Flat Roof Design
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.

grey mineral torchon flat roofDesign 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.
For a conversion table showing the relationship between ratio, angles and percentage slope click here (table will show on left column of this page).

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.

black mineral torchon roof on timber constructionFully 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, PVC Decking 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, with varying degrees of success long term.

grey mineral torchon to sloping roofSemi- 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.
Leaks in the covering are hard to trace.
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. See example here
Fall (Slope) Conversion Table 
Fall Ratio
Slope angle
% Slope
1: 120
0.5 o
1: 100
0.6 o
1: 80
0.7 o
1: 60
1.0 o
1: 40
1.4 o
1: 38.2
1.5 o
1: 28.6
2.0 o
1: 19.1
3.0 o
1: 14.3
4.0 o
1: 11.4
5.0 o
1: 9.5
6.0 o
1: 8.1
7.0 o
1: 7.1
8.0 o
1: 6.3
9.0 o
1: 5.7
10.0 o


 Available on this web site for direct download is the Ruberoid "Blue Book" (Flat Roofing,  A Guide To Good Practice).
The Ruberoid "Blue Book" has  become the industry standard for the design and installation of built-up roofing and mastic asphalt.
The "Blue Book" covers all aspects of roof design including wind uplift, drainage and insulation requirements. It also includes information relating to the manufacture and installation of roofing systems, together with typical details.
First published in 1982, the current "Blue Book" has been totally updated for 2002  in accordance with revised Building Regulations and relevant changes in British and European Standards.

The book has been converted to .pdf  files, so you will need Acrobat Reader to access the information.
If you do not have Adobe Acrobat Reader on your computer, click on the logo below to load the free programme

Due to the size of data involved the book has been broken down into sections as follows;

Falls and Drainage
Thermal Design
Vapour Control Design Guide
Trapped Moisture
Movement and Membranes
Wind Attachment Design Guide
Roof Renewal
Structural Decks
Insulation Boards

Built Up Roofing
Built Up Roofing - Materials
BUR - Application Techniques
BUR - Surface Protection
BUR - Typical Specifications
BUR - Detail Design
BUR - Typical Details

Mastic Asphalt

Mastic Asphalt- Materials
MA - Asphalting Techniques
MA - Surface Protection
MA - Typical Specifications
MA - Detail Design
MA - Typical Details

Maintenance & Repair

Appendix A - U- value calculation method

References - BS & Codes of Practice

For detailed Technical Information and Specifications for fixing of Plywood as a substrate click here

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