Flat roofing and materials used

Flat roofs are increasing being used in building construction and as matter of specifications have a pitch of 10 degrees   or less. These pitch angles ensure effective drainage of rain water, something that would be difficult if the roofs were to be flat like an artificial football pitch.

Embed from Getty Images

A random sampling shows that, flat roofs are often used on domestic building extensions and commercial buildings like shopping malls(see above image). So this begs the question: why are more people and entities deploying   flat as opposed to a pitched roofs? Well  flat  roofing   is  cheaper, easy  to maintain  and  may provide  extra  space  for  (a) erecting tents  for events  like parties,  (b)swimming  pools (c) water storage Tanks (d) car  parking and  many more. On the flip-side, these roofs have lower lifespans, loose more heat through radiation to the night air, and are less waterproof. But these are issues that can be fixed with superior workmanship.

The structure of flat roof has four parts, namely

  • Roof deck-This provide   a  base  for  the  waterproof membrane
  • Thermal insulation layer-This work to reduce heat loss through the roof.
  • Load bearing structure-This transfer the weight of the roof and wind pressure acting on the roof to the load bearing walls. Its constructed using timber, concrete   or structural steel.
  • Waterproof membrane-This prevent water from percolating through the roof structure onto the ceiling board giving them unsightly stained patterns.

This multi-layer structure forms an impermeable waterproof roof envelope that protect people and equipment that occupy these building   from the weather elements.

The materials used in flat roofing are (1) built-up bitumen sheets (2) Mastic asphalt (3) single ply-plastic membranes and (4) liquid coating.

(1) Built-up bitumen sheets

Bitumen is made from the non-volatile part of crude oil. The Bitumen sheets are made by bonding together two or more membranes of bitumen by hot bitumen. Then the  sheets  are coated  with  sand  to  produce  the finish  according  to  specifications .There  different  type of  bitumen sheets, but the class  3  and  5  are preferred  for built-up  bitumen  sheets  roofing  system because they  are very strong and  durable. These  may  be used  on roof structure made of (a)reinforced  concrete (b)Plywood (c)Timber and (d)  Galvanized  Steel or Aluminum  among others .The  insulation material  may be cork, glass  wool or  polyurethane  ,to  name a few  examples.

The  bitumen  sheets  are  fixed  on the roof  by heating the  bonding bitumen to  between 200 and  250 degrees Celsius, then pouring it  in front of the  sheets as its unfolded   effectively  ticking the sheet and the substrate  together  permanently.

The  bitumen sheet  can be  covered  with  copper foil  0.08 in thickness to  give the  sheets  a more appealing look  and higher  durability.

Built up bitumen sheets  may  be  used  on three types  of  flat  roofs, namely cold deck roofs, warm deck roofs  and  inverted  roofs.

Cold deck roofs

Bitumen layer/waterproof membrane
Roof deck
Load bearing  structure
Thermal insulation sheet/layer
Ceiling board

This roof (see a simple structure above) has the waterproof layer of bitumen being fixed directly   on the roof deck made of plywood that must be under continuous support of a timber load bearing structure. A thermal insulation material is then layered below the load bearing roof structure in way that creates a space between the roof deck and the thermal insulation layer. Then finally the ceiling is fixed on the roof.

Warm deck roofs

This roof has  the waterproof membrane fixed on thermal  insulation layer  that is  under continuous  support  of the plywood roof deck   which in turn is  supported by   the  roof  load  bearing   structure. This is then under-layered with ceiling board but with a pace left between the plywood deck and the ceiling board. This ensures that roof load bearing structure and the roof deck is insulated from extreme temperature swings which may cause damage to the roof system. The  top  layer  usual  need  additional    mechanical  fixing in order to withstand  strong  winds  .Warm deck  roof  technique  is applied  mostly   when constructing  lightweight roofs.

Below is an image of a timber load bearing structure  for a flat roof.

Embed from Getty Images

Inverted roofs

Here  the roof  load bearing structure supports the waterproof  membrane , the thermal  insulation sheets  and  the  three layers  are protected  by  an externally    fixed  insulation  of ballast, gravel   or paving slab. This  ensures that the whole  roof  is insulated  from extreme temperature  swings , the  damage from  maintenance traffic  and  ultra-violent radiation from the  sun. However  this  roof  has  a lot of  dead weight  and  it’s  also  not easy to  locate the exact source of leaks. Inverted roof are often used to construct heavyweight, high performance roofing system.


It is manufactured by blending bitumen with limestone powder and fine limestone aggregates. Mastic Asphalt   is delivered   as blocks for melting on site before fixing it on a flat roof or pitched roof.

Mastic asphalt is brittle and must be supported by either (1) Plywood (2) Timber or (3) Profiled steel decks. The material is then laid to 20mm thickness   in two layers. Sands  is then rubbed into  the  top layer  to break up the skin of the  material  .This  material  can be used on inverted  roof  structures.


It rely on a single waterproof membrane to protect the roof and as such requires highly skilled workmanship by the installer. A steel structure support the vapor barrier sheet, Thermal insulation sheets and all these is covered externally by a single waterproof membrane. The membrane is held on the roof by mechanical fastening. The membrane is made of modified bitumen that is 5 mm in thickness. The membranes are also made of other alternative materials.


Liquid waterproof coatings may be installed on new roof or as a part of repair and maintenance of an existing roof. The coatings are manufactured from bitumen or polymers. They are often used when the source of water leak is difficult to locate or   when a new roof on an existing building is not feasible because of chaos it may create.

©Copyright 2017 eastafricaminihardware.com


  1. Material for Architects and Builders by Arthur  Lyons
  2. The construction of houses by Duncan Marshall and Derek Worthing

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