Membrane Surfaces

Jump to: navigation, search

Membrane Surfaces

1 General

Roof surfacing materials are required to protect bituminous roof membranes from direct sunlight (ultraviolet rays). This includes bituminous, modified bituminous, and built-up roof membranes. In addition, the top ply or felt in a BUR must be protected from weather exposure to prevent “wicking” of moisture into the system. The surfacing material can also provide fire resistance, reflectivity or impact resistance. Basically, there are three surfacing options:

  • Aggregate (gravel, stone, slag, etc.)
  • Smooth Surface Coating
  • Mineral Surface (Cap sheet)

The use of protective walkway pads both to define rooftop traffic areas and around mechanical units is good roofing practice and should be considered.


2 Aggregate

When modified bitumen membranes are surfaced with aggregates, refer to the RoofStar Guarantee Standards under Low Slope Roofs for surfacing requirements. For slope limitations refer specifically Low Slope roof membranes.

Aggregate surfacing usually consists of a suitable roofing gravel embedded into one or two pour coats of hot asphalt.

A “single pour and gravel” consists of a pour coat of 3.0 kg / m2 (60 lb / square) asphalt with 20 kg / m2 (400 lb / square) of gravel embedded into the pour coat. Care should be taken to ensure that a complete film of asphalt covers the felts in a built-up roof membrane (i.e. no “skips”) and that, in all cases, the gravel covers the asphalt without leaving spaces. A single pour and gravel is to be used only for slopes exceeding 1:24 (1/2" in 12") but not exceeding 1:5 (2-1/2" in 12"). This will prevent the weight of the asphalt and gravel from causing slippage of itself or the underlying membranes.

A “double pour and gravel” consists of a pour coat of 3.0 kg / m2 (60 lb / square) asphalt with 20 kg / m2 (400 lb / square) of gravel embedded into the pour coat. The loose gravel is then swept off and another pour coat and gravel (of the same quantities) is installed. A “modified double pour and gravel” is similar to a double pour except the first pour and gravel only consists of 2.0 kg / m2 (40 lb / square) of asphalt and 10 kg / m2 (200 lb / square) of gravel. An alternate method of a modified double pour and gravel is defined as 2 kg / m2 (40 lb / square) of asphalt flood coat followed by an additional 3 kg / m2 (60 lb / square) pour coat of asphalt followed by a 25 kg / m2 (500 lb / square) application of gravel. These methods are not recommended on slopes over 1:24 (1/2" in 12"). Use a double or modified double pour at all roof corners exposed to wind and at the corners of penthouses where severe wind eddies occur.

On protected membrane or modified protected membrane roof assemblies (PMRA or MPMRA), where the insulation is above the membrane, the gravel is used to protect the insulation from ultraviolet rays, wind uplift, and flotation. Refer to the Protected and Modified Protected sections under Low Slope roof membranes for (insulation manufacturer) ballast weight requirements for the negative flotation of extruded polystyrene insulation.

Aggregate or roofing gravel for conventional built-up roofing should generally be 6 mm (1/4") to 19 mm (3/4") washed crushed gravel or washed river bottom gravel. It should be dry and free of sharp angular pieces. It is important that the gravel is opaque and that white rock chips (frequently transparent) should only be used over roofing gravel. In all cases the gravel must cover the asphalt without leaving spaces. For PMRA's or MPMRA's the stone ballast should generally be 25 mm (1") clear size, free of fines or stones smaller than 15 mm (5/8") or larger than 35 mm (1-3/8"). (Consult the RoofStar Guarantee Standards for exact sieve size.)

An aggregate-surfaced roof provides the following properties and advantages:

  • protects the bitumen from premature aging due to ultraviolet rays and weathering
  • increases the wind uplift resistance
  • stabilizes the asphalt, allowing heavier pour coats
  • improves fire resistance
  • moderates membrane temperature fluctuations through reflectivity
  • protects against hail damage
  • requires less periodic maintenance


The possible disadvantages or precautions involved in the use of an aggregate-surfaced roof include:

  • it increases the weight of the roof assembly
  • it obscures damages or defects, making it more time-consuming to repair
  • complete gravel coverage over the bitumen must be maintained

3 Smooth Surface Coatings

A smooth surface roof coating is usually asphalt that has been liquefied to suit cold application. This can be achieved by two methods:

  1. The asphalt can be dissolved in organic solvents to produce cutback asphalt.
  2. The asphalt can be emulsified in water to produce an asphalt emulsion.


These products are usually modified with mineral fillers, such as clay and asbestos fibre, to stabilize the mixtures and produce a material that is easier to trowel. When applied the liquids evaporate, and the bitumen and mineral mixture remains as the coating.

Smooth surface coatings for roof membranes include:

  • Filled Cutback Asphalt Roof Coating: conforming to CGSB 37-GP-8Ma, is defined by CGSB as being used “as a sprayed or brushed-on coating for asphaltic built-up roof surfaces.” (Apply according to CGSB 37-GP-2M and manufacturer's latest printed instructions.)
  • Unfilled Mineral Colloid-Type Emulsified Asphalt for Roof Coating: conforming to CGSB 37-GP-2M, is defined by CGSB as being used as a “sprayed, troweled, or brushed-on coating for damp-proofing and waterproofing concrete or masonry surfaces, and for built-up roofs and other roof surfaces.” (Apply according to CGSB 37-GP-31M and manufacturer’s latest printed instructions.)


In addition, there are protective and reflective coatings for use on smooth-surfaced roofs. The most common are aluminum or light coloured for reflectivity, prolonging the life of a smooth surface roof by reducing:

  • surface temperatures
  • evaporation of light oils
  • rapid temperature changes
  • ultraviolet degradation


Reflective coatings must be kept clean and in good condition. They must not be installed over just a mopping of asphalt or over leaking, blistered, graveled, or shingled roofs. Cure cutback or emulsion a minimum of 48 hours prior to application of reflective / protective coatings.

Aluminum cutback asphalt roof coating tested to CGSB 37-GP-42M comes in two types: Type 1 non-fibrated and Type 2 fibrated, and is designed for asphalt roof surfaces.

The RoofStar Guarantee Program recommends that smooth surfaced roofs only be considered where aggregate surfacing is impractical:

  • roof slopes exceeding 1:4 (3" in 12")
  • air intake or exhaust causes concern over loose aggregate
  • proper aggregate is unavailable


Smooth surfaced roofs provide the following properties and advantages:

  • lighter weight
  • easier location and repair of damages or defects
  • lower re-roofing cost


The possible disadvantages or precautions involved in the use of smooth surfaced roofs include:

  • frequent maintenance
  • easily damaged by impact

4 Mineral Surface (Cap Sheet)

It is possible, but not popular, to use a modified bituminous mineral-surfaced cap sheet (see Low Slope Membranes). as the finished surface on a built-up roof membrane. It requires phased construction and, therefore, is not often used for built-up roofs in colder and wetter climates. Mineral-surfaced roofing provides the following properties and advantages:

  • lighter weight than aggregate
  • easier location of damages and defects for repair
  • less periodic maintenance


The possible disadvantages or precautions involved in the use of mineral surfaced roofing include:

  • higher initial cost



Protected and Modified Protected Roof Systems

Home