Trussed Rafters (Roof Trusses)
What are Trussed Rafters (Roof Trusses)?
Trussed rafters are individually designed components, manufactured from quality controlled, kiln-dried, strength graded timber joined with steel nailplate fasteners engineered to provide a structural frame to support the roof fabric, ceilings and/or floors together with other features, such as dormer windows and attic roofs. Combined with other ancillary components, they are used to construct trussed rafter roofs on a wide range of building types.
Trussed rafters have proved to be an efficient, safe and economical method for supporting roofs since their introduction into the UK in 1964. They are used in around 95% of all new housing and in an increasingly large proportion of roofs for non-domestic premesis such as offices, retail outlets, hospital extensions, leisure developments etc.
With the continuing trend toward individualism in domestic house styling, let alone the reflection of this in new inner city estates, the facility to introduce variation to the standard design is vital. The provision of many character differences by designing and then constructing L returns, doglegs, hips for example, satisfies the inherent need for individuality at affordable prices.
Experienced roof designers and trussed rafter manufacturers are therefore in an ideal position to assist the architect or specifier in achieving affordable solutions throughout the building industry. Simply provide a brief sketch or description of that being considered, including alternatives, and we will do the rest. The whole roof is designed and specified using state-of-the-art computer aided design technology supplied by Wolf Systems Ltd.
Benefits of Trussed Rafters
- Suitable for a wide range of roof structures and building types
- Speed of construction allows just-in-time delivery to site
- Space saving - no on-site storage area required
- Prefabricated components mean reduced labour costs
- Up to 40% more energy efficient than traditional construction methods
- More thermally efficient than alternative materials such as aluminium, steel or concrete
- Made of timber, the only truly renewable building resource
- Low-tech manufacturing process reduces emissions and pollution
- Minimal environmental impact from installation
Room in Roof Trussed Rafters
For many years the roof space has been largely ignored, but not anymore. Room in the Roof, attic trusses, are the ideal way to increase the habitable area of a house, or improve the storage area.
Provision for creating extra rooms may be incorporated at the inital design stage for immediate use, or conversion later into living accommodation to suit the house owner's changing circumstances. No extra strengthening of the trusses will be required.
Attic trusses will provide living accomodation in the roof space; therefore, care has to be taken with respect to stairwells, rooflights and dormers. Usually these are wider than the truss spacings, but difficulties can be overcome by grouping trusses together to form compound girder trusses on either side of the openings. Common rafters and floor joists can be supported by purlins and binders between the compound trusses.
Stairwells should lie along the line of trusses to avoid cutting across them, and similarly, to avoid too many compound trusses, dormers and rooflights should be situated opposite each other.
Always inform us of any dormer windows or stairwells to be installed. Even items to be installed at a later date must still be designed for from the beginning. Trusses can not be cut at a later date. Never cut trusses!
Attic trusses for roofs over 3.7 metres in height must be made in two parts due to delivery height restrictions.
Loadings to attic trusses are usually greater than normal trusses, consequently, timber sizes are larger and spacings may need to be reduced.
And don't forget to brighten up your new room by installing VELUX® Windows.
Design
Trusses are designed in accordance with the current code of practice, which is BS 5268: Part 3, and the relevant building regulations.
Spans
We can design and manufacture trusses with a span of up to 12 metres depending on the size of any overhangs. Trusses requiring spans greater than this can be designed and are supplied as multiple trusses to be fixed together on site.
Heights
We can manufacture trusses up to 3.7 metres but if you require trusses larger than this, two-part trusses could be designed e.g. Attic or Room in Roof trusses which require large room sizes can be top hatted.
Pitches
It is more economical to standardise the range of pitches between 15 and 40 degrees, however, trusses may be supplied outside this range. Care should be taken when specifying because deflection problems may arise with e.g. very shalow pitches. The pitch also depends on the loadings e.g. tile/slate loads and snow loads.
Eaves Overhangs
Any overhang can be supplied to suit the customer's requirements, a standard overhang is around 450mm. NB. Very large overhangs may cause the trusses to be uneconomical.
Spacing
Trusses are usually spaced at 600mm, but can also be positioned at 450mm or 400mm to support heavier loads. Truss spacing also depends on the loadings, the heavier the load the less spacing between trusses there is.
Timber
We use TR26 grade softwood which is kiln dried and stress graded, and which complies with current European and British Standard BSEN519.
