SEPARATING LAYER

The purpose of the separating layer is to isolate the asphalt from joint movement in the substrate but still provide a significant friction to help restrain the asphalt against contraction in cold weather.  It must also allow a free lateral passage for hot air and moisture vapour during the application of the hot asphalt and act as a long term vapour pressure release layer.

Separation is provided by Black Sheathing Felt which is laid entirely loose with lap joints of 50mm.  However because Black Sheathing Felt is liable to compress slightly under heavy loadings, a separating membrane of Glass-Fibre Tissue should be specified in buried specifications such as Roof Gardens.

TYPES OF MASTIC ASPHALT

Mastic Asphalt for Roofing (limestone aggregate BS 6925:1988 Type R 988/T.

Although Kitemarked and covered by BSI Certification the following grades are manufactured to within much narrower requirements than those set out in the British Standard specification.

Polymer modified mastic asphalt is now available and is designed to provide even higher performance characteristics than standard grades of asphalt roofing.  It provides increased high temperature stability coupled with low temperature flexibility in all roofing situations, particularly over insulated substrates.

APPLICATION OF ASPHALT

The application of roofing asphalt should be in accordance with BS 8218 and BS 8000: Part 4.

FLAT ROOFS UP TO 10°

Roofing asphalt is normally laid in two coats, breaking joint, to a nominal thickness of 20mm on an underlay of black sheathing felt laid loose with 50mm lapped joints.

Where thermal insulation is laid beneath the weather proofing it is recommended that the roof pitch does not exceed 5°.

SLOPES OVER 10°  AND VERTICAL SURFACES (EXCLUDING SKIRTINGS)

Roofing Asphalt laid on concrete or screeded sub-structures of 10° – 30° slope is applied in two coats to a nominal thickness of 20mm direct to the concrete.

For vertical work over 300mm and slopes over 30°  the asphalt is applied in three coats, the first coat being applied very thinly with a steel trowel or small wooden float.  A further two coats are then applied, breaking joint, to give a nominal thickness of 20mm.  For these applications the concrete must be left with a roughened surface to form a key for the asphalt.

In the case of sloping timber surfaces over 10° a layer of black sheathing felt is first nailed to the timber.  Bitumen primed expanded metal lathing is then fixed at 150mm centres with galvanized clout nails or staples to form a key for the asphalt which is applied in three coats to a nominal thickness of 20mm.

SAND RUBBING

With the exception of vertical and steeply sloping work, clean sharp sand should be rubbed evenly into the finished surface of the asphalt with a wooden float while it is still hot.  This rubbing breaks up the skin of bitumen brought to the surface at the time of application.  Gradual crazing of the surface due to the action of the sun is minimized by sand rubbing in this way.

KEYING TO SURFACES

CONCRETE

Apply a light brush coat of High Bond Primer and allow to dry thoroughly before applying asphalt.

Where vertical or sloping concrete is very smooth (eg. where steel shuttering has been used), the surface laitance should be removed by wire brush or suitable mechanical means to provide a satisfactory key for the asphalt.  Where excessive blowing is experienced the fixing of bitumen coated expanded metal lathing over black sheathing felt may be required.

NOTE

Damage to asphalt and loss of key will be caused by excessive use of mould oil.

Asphalt cannot be applied directly to lightweight concrete of lightweight concrete block work which should be rendered with a suitable sand cement facing or have bitumen coated expanded metal lathing on sheathing felt fixed at maximum 150mm centres.

BRICKWORK

Joints in brickwork should be flush pointed.  The surface must be cleaned and high bond primer applied to avoid blistering or loss of bond.

TIMBER

The key to vertical or sloping timber surfaces if obtained by fixing expanded metal lathing over black sheathing felt fixed by nailing with extra large head galvanized felt nails to BS1202 or with galvanized staples at maximum 150mm centres.

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