Roof Truss Glossary
Apex/Peak
The upper most point of a truss.
Asymmetric Truss
A truss that has two rafters that meet at the APEX, which has a different pitch on both sides
Attic Truss/Room-in-the-Roof
A Truss forming the storey of a dwelling. Characterised by a central, habitable area free from web members. Timber sections are generally larger than a standard Truss.
Balloon Frame
Uncommon build type in the UK for timber frame where the panels are built to roof/wall plate level and the floor is then hung internally from pole plates or hangers.
Battens
These are small timber members spanning over roof trusses to help support tiles, slates and any other type of material that may be used.
Bearer
Designed to distribute loads across a number of trusses
Bearing
The part of a truss receiving structural support, usually a wallplate but can be an internal wall etc.
Birdsmouth
A notch in the underside of a loose rafter to allow a Horizontal setting at a point of support (not usually used On Truss Rafters except with a Raised Tie Truss).
Bobtail
See STUB END.
Bottom Chord/Ceiling Joist/Tie
The lower member of a Truss, normally horizontal, which carries the ceiling construction, storage loads and water tank.
Bracing
See STABILITY BRACING.
Breather paper
A man made materials that is usually attached to the outside face of an external timber frame panel that allows water vapour to pass through from the inside to the outside of a timber frame wall. It does not however allow the passing of vapour in the opposite direction.
Brickwork Lintels
Galvanised steel lintels that get fixed to the external timber frame panel and support brickwork, blockwork over the window and door openings in timber frames.
Building Designer
The person responsible for the structural stability and integrity of a building as a whole.
Camber
An upward vertical displacement built into a truss that will compensate for any anticipated deflection caused by applied loads.
Cantilever
The part of a structural member or truss that extends beyond it’s bearing.
Cavity Barrier
Either timber/wire reinforced mineral wool blanket or polythene sleeved mineral wool used to seal off the cavities into zones
Chevron Bracing
Diagonal bracing that is nailed to the truss in the plane of specified webs to add stability to a roof
Concrete Screen
An in situ flooring of cement mortar that is laid to an accurate flat surface through screeding. Screeds are laid on the structural floor and usually have no reinforcement; in a timber from situation that would be laid after the structural shell has been erected.
Connector Plate/Fastener/Nail Plate
Metal plate having integral teeth punched from the plate material. Used for joining timbers in one plane with no overlap within Truss over Joints/node points. Usually in Galvanised steel. Not usually for site application.
Jack Rafter
An infill rafter completing the roof surface in areas such as the corners of hip ends.
Dead Load
The permanent load produced by the fabric of the building
Design Load
Collective term for the loads for which the unit is designed. This takes into consideration the duration of the loads-long term, medium term, short term and very short term.
Deflection
The deformation caused by the loads.
Differential Movement
Timber being a natural resource is prone to some shrinkage across the grain, although is stable in the longitudinal section – and an allowance is made in the floor where joists have to lie across the grain. The difference between the timber frame movement and that of the external brickwork is the differential movement.
Dormer Roofs
A dormer is a roofed structure, that will often contain a window, that projects vertically beyond the plane of a pitched roof. These popular roof extensions will provide additional living space in any loft or attic conversion.
Eaves/ Heel Joint
This is a section of the roof truss where the rafter and the ceiling tie intersect which is usually where the truss is supported.
Extended Rafter/Raised Tie Truss
A Truss which is supporting at a point on the extension of the rafter, beyond the point where the bottom chord meets, creating a part sloping ceiling within the rooms below. Not only used as a decorative feature but to achieve greater overall ceiling height where wall height is restricted.
Fascia/ Barge and Soffit
This is a board that is generally timber of upvc set on edge along the eaves. This covers the rafter ends and will usually carry the gutter. Bargeboad is used on the sloping areas of the roof and are fixed in pairs along the edge of the gable to cover the roof timbers and protect them from rain. They can be ornately carved or moulded if required to create a feature. Soffit board – a horizontal sheet fixed under the eaves, concealing the rafters and the underside of roofing. This runs between the back of the fascia and the face of the outer wall.
French Heel
A heel joint where the top chord sits on the top of the bottom chord.
Fink Truss
Named after the original designer. A duopitch truss with a wed configuration within forming a letter W. The most common truss type used for dwellings.
Gable End
The end wall (parallel to the individual trusses) extended up vertically to the plane of the top chords of the trusses.
Gable Ladder
Components used to bridge across a gable end to form the roof overhang
Girder Truss
A truss comprising of two or more individual trusses fixed together and designed to carry exceptional loads, such as those imparted by other trusses.
Hanger
A metal component that is designed to provide a connection between a truss or other component and its support to benefit the structure.
Hip End
An alternative to a gable end. The end wall is finished to the same height as the adjacent walls. The roof inclines from the end wall usually at the same pitch as the main Trusses.
Pitch
The angle of the chord (usually rafter) to the horizontal measured in degrees.
Imposed Load
The load produced by occupancy and use including storage, inhabitants, moveable partitions and snow, but not wind. These can be long, medium and short term.
Insulation
This is the material that is used in walls and floors and roof space to create a warm structure to satisfy the client’s requirements and comply with part L of the building regulations. Types of insulation include timber batts, a semi-rigid unfaced slab designed to give thermal and acoustic insulation and fire resistance. This is push fitted between studs at 600mm centres and is made from natural materials and recycled glass which is generally used for external and internal panels.
Rolls
Products that are produced from fine and non-combustible glass mineral wool and generally used in horizontal applications like roof and floor areas.
Polyurathane
Expanded plastics that have excellent insulation properties. It is generally used as a rigid board for installation in sloping roofs. They can even be used in between the studs in external wall panels. It can also be sprayed in situ as expanding foam to follow complex shapes.
Warmcell
Warmcell is an insulation material made from recycled materials such as paper. This product is blown into the required area pneumatically by specialist contractors on sire from a van mounted machine. It is used in wall panels, horizontal and sloping roof areas and can result in high U values.
Span
Span over all wall plates. The distance between the outside edge of the two supporting wall plates situation on the cavity walls.
Stability Bracing
An arrangement of loose timbers installed in the roof space to provide lateral support to truss members and to the trusses.
Stub End
A truss type formed by the truncation of a normal triangular truss.
Top Chord/Rafter
The uppermost member of a truss normally carrying the roof covering and snow
Truss/Trussed Rafter
A lightweight framework, normally triangulated, spaced at intervals generally not exceeding 0.6m and made from timber members of the same thickness fastened together in one plane by metal fasteners.