The following post includes excerpts from our book Understanding Architectural Details – Residential.
Introduction to Flat Roof Details
The roof is a key element of the building structure, providing protection from the elements and has a vital role in reducing heat loss from the building. Roofs tend to be either pitched or flat, and typically in residential construction, timber is the most common material used.
The Building Regulations in the UK provide guidance on roof design. You can find the Building Regulations here.
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Functional Requirements:
The functional requirements of the roof are:
• Strength and stability
• Protection from the weather
• Durability
• Resistance to passage of sound
• Thermal insulation
• Resistance to air leakage
• Fire safety
• Security
• Aesthetics
Flat Roof Construction for Residential
A roof is considered to be flat when it is of a slope of 1-5o to the horizontal. Although flat roofs are cheaper to build than pitched roofs, they have a shorter life span.
A flat roof usually consists of the following:
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Waterproof membrane to prevent water penetrating the structure and interior of the building
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Roof deck, providing the base for the waterproof membrane, and in some cases the insulation
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Thermal insulation
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Load bearing or primary structure, usually constructed in timber in a residential construction.
The construction of the flat roof is similar to that of the timber upper floor. Joists are placed on edge, spaced at 400-600mm centres, supported by external and internal load bearing walls. TRADA and Approved Document Part A give guidance on sizes of joists for flat roofs relating to spans and loading. Strutting is fixed between the joists to provide lateral restraint.
Depending on the roof build up, a roof deck, is then fixed to the joists over firring strips, to provide the fall of the roof. The roof deck material is usually chipboard or plywood. The firring strips allow the roof to have the slope required to enable water run off to the rain water outlets. An alternative to this method sees insulation boards made into shallow wedge sections to provide the fall.
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Insulating the roof
Cold Roof/Deck
Warm Roof/Deck (Sandwich roof)
Inverted Roof
Flat Roof Detail Examples
The following images provide examples of flat roof details.
Flat Roof Detail – Masonry cavity wall, flat roof, insulation above and between joists
Flat Roof Detail – Masonry cavity wall, flat roof, insulation above and between joists
Parapet Detail – Masonry cavity wall, flat roof with parapet, insulation above joists
Parapet Detail – Masonry cavity wall, flat roof with parapet, insulation above joists
Timber Frame Flat Roof Detail
Timber Frame Flat Roof Detail
Notes from the Building Regulations on Roofs
General Requirements – Roofs
The following information is a partial list of requirements from the Building Regulations Approved Documents – for full and detailed explanations and requirements please consult the full publications.
The building shall be constructed so that the combined dead, imposed and wind loads are sustained and transmitted by it to the ground.
(Approved Doc A1)
Precautions against moisture
The roof of the building shall be resistant to the penetration of moisture from rain or snow to the inside of the building.
All floors next to the ground, walls and roof shall not be damaged by moisture from the ground, rain or snow and shall not carry that moisture to any part of the building that it would damage.
(Approved Doc C2)
Rainwater
Rainwater from roofs shall be carried away from
the surface either by a drainage system or by other means.
The rainwater drainage system shall carry the flow of rainwater form the roof to an outfall.
(Approved Doc H3)
Fire precautions
As a fire precaution, all materials used for internal linings of a building should have a low rate of surface flame spread and (in some cases) a low rate of heat release.
(Approved Doc B2)
External fire spread
The roof shall be constructed so that the risk of spread of flame and fire penetration from an external fire source is restricted.
The risk of a fire spreading from the building to a building beyond the boundary should be limited.
Internal fire spread
Ideally the building should be subdivided by elements of fire resisting construction into compartments.
All openings in fire separating elements shall be suitably protected in order to maintain the integrity of the continuity of the fire separation.
Any hidden voids in the construction shall be sealed and sub-divided to inhibit the unseen spread of fire and products of combustion, in order to reduce the risk of structural failure, and the spread of fire.
(Approved Doc B3)
Ventilation
There shall be adequate means of ventilation provided for people in the building.
(Approved Doc F)
Conservation of fuel and power
Reasonable provision shall be made for the conservation of fuel and power in buildings by:
* limiting heat gains and losses through thermal
elements and other parts of the building fabric,
* from pipes ducts and vessels used for space
heating, space cooling and hot water services.
* providing fixed building services which are
energy efficient and have effective controls and are commissioned by testing and adjusting as necessary to ensure they use no more fuel and power than is reasonable in the circumstances.
(Approved Doc L)
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