ceiling, resulting in the roof covering and structural deck being substantially colder in winter than the interior of the building. It is important that a minimum 50mm void is allowed between the top of the insulation and the roof deck; this void must be ventilated and cross ventilation must be provided with a minimum 25mm continuous opening at eaves on opposing sides of the roof and the provision for venting between joists. If eaves cross ventilation is not possible proprietary roof vent systems must be installed.

3. Inverted roofs – thermal insulation is positioned above the roof covering, resulting in the roof covering, structural deck and ceiling being at a temperature close to that of the interior of the building. The insulation type for use on inverted roofs must be Extruded Polystyrene (XPS) and is secured by ballast loading. Note: the roof deck must be capable of withstanding the additional loading of a ballasted roof.

Thermal Insulation

New and refurbished roofs will need to be insulated to meet the requirements of the current Building Regulations. Specific guidance on the calculation of thermal transmittance (“U”) values and condensation risk within a structure can be found in the current issue of BS6229. Specific U-Valve calculations can be provided by Icopal Technical Services.

Wind Action

The design of the roof structure and choice of waterproofing system must make allowance for wind forces which can have a significant effect on roof coverings, particularly during winter gales and in exposed locations. Attachment of roofing systems should comply with the current recommendations of EN1991-1-4. Specific wind uplift calculations can be provided by Icopal Technical Services.


The fire requirements of the current Building Regulations must be taken into consideration when designing the roof and waterproofing system. These requirements are based on the performance of the roof structure as submitted for assessment in accordance with PrEn 1187 B Roof T4 (BS476:Part 3:2004) – fire tests on building materials and structures – Part 3: Classification and method of test for external fire exposure to roofs. An Ext. F.A.A,
F.A.B or F.A.C result will normally allow unrestricted use and Icopal High Performance Waterproofing Systems will achieve one of these results.

Installation and Site Work


Cover Flashing

Prior to the application of the waterproofing membranes suitable chases should be cut to receive FormFlash flashing installed in accordance with current Icopal instructions. All chases must be positioned a minimum of 150mm above the proposed finished level of the roof surface.


All substrates to receive the waterproofing membranes must be clean, free from oil, grease and sharp projections, all mechanical fixings used to secure the roof deck must be well driven to prevent fixings puncturing the membranes.

Surface Preparation

Metal, concrete, and screeded surfaces, plywood, OSB3 and masonry should be primed with either; Xtra-Seal SF Bitumen Primer or Xtra-Seal QD Bitumen Primer in accordance with the coverage rates and instructions on the containers.

Membrane Attachment

When installing a 2 layer waterproofing system over plywood, OSB3, concrete, and screed surfaces the first layer of waterproofing must be partially bonded using the appropriate preparation layer (refer to Specification build ups).

When applying the waterproofing system over Polyisocyanurate (PIR) the waterproofing must be partially bonded with the appropriate preparation layer.

Timber boarded decks require a first layer of Xtra-Gard Toughbond 180 Underlay to be nailed at 150mm staggered centres over the field area and at 50mm centres along side and end laps.

On metal decks the waterproofing system is laid over the chosen insulation, which is laid onto a vapour control layer bonded to the primed metal deck.

Waterproofing layers are laid with minimum 75mm side laps and minimum 100mm end laps (or as otherwise specified) and fully sealed. Successive layers of waterproofing are applied with side lap joints offset by 300mm and end laps must also be offset from the previous layers to avoid a build up of joints in successive layers. When applying waterproofing membranes using the pour and roll method of application 95/25 grade bitumen should be used on roofs up to and including 5° pitch and 115/15 grade bitumen over 5° pitch. End/head laps on membranes applied on roofs over 5° pitch must be mechanically fixed.

When applying waterproofing membranes by torching it is essential that no torching flame is allowed to contact any timber or combustible surface. If in any doubt apply a sacrificial nailed layer first. It is essential that the thermofusible film is completely dispersed and a continuous 5mm bead of bitumen extrudes from all laps as work proceeds.

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