Flat roofs represent the most efficient use of urban space, offer architectural freedom, control of the down flow of water to the drainage system and energy savings.
Roofs of very low slope are usually selected for one or more of three reasons; aesthetic design, provision of a promenade roof /deck and ease of covering for structures of complex plan.
It is generally accepted as good practice for “flat roofs” to be designed to clear surface water as rapidly as possible and it would be exceptional nowadays for a flat roof to be designed without falls.

Design of Falls

Flat roofs should be constructed to a minimum fall (slope) of 1 in 80. To achieve this the designer needs to adapt a design fall which will allow for deflections and inaccuracies in construction.
Some designers arbitrarily double the finished fall and adopt 1 in 40 as the design fall, assuming that this will always produce a finished fall of at least 1 in 80.
The fall is most commonly expressed as a ratio, such as 1 in 80, or as an angle, although it is sometimes convenient to describe it in terms of a percentage slope where by definition 1 in 100 is 1%. This is convenient for calculation as it expresses the fall in centimetres per metre run.

Attachment of Roofing Membranes to the Substrate.

Note: Exterior timber constructed decks are susceptible to structural movement due to temperature and moisture content fluctuations, which could cause plywood sheathing to deflect. Ridging and uneven surfaces may occur on the deck, however these occurances are not generally attributed to improper membrane installation, rather a result of the membrane attachment method chosen.

There are three methods of attachment
Independent – Fully Adhered – Semi-independent


Independent applications are simply laid on top of the supporting surface; indeed, an “independent layer” is often laid dry on the laying surface first so that the membrane covering will not adhere itself under the heat of the sun. A 100g/m2 glass fibre “veil” performs this task as well as fibre backed base sheet membranes with open non woven polyester on the lower side.
Independent applications cannot be executed on roofs where the slope exceeds 5% and the membrane covering must be ballasted with gravel protection or pavers, which prevent the wind from getting underneath and lifting the membrane off the substrate. Independent applications are most commonly used where concrete slabs or concrete precast panels are used and are able to support the heavy weight of the roof ballast.
The membrane covering is independent from the movements of the supporting surface below and cracking of the membrane is very rare, however leaks are difficult to trace.
IRMA (inverted roof membrane assemblies) are often applied by this method and BSS Mastic Asphalt Roofing can be considered a self-ballasted “independent” roof membrane system.

  • Quicker to apply
  • Less sensitive to cracking of the substrate
  • Vapour is diffused without creating local areas of pressure


  • Less reistant to foot trafic and shock
  • Only suitable for slopes less than 5 %
  • Requires heavy protection
  • Difficult to find leaks
  • Free to contract (shrink) with heat and cold

Fully Adhered

The covering is completely adhered to the laying surface. The surface needs to be very stable with minimum flex, while the covering must be both resistant and elastic (resilient) to cope with any substrate movements.
The fully adhered system is usually used on lightweight construction (i.e. plywood, cement sheet) , on sloping roofs over 5% or where heavy ballast cannot be used.
Leaks in the covering are generally easier to trace or isolate, and the quantity of water, which may enter the building during a leakage, will be modest.
In fully adhered systems Bridging Strips are often used, where they are laid over edging lines between pre fabricated panels / sheets and adhered to one side of the join only, before the membrane covering is applied. Bridging strips help counter the effects of differential movement between the panels by distributing the force of the expansion/ contraction cycle of the joint over an area as large as the width of the strip itself and not directly onto the membrane. Bridging strips may be made from the main membrane covering material itself, depending on the type. See example here
Butynol or Butyl Rubber, Acrylic Fibreglass, Dec-k-ing PVC, Sarnafil PVC and Torchon membranes are all considered capable fully adhered systems, and each type of membrane handles the movement at the sheet joints by varying methods.

  • Better resistant to foot traffic and shocks
  • Applicable on any slope
  • Does not require heavy balast
  • Resists wind uplift well
  • Easier to trace leaks
  • Better transmission of thermal stresses to laying surface
  • Membrane more stable in cold and hot contractions


  • Application takes longer
  • Greater sensitivity to substrate cracking
  • Generally costs more
  • Blisters more easily

Semi- Independent ( Partial Bonding)

When the laying surface is not sufficiently stable and expected to create stress to the membrane or may be damp and therefore creating blisters under the adhered membrane covering, the semi-independent system should be used.
This is a compromise between the first two systems and partially adheres the membrane covering at particular points in a variety of ways:
-adhesion through perforated vent sheet underlays.
-strips or areas created by partial application of adhesives (e.g. trickle application) or in the case of Torchon by partial flame bonding of the lower side of the membrane.
-mechanical fixing on timber or in special cases concrete screeds, the underlay sheet is nailed/screwed onto the substrate then the cap sheet is applied on top.

The most common use of semi-independent systems is in roof overlays of existing failed membranes, and in re roofing, where the original membrane is totally removed and the new membrane is applied to the existing timber sarking boards.

  • Does not require heavy protection
  • Vapour can be diffused


  • Only suitable for slopes less than 20%
  • Leaks are difficult to trace

Fall (slope) Conversion Table

Fall ratio angle (degrees) slope (%)
1:120 0.5 0.8
1:100 0.6 1.0
1:80 0.7 1.3
1:60 1.0 1.7
1:40 1.4 2.5
1:38.2 1.5 2.6
1:28.6 2.0 3.5
1:19.1 3.0 5.2
1:14.3 4.0 7.0
1:11.4 5.0 8.7
1:9.5 6.0 10.5
1:8.1 7.0 12.3
1:7.1 8.0 14.1
1:6.3 9.0 15.8
1:5.7 10.0 17.6

Leave a Reply

Please log in using one of these methods to post your comment:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s