A Guide to the Architecture of Roof Construction for Ecologists

by Ashley Dale

As licensed ecologists we spend a lot of our time crawling around in loft voids looking for evidence of bats or the bats themselves!

However, it is not just the bats that we are looking for. In every bat preliminary roost inspection, we have to record the architectural design and materials which are used in making the loft void to gain an insight for its suitability for bats and to support our overall justification for why it is suitable or unsuitable for roosting bats to utilise.

For other architectural terms read our Guide to Architecture blog post!

List of Components within the Loft void

BATTEN –a small timber section which is fixed across the rafters to support tiles or/and slates.

BEAM – a long timber, can be made of steel or concrete, which carry loads over a span between the supports.

BINDER – timber joist at the bases of the beams to strengthen them.

MORTIS AND TENDON JOINTS – joints that slot together and make up the structure of the loft space. They are traditionally truss timbers that have been bolted together or mortise and tenoned and then pegged.

PURLIN – a horizontal timber or steel beam which creates support for the roof rafters.

PEGGED ‘A’-FRAMED TIMBERS – Timbers that are pegged or slotted in together and in a shape of an ‘A’. They support the roof structure of the building and loft space.

RAFTER – a parallel timber or steel member, which slopes to form the roof of the building.

SARKING – a felting sheet which can be laid on top of the rafters to provide a weatherproof barrier under the tiles/slates (see loft void insulation).

STRUT – a beam positioned at an angle used to support the purlins.

TIE BEAM– a horizontal beam to reduce stress and used in roof trusses.

  • Collar ties: horizontal ties in the upper third of the loft void or roof.
  • Mid-span rafter ties: horizontally based in the middle of rafter.
  • Low rafter ties – located bottom third of the rafters run.

HANGER – a vertical beam often found connected to the strut to help support the purlins.

RIDGE BOARD– is located at the very top of the roof and is the main support of the ridge.

RIDGE COLLAR – it is the beam located horizontally at the very top of the roof structure.

DOVETAIL JOINT – A joint that slots together often in a triangular shape. It is used as a structural support within the roof and supports the main beam.

standard roof structure

Roof Structure Types

ROOF TRUSS/ TRUSSED RAFTERS – an assembly of frames structural members consisting of ceiling joists, rafters, struts and ties. They use pre-fabricated triangular frames made with small section timbers fixed together with metal plates. They are placed at regular intervals along the supporting walls of the building and are used to replace the purlin and rafter construction. They come in a variety of styles and a pre-designed to fit each type of building. They can be built with wooden or metal joints. The shape of this roof is like shaped like triangles next to each other or teeth.

BOARDED ROOF – is a roof where the rafters are covered in boarding just before the roofing felt and tiles/slates are fixed on to the structure.

POST ROOFS – are internal support struts in the loft, it props up the rafters and purlins in traditional cut and pitched roofs. It also makes up a web of braces in modern trussed roofs.

MODERN TIMBER ROOFS – made up of framed with pairs of common rafters or wooden trusses fastened together with truss connector plates.

SIMPLE RAFTER ROOF – this type of roof has rafters that has horizontal wall plates on top of the walls. The rafter tops meet at the ridge beam but can be built into another rafter to form a pair of rafters or a ‘couple’.

ROOFS WITH PURLINS – often purlin roofs need support at intervals along their length. A method of providing support is via using large sections of timber creating a truss.

KING POST ROOF – this roof structure is often used in barns, and the main king post is a vertical post in the centre of the main structure.

Kings post roof

QUEEN’S POST ROOF – the main post is horizontal in the middle of the main structure.

CROWN POST – the upper part of the roof is tied together by the high-level collars but the collars are in supported by the crown plate which holds up the centre of the roof structure in the same way as a ridge beam but at a lower level.

Crown post roof

A’-CUT ROOF – this is a traditional method of cutting the timber on site and building up the roof using rafters, ridge boards, joists and purlins the exact details are determined by the size of the roof and the size of the timbers.

‘A’-FRAME ROOF – where regular timber members (called collars) are provided further up the rafters than at eaves level. This allows the use of some of the space within the roof as living space giving a room shape with more character and reducing the height of the roof and the ‘bulk’ of the building. The rafters are tied together at collar level but tend to spread below this level.

A-frame roof

COLLAR AND PURLIN ROOF – In order to reduce the size of the rafters, purlins (timbers running along the length of the roof) were added to support the rafters at mid span and these were in turn supported by the larger principal rafters joined by collars at intervals of usually about 2 – 3 metres.

Collar and purlin roof

Loft Void Materials

SAWN TIMBER – in its rough state before being processed.

SOFT WOOD – used in sections such as the floor or ceiling joists – made up of conifer trees.

HARD WOOD – forms beams and trusses – made up of deciduous trees.

LAMINATED STRUCTURAL TIMBER – is well sourced environmentally, often used on offices and public spaces rather than houses.

ENGINEERED TIMBER – such as Masonite beams – it has the same structural strength as traditional beams and joists but is much lighter and uses less timber.

Roof Membranes in the loft void

Battens are laid parallel to the ridge to support the covering and keep the felt in place, it is there to provide a fixing point between the covering and the main structure. There are currently two types of felt used within the roof space, Bitumen 1F felt and breathable membranes (see our roof felt blog). The felts create an impervious layer to keep the roof waterproof.

References

https://www.theloftconversionshop.co.uk/glossary/
https://www.self-build.co.uk/how-convert-loft/
https://www.designingbuildings.co.uk/wiki/Domestic_roofs