Fixing of membrane roofing

The roof designer shall specify the method of fixing, fixing types and distribution in different parts of the roof, taking into account the properties of the membrane, the decking and the loads the structure is exposed to. Insulation slabs and roofing are usually mechanically fixed to the decking due to the wind suction load and other forces affecting the structure. Each decking material requires its own fixings and fixing methods.

Membranes are fixed to the substrate with bitumen, special mechanical fixings or a combination of both of these.

Bonding with bitumen is either done over the whole surface or in parts, depending on the substrate. In a built-up system the membranes are bonded to one another with bitumen over the entire surface area.

It is recommended that the pressure equalizing membrane is attached so that fully bonded membranes form controllable pressure-equalizing zones with a practicable surface area (10 - 100 m²) (this ensures that in the event of a damage to the structure, water does not spread over the entire roofed area along the pressure equalizing layer. A pressure-equalizing membrane is used as a base layer in systems where sufficient moisture to cause moisture pockets might accumulate on the top surface of the decking.

Modified bitumen membranes are bonded with either blown or modified bitumen. The modified bitumen’s currently used are in practical terms rubber bitumen (e.g. KB 100).

In normal conditions, however, the use of modified bitumen (in practice rubber bitumen) is not practicable in part of the base sheet. Experience has shown that blown bitumen meets the necessary bonding requirements and, if needed, mechanical fixing may be used to secure the roofing.

The Finnish Roofing Association recommends that the base membrane of a roofing system be bonded to the decking with blown bitumen (e.g. B 100/30 tai B 95/35).

The heating of rubber bitumen and its use as an adhesive requires great care and attention, as there is only a slight difference between the highest permissible temperature and the minimum temperature for workability. The margin of working temperatures is significantly wider with blown bitumen. If overheated, the properties of rubber bitumen deteriorate significantly. If not sufficiently heated the workability and adhesion of rubber bitumen suffers.

Cap sheets are mainly bonded by torching, and standardized products always have a bonding bitumen that meets applicable requirements.

In mechanical fastening the fixings are specified according to the substrate material, the tear resistance of the membrane, and the thickness and compression resistance of the underlying insulation. If thick flexible materials (e.g. mineral wool) are used below the roofing, flexible fixings that compress under load (e.g. loads imposed by walking on the roof or by snow) should be used.

Wind exerts both pressure and suction loads on a roof. For membrane roofing suction load is generally a greater stress factor than wind pressure. The greatest stresses exerted by wind are exerted on the corner areas of a roof, and therefore the distribution of fixings must be densest there. There is lesser need for extra fixings at edge areas than at corners. The recommendation for roofs is 2 fixings/m², unless wind load calculations indicate a higher frequency.

The number of fixings and their types are specified on a case-by-case basis with consideration for the above factors. A basic number of fixings is used in the central areas of the roof. The specification of the membrane through which the fixings are to be driven may not be changed without revising the fastening calculations. The weakest point and the point subject to the greatest stress of a structure should always be determined in the calculations.

Roofing fixings are divided into three use classes depending on the corrosion resistance required of them in different roof systems. The use classes are K, KL and KLA. In practice the only ones used in the Nordic Countries are the KLA class fixings, which have the best corrosion resistance.

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