Also known as bituminous membrane sarking and is an application known as built-up roofing. 

Bituminous felt is a flexible sheeting applied to the top of continuous surfaces, such as timber and concrete, of any shape, and flat, medium or steeply pitched. More commonly applied to flat roofing.

Manufactured by impregnating or saturating asbestos felt with asphalt. Was manufactured in several thicknesses known by weight (30lb, 40lb or 50lb) or by ply (1 ply and 2 ply) and supplied in rolls around 3′ wide which varied in length: 36′, 72′ and 144′.

Two to 4 layers were lapped and bonded together with hot bitumen or cold bitumen emulsion, although hot was preferable. Angle fillets were placed at the junction of the roof and any walls.

Non-ACM equivalents contained rag felt, paper, glass fibre or hessian. The asbestos containing product was considered superior. Asbestos provided stability and rotproofing.

May be an exposed top layer or installed as an underlayer beneath gravel or a roof deck. A variety of additives were sometimes applied to the exposed top surface for extra protection such as marble chips, fine gravel or metallic foil embedded in bitumen. These provided resistance to the drying effects of the sun and provided a degree of heat reflection.  

Older industrial applications may consist of up to three layers of asbestos felt interleaved with asphalt, sometimes with a lower layer of AC sheeting.