We are fully committed to ensuring that where felling takes place, replanting is undertaken to ensure the continuity of our raw material, together with the provision of living forests for future generations to enjoy.
Preservation
Trusses may be treated with a water-based microemulsion industrial preservative which is a dual purpose fungicidal and insecticidal formulation. Due to the fact we don't have our own treatment plant trusses will only be treated if requested.
Nailplates
We use Wolf 101 nailplates which are precision punched 1mm thick metal plates with integral teeth and are manufactured from structural grade galvanised mild steel to BS EN10326 Fe E 280 GZ275. Also available and often specified for splice joints are the Wolf 126 nailplates in 1.25mm thick galvanised mild steel.
Wolf101 & wolf126 have been tested to EN1075 and therefore comply with BS EN 14250.
They carry a unique 60 year performance warranty and are covered by certificate No.89/2290 issued by the British Board of Agreement
Trussed Rafter Roof Shapes
The table below shows some of the most common roof shapes, although the majority of roofs designed today tend to incorporate combinations of multiple roof shapes.
The Port Logan Self Build (3D image on right) for example is a combination of different roof shapes. The rear of the property, bottom left side, has a hexagonal hip end over a garden room, next is an attic roof including one dormer, together with a raised tie T-intersection with dormers, and a small hip over the front door.
Click on the 3D image to the right to open the interactive 3D model.
Both the roof layout/plan and interactive 3D model are PDF files, they can be viewed by clicking on the image.
| Roof Shape | Roof Layout/Plan | Interactive 3D Model |
|---|---|---|
Mono-pitch |
 |
 |
Duo-pitch |
 |
 |
L-return |
 |
 |
Hipped |
 |
 |
Dutch or Barn hip |
 |
 |
Gablet |
 |
 |
Mono L-return |
 |
 |
Mono L-return |
 |
 |
Overlaid hip |
 |
 |
Dormer |
 |
 |
Dogleg |
 |
 |
T-intersection |
 |
 |
Instructions for Use
Adobe Acrobat Reader is required to view PDF files, although interactive 3D PDF's will work in Acrobat Reader version 8, we do recommend Adobe Reader X to get full functionality.
If you do not wish to open the file now, right click on the download arrow and select...
- 'Save Target As...' in Internet Explorer
- 'Save Link As..' in Mozilla Firefox and Google Chrome
- 'Save Linked Content As...' in Opera
- 'Download Linked File As...' in Safari
Once the PDF has been opened click on the image to activate the 3D model, using your mouse to navigate and the scroll wheel to zoom.
Roof shape pictures (left column) © Wolf Systems Ltd and are taken from their roof truss Technical Manual.
Roof designs produced by E & H Baxendale Ltd, 3D PDF produced by Wolf Systems Ltd.
Trussed Rafter Types
Trussed rafters come in all shapes and sizes, they are specifically designed for each project and as such there is no standard off the shelf truss. Trussed rafters fall into one of three styles:-
- Duo-pitch trusses which have two rafters (slopes) meeting at an apex, this is the most common shape
- Mono-pitch trusses which have only one rafter
- Hip trusses like those used within a hipped roof, they can be pitched on one or both sides with a flat middle or they can be completely flat topped with no slopes
The configuration of a trusses internal webs and the timber sizes used is determined by the span, the pitch, and the loads applied to the truss, such as snow and wind loads, the roof covering loads, water tank loads, hoist loads etc.
These are some of the most common types of trussed rafters manufactured today, there are others, but most are just a variation of one these below. TRA Data Sheet 2 contains two pages of trussed rafter profiles.
King Post- A King Post truss is the simplest form of truss, it consists of two rafters that meet at the apex of the truss, a ceiling tie that ties the two rafters together and one centre web that connects the ceiling tie to the apex. The King Post trusses we manufacture usually span up to 3100mm.
Queen Post- A Queen Post truss is similar to the King Post truss, the only difference being that it has two additional webs connecting the rafters to the centre web at the ceiling tie. The Queen Post trusses we manufacture usually span up to 4800mm.
Fink- The Fink truss is the most common truss used today, it's webs are configured in a W shape with the centre bay most commonly being used to carry a water tank. The Fink trusses we manufacture usually span up to 9000mm.
