Detail Post: Pitched Roof Details

The following post includes excerpts from our book Understanding Architectural Details – Residential.

Introduction to Pitched 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

Pitched Roof

The majority of residential houses in the UK are constructed with pitched roofs. These pitched roofs are often constructed as a symmetrical roof with equal slopes that meet at a central ridge. Roof covering materials often determine the angle of the pitch, being the minimum slope that is necessary to prevent rain and snow ingress.

 

 

Roof Parts Labelled

 

 

A pitched roof is usually covered with a roof tile. Beneath the roof tiles, a secondary weather proof barrier is provided using a sarking membrane, usually in the form of a bituminous felt or sheet, or a proprietary reinforced plastic sheeting. The membrane is placed over the rafters with the tile battens nailed on top. At the eaves, the membrane is dressed into the gutter so that any moisture that has penetrated the tiles can run freely into the gutter.

purlin roof 2

​Eaves

 
 
The eaves of a roof can be described as the lowest courses of tiles and the timber that supports them. The eaves of a pitched roof tend to extend around 150-300mm beyond the external face of the wall, to provide protection from rain. In some cases a closed or flush eaves can be designed, which stops the rafters and ceiling joists at the face of the external wall, where a fascia board is then fixed to the ends of the joists and the rafters.
 
A closed eaves provides soffit boards with ventilation gaps or channels to allow air into the roof space, to be expelled at ridge level.

 

Insulating the roof

 

Providing sufficient roof insulation design is key to minimising heat loss through the fabric of the building. The Building Regulations specify a minimum U-value for the insulation of roofs of dwellings to be 0.13W/m²K target.

 

Cold Roof

 

The roof can be insulated in a number of ways. Insulation can be fixed between or across the ceiling joists, an economical option as the area of the ceiling is less than the slopes of the pitched roof. This is defined as a cold roof. Any water carrying service pipes, water storage cisterns in the roof will need to be insulated to prevent damage from freezing, as the roof space itself is a cold unheated space. This type of roof system requires ventilation, in order to prevent a build up of condensation in the roof space.

Cold Roof Detail Example Cold Roof Detail Example

 

Warm Roof

 

A warm roof construction is used when the roof space is used for storage or as part of the building itself. The insulation is fixed either between the rafters, or above or under the rafters in varying combinations. The advantage of this system is that the roof space will be warmed by heat rising from the rooms below.

Warm Roof Detail Example Warm Roof Detail Example

 

Warm roof and cold roof construction is also seen in flat roofs, discussed later in our flat roof detail article.

Pitched Roof Detail Examples

The following images provide examples of pitched roof details.

MR1 Cold Roof pitched roof detail

Roof Eaves Detail – Masonry cavity wall, ventilated cold roof, insulation between and over ceiling joists

MR1 roof detail 3D

Roof Eaves Detail – Masonry cavity wall, ventilated cold roof, insulation between and over ceiling joists

MR3 Warm Roof pitched roof detail

Roof eaves detail – Masonry cavity wall, warm roof, insulation between and under rafters.

MR3 pitched roof detail

Roof eaves detail – Masonry cavity wall, warm roof, insulation between and under rafters.

MR4 Warm Roof pitched roof gable detail

Roof gable detail – Masonry cavity wall, warm roof, insulation between and under rafters.

MR4 Gable end detail 3D

Roof gable detail – Masonry cavity wall, warm roof, insulation between and under rafters.

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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