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.
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|
|Load bearing structure|
|Thermal insulation sheet/layer|
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.
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.
(3) SINGLE-PLY ROOFING SYSTEMS
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.
(4) LIQUID COATING
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.
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- Material for Architects and Builders by Arthur Lyons
- The construction of houses by Duncan Marshall and Derek Worthing