Howe- The webs in a Howe truss are configured similar to those in the Queen Post, the only difference being that it has an additional vertical web connecting each rafter to the ceiling tie. Howe trusses are usually used as multi-ply girder trusses. The Howe trusses we manufacture usually span between 4000mm to 9000mm.
Fan- The webs in a Fan truss are configured similar to the webs in a Fink truss with the addition of an extra vertical web connecting each rafter to the ceiling tie. The Fan trusses we manufacture usually span up to 11000mm.
Double W- The Double W truss is most commonly used for large spans or to reduce the timber sizes on smaller span trusses in high load situations. The webs are configured in a WW shape with the two W's meeting at the apex. The Double W trusses we manufacture usually span up to 12000mm.
Cantilever- Cantilevered trusses are used when the ceiling tie is required to extend past the supporting wall plate, you often see this type of roof construction on stable blocks where the roof is cantilevered to provide some shelter. Although cantilevers can most commonly be found in standard fink trusses, they can also be found in bobtail, flat top, mono and attic trusses as well. Cantilevered trusses will usually have an extra web that will be positioned over the support point.
Bobtail / Stub End- A bobtail or stub end truss is a truss that has been shortened or truncated on one or both sides. It is commonly used to form the ‘Horn Detail Built Into Wall’ eaves detail. Normally the ceiling tie will be supported by the wall plates but occasionally the support point can be moved so it is supported by a web post under the rafter instead.
Raised Tie- Raised tie trusses are used when the room height is required to be higher than the wall plates. Unlike other trusses which are supported on the ceiling tie, Raised Tie trusses are supported part way up the lower end of the rafters. The webs in a standard Raised Tie truss are usually configured like those in a Queen Post truss but in higher load or girder situations they can also be configured like those in Howe or Flat Top trusses.
Scissor- Like Raised Tie trusses Scissor trusses are used when the ceiling is required to be higher than the supporting wall plates. Unlike most trusses the Scissor truss uses two ceiling ties, these are pitched with them meeting at the centre of the truss with the centre web, giving a vaulted appearance. The pitch of the rafters determines the pitch of the ceiling tie slope.
Attic / RiR (Room-in-the-Roof)- Unlike other truss types Attic trusses are designed with a room built into them, they are generally designed to carry a 1500 N/m² live load, because of this the timber sizes used in attic trusses are deeper and wider than those used in standard trusses. When transport height is a problem Attic trusses are sometimes made in two sections, the lower half containing the room will have a flat top onto which a smaller king or queen post truss will sit forming the top half of the truss.
Flat Top (Hip End)- Today Flat top trusses are most commonly used to form girder trusses in hipped roof construction, carrying incoming mono trusses, hip boards and infill timbers. They are also used when the roof design calls for a duo pitched roof with a small flat area to the top instead of a ridge. The webs in flat top trusses are most commonly configured like those in a Fan truss.
- Parallel
- Parallel trusses are generally used in more complicated roof designs as girder trusses providing support to other incoming trusses, or occasionally to create flat roofs. An example of their more advanced use would be in complicated attic roof designs for which standard attic trusses may not be suitable for. The parallel truss would be used to create the small vertical side walls in the room supporting incoming mono trusses with extended top rafters forming the pitched side of the ‘attic truss’, the missing ceiling tie between the two parallel beams would be formed by loose floor joists.
Mono 2/1- Mono trusses with a single web are common in hipped roof designs and for use over short spans, such as porches. In hipped roof designs mono trusses are usually supported by a girder truss such as a flat top, it is also common for the rafter to continue past the last vertical web.
Mono 3/2- Mono trusses with multiple webs are mainly used over larger spans or in higher load situations. It is not uncommon to find two mono trusses positioned back to back to create a full truss, especially if a duo-pitched truss would have been unsuitable for manufacture or delivery.
Trussed Rafter Eaves Details
These are the most typical eaves details used.
Standard Eaves Joint- Description coming soon.
Horn Detail Built Into Wall- Description coming soon.
French Heal- Description coming soon.
Cladding to Vertical Face- Description coming soon.
Hanger Detail- Description coming soon.
Blocked Heal- Description coming soon.
Scissor- Description coming soon.
Relief Rafter- Description coming soon.
Raised Tie- Description coming soon.
Cantilever- Description coming soon.
Here are some of the latest photos of trussed rafters from our Flickr photostream
