Premier Mastic Asphalt

Protected Membrane Roof

Consideration should be given to this form of construction especially where high thermal insulation is required.

The basic difference between the Protected Membrane Roof (also known as the Inverted Roof) and traditional flat roof construction is that the insulation is located above and not below the #Asphalt membrane.

The system has significant advantages:

  • · The insulation acts as a protective layer to the asphalt waterproof covering.
  • · The asphalt and structure are protected against extremes of thermal change during winter and summer periods, giving considerable thermal stability to the roofdeck system
  • · It is the ideal solution to the control of condensation problems.
  • · No separate vapour control layer is required.
  • · Drainage falls may be omitted.
  • · The system forms an ideal specification for insulated balconies and rooftop terraces.

REQUIREMENTS FOR THERMAL INSULATION

Extruded polystyrene insulation boards are used in the Protected Membrane Roof system, loose laid on a geotextile isolating layer and ballasted in accordance with manufacturer’s instruction.

When calculating board thickness to achieve a specific U-value, it is necessary to allow for loss of efficiency once the board is installed due to the effect of rainwater draining below the insulation.

Manufacturer’s take this factor into account by recommending a 20% increase in board thickness.

This is allowed for in the table below.

Deck

U-Value (W/m2k)

 

0.45

0.35

0.25

150mm in situ cast

slab and screed

75mm

100mm

150mm

100mm precast

lightweight

concrete units

50mm

75mm

140mm

fig145
fig146

SURFACE PROTECTION

Protection for insulation in a traditional protected membrane roof is provided by minimum 50mm thick paving slabs or 20 to 40mm washed rounded pebbles. The pebble layer should be at least 50mm deep and a geotextile isolating membrane should be laid above the insulation.

LIGHTWEIGHT SYSTEMS

Extruded polystyrene boards which incorporate a cementitious topping are used in lightweight protected membrane systems. The use of these boards represents a considerable weight saving over conventionally fully loaded systems.

fig147

ROOF DECKS WITHOUT DRAINAGE FALLS

Where the substrate does not have a minimum finished fall of 1 in 80, 30mm three coat mastic asphalt should be specified for the horizontal areas