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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.

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:

Waterproof membrane to prevent water penetrating the structure and interior of the building
Roof deck, providing the base for the waterproof membrane, and in some cases the insulation
Thermal insulation
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.

Insulating the roof

Similar to the pitched roof, a flat roof can be insulated as a cold or a warm deck. The position of the insulation dictates which type it is.

Cold Roof/Deck

A cold roof system sees the insulation placed immediately above the ceiling between the joists, with ventilation space above the insulation. The disadvantage of this method is wasteful cutting of insulation to install between the joists, labour required to install the material and increased electrical cables required to prevent overheating, that run within the insulation. It is also difficult to provide suitable ventilation of the void above the insulation to prevent condensation. This roof system is rarely used these days.

Warm Roof/Deck (Sandwich roof)

Warm roof systems benefit from the insulation sitting above the deck. This enables the temperature of the structure and the deck to be kept close to the temperature of the inside of the building (hence warm). The insulation is tapered to provide a fall to the roof to enable water to run to the water outlets.

With the warm roof deck, there is less likelihood of condensation forming in the warm roof space, so ventilation is not required. A vapour control layer is still installed to minimise any moisture movement. This system is probably the most common arrangement for residential flat roofs.

The disadvantage of this roof system is that the insulation is directly under the roof covering, so the covering suffers considerable temperature fluctuations. An inverted roof system avoids this issue.

Inverted Roof

The inverted roof puts the insulation above the roof covering. The insulation is then protected with a layer of chippings or concrete paving. This system is often seen in more commercial projects.

Both of the warm roof options prevent wasteful cutting of insulation and decrease installation time, therefore labour costs.

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

Parapet Detail – Masonry cavity wall, flat roof with parapet, insulation above joists

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)