Premier Mastic Asphalt

Detail Finishes

SKIRTINGS AND FLASHINGS

In the case of concrete, brickwork and similar sub-structures a two-coat asphalt skirting is necessary at all upstands to a nominal thickness of 13mm and a minimum height of 150mm above finished roof level, a two coat angle fillet being formed at the junction of the vertical and the flat. The top of the skirting is splayed and turned into a chase 25mm x 25mm unless the asphalt continues horizontally. Skirtings above 300mm high are regarded as “shown vertical” and asphalt is applied in three coats to a nominal thickness of 20mm.

Fig 1.3
Fig 1.4

Where the substructure consists of timber board, plywood, woodwool slabs or decking units which are liable to slight lateral movement require a free-standing splayed timber kerb fixed to the sub-structure minimum 12mm clear of walls and upstands. The asphalt is then applied in three coats to a total thickness of 20mm on to bitumen coated expanded metal lathing fixed over black sheathing felt. The skirting is cover-flashed and protected by an application of solar reflective paint. (Fig. 1.4).

Where insulation is used beneath the asphalt a minimum 25mm wide support leg to the skirtings is essential.(Fig. 1.4)

VERGES

Perimeter kerbs may be completed with a G.R.P. edge trim fixed at 300mm max. centres (Fig. 1.5). Alternatively an asphalt apron with an undercut drip may be formed (Fig. 1.6 and 1.7).

Fig 1.5
Fig 1.6
Fig 1.7

EAVES GUTTERS

Where the roof fall is into an eaves gutter the asphalt can be finished over a lead flashing set into a rebate in the sub-structure. The lead must be welted at the back and the depth of the rebate must allow for a full thickness of asphalt over the welt. (Fig. 1.8).

Alternatively, an asphalt apron or purposely made GRP Edge Trim can be used.

Fig 1.8

RAINWATER OUTLETS

A number of outlets can be used in conjunction with asphalt roofing but cast iron or spun aluminium outlets with a bellmouth and internal clamp ring are particularly recommended (Fig. 1.19)

Where a Syphonic drainage system is required, the outlets must be designed sopecifically for use with mastic asphalt.

At outlets through parapet walls asphalt is lapped onto a pre-formed lead chute which is taken through the wall into hopper head.

Fig 1.19

PROJECTIONS THROUGH ROOFS

Projections fixed in or passing through the roof, such as handrails, stanchions and metal pipes, can usually receive an asphalt collar direct. The asphalt is dressed 150mm above finished roof level and the upper edge protected with an apron flashing (Fig. 1.10). In situations where asphalt can not be applied directly to the projection a lead sleeve should be fixed to the substrate.

Fig 1.10

GUTTERS

These can be lined in asphalt to follow any shape of contour in the sub-structure. Where a gutter is formed between a parapet wall and a tiled or pitched roof, the asphalt is carried up the slope and over the tilting fillet. (Fig. 1.11).

It is generally preferable to avoid the use of integral gutters on flat roofs, using, instead, falls and cross falls to direct the flow of water to rainwater outlets.

Fig 1.11

ROOFLIGHTS AND VENTILATORS

Rooflights and ventilators should be mounted on kerbs minimum 150mm above the roof finish. Asphalt roofing is then taken up the side and over the top of the kerb (Figs. 1.12 and 1.13). Proprietary kerb adaptors are recommended for such details and these must be applied before the roof light is fixed. Advice on the suitability of PVC and metal roof lights kerbs to receive asphalt directly must be obtained from the manufacturer. However, a timber facing fixed to the kerb followed by Expanded Metal Lathing fixed over Black Sheathing Felt would normally be required. (Fig. 1.21).

Fig 1.12
Fig 1.13

MOVEMENT JOINTS

Twin kerb movement joints are recommended with a metal cap flashing fixed to one kerb only, or a capping system held by cleats or spring clips (Fig. 1.14). In either case suitable fixings should be provided to avoid penetrating the asphalt.

All ends should be boxed as necessary to complete the waterproofing but still allow movement.

The design of the structure should avoid flush surface movement joints if at all possible. However, proprietary systems are available for this application and specialist advice is necessary.

fig114

PLINTHS

Plinths to receive rooftop equipment must be constructed off the deck or screed level. Asphalt is dressed a minimum 150mm above finished roof level and the top protected with a cover flashing (Fig. 1.24).

Fig 1.24