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Mastic Asphalt Detail Design

Here we will have an in-depth look at mastic asphalt detail design. Areas we will cover includes adhesion, thickness, asphalt fillets, skirting, gutters, expansion joints and rainwater outlets.

Mastic asphalt follows the same fundamental principles as built up roofing when it comes to using upstands for waterproofing at skirting and turn downs on open roofs.

The minimum required height of an upstand for waterproofing at skirting’s is 150mm. Whereas for a turn down on an open roof the size should be 50mm. This includes the need to form independent upstands where differential movement between the roof and wall is likely.

A complexity of asphalt detailing is the main area of asphalt roofing is applied horizontally on top of a separating layer in the form of sheathing felt. The reason for this is to isolate the asphalt from the movement of the roof deck.

This means there is a requirement for the horizontal layer of asphalt to have full adhesion to the vertical upstands and the open roof edge details. The reason for this is to prevent the shrinking back of the asphalt.

Adhesion at Details


The techniques used for achieving adhesion with mastic asphalt detailing is dependent on the substrate.

If the substrate is concrete, brick or sand/cement the surface will need to be prepared appropriately then the mastic asphalt will need to be applied directly to the surface.

In some cases, it will not be possible to prepare the substrate in a manner which will allow reliable adhesion of the mastic asphalt. If this is the case expanded metal lathing will need to be secured to the substrate. The reason for this is to provide a key for the asphalt to adhere to.

For substrates such as plywood, particleboard, timber of lightweight concrete blocks direct application of mastic asphalt is not possible. For the application of mastic asphalt to any of these substrates expanded metal lathing1 on sheathing felt must first be secured to the substrate.

Vertical metal substrates (apart from small metal pipes) will not allow a reliable adhesion of mastic asphalt if directly applied. Expanded metal lathing must be secured to the vertical metal substrate before asphalting commences.

Securing Metal Lathing:

Expanded metal lathing must be secured with nails or screws. The nails or screws must have centres of no more than 150mm in either direction. Expanded metal lathing is manufactured to provide a high number of keys per square yard. The cupped shape across the lathing must point upwards to provide an adequate key for the mastic asphalt to adhere to.

Preparing the Substrate for Aspahlt:

High bond primer is always applied to all vertical brick, concrete and sand/cement surfaces prior to the application of asphalt. The application of this primer overcomes most major difficulties encountered with these substrates. These difficulties include mould oil, extremely smooth surfaces and concrete laitance. Before the application of primer every effort should be made to remove an excessive mould oil2. And extremely smooth surfaces should be roughened.

Thickness of Asphalt at Details

Vertical asphalt roofing work will in general carried out to the three following nominal thicknesses:

1) Asphalt detailing applied directly to a suitable substrate - 2 coats for a total thickness of 13mm.

Two coats of asphalt are required to be applied to brick, concrete or sand/cement substrates. First any excessive mould oil is removed and if the surface is extremely smooth roughness is created. Next a high bond primer is applied to the substrate.

Once this is done the first coat of asphalt is applied. This coat of asphalt is relatively thin and is used to remove any irregularities in the substrates surface. And form a key with the substrate. Once this coat of asphalt has cooled, a second coat must be applied. This second coat brings the thickness of the asphalt to a total of 13mm. The finish is smooth and of high quality.

2) Asphalt detailing applied on expanded metal lathing - 3 coats for a total thickness of 20mm.

Three coat work is required when mastic asphalt is applied to metal. The need for a complete covering of the metal lathing is the reason a third coat is needed as it is difficult to achieve a satisfactory finish with just two coats.

The first coat of asphalt applied is a thin one which is used to form a key with the lathing. Once this coat has cooled a second coat is applied to build up the thickness of the detail. The finish of this second coat is a rough one. The final coat brings the thickness of the asphalt up to 20mm and is finished in a smooth high quality manner.

3) Asphalt detailing which is greater than 300mm in height on any substrate - 3 coats for a total thickness of 20mm.

If the height of the asphalt detailing work exceeds 300mm in height it is not possible to achieve a good finish with just two coats of mastic asphalt. The third and final layer of this type of work brings the nominal thickness to 20mm.

*Three coat work may also be required when applying asphalt to old brickwork. The extra coat may be needed as the first coat of asphalt may need to be used to establish a suitable base for the mastic asphalt.

Asphalt Fillets

Horizontal and vertical asphalt work are carried out as separate applications. To form a fully efficient junction between the horizontal and the vertical a fillet must be applied. The application of an asphalt fillet is carried out independently of the application of both the horizontal and vertical asphalt. The use of an asphalt fillet fuses both the horizontal and vertical together. The fillet also provides reinforcement to the joint due to the considerable thickness of the asphalt used.

To achieve a fully efficient fusion between the horizontal and vertical an asphalt fillet must be created with two coats of mastic asphalt.  Before a fillet is applied the area around the joint between the horizontal and vertical plane of asphalt must be cleared so it is free of and sand or debris. A hot poultice of asphalt is applied to the joint to soften both the vertical and horizontal plane at this intersection.

Once this has been done the hot poultice is removed. The junction is scraped and cleaned before a fresh coat of asphalt is applied with a filleting tool to the junction. A second coat of mastic asphalt is now applied. This coat is also laid with a filleting tool. The filleting tool will be applied with pressure to cut into softened asphalt of both the horizontal and vertical asphalt. The result will be an indentation of 2-3mm. There is no need to worry about any loss of thickness in the asphalt. This indentation is good practice and an indication of a well-formed asphalt fillet. The most important aspect of the fillet is to achieve a first-class bond between the horizontal and vertical plane of the mastic asphalt.

To maximise the longevity of the asphalt fillet, a layer of solar reflective paint should be applied (it is good practice to renew this solar reflective paint every 3 to 4 years). This protects the asphalt from the natural movement caused by the surface being heated in sunlight. If roof is covered in chippings it is good practice to stop the chippings 75mm from the fillet and finish this part in solar reflective paint. This makes it easy to visually check the condition of the asphalt fillet.

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Asphalt Upstand Detailing (Westminster, London)