What is a sprocket (roofing)?
A sprocket is a short length of timber fixed to the lower end of a rafter to extend the roof slope beyond the external wall, creating an eaves overhang. Sprockets are used when the rafters themselves are not long enough to project past the wall.
How does a sprocket work?
In a standard roof construction, the rafters rest on the wall plate. If you want an eaves overhang without extending the full rafters, you can use sprockets:
- The sprocket is fixed to the underside or side of the rafter near the wall plate
- The short piece projects beyond the wall
- Roof battens or boarding are carried on the sprockets to support the roof covering
A sprocket also has the effect of reducing the roof pitch slightly at the eaves, giving the roof line a gentle kick — a characteristic feature of traditional buildings.
Application
Sprockets are used in:
- Eaves overhangs — when the rafters are not long enough to project past the wall
- Renovation work — when adding an overhang to an existing roof
- Purlin roofs — where the purlins sit on the wall and sprockets extend the roof surface
- Sheds and outbuildings — an overhang protects the walls from rain
Sprocket vs. extended rafter
| Method | Advantage | Disadvantage |
|---|---|---|
| Extended rafter | Stronger, one piece | Longer timbers needed |
| Sprocket | Shorter timber, flexible | Additional joint needed |
Fixing
Sprockets must be securely fixed, because the overhang catches wind loads — uplift forces can pull the sprocket upward.
Related terms
- Overhang
- Rafter
- Wall plate
- Roof structure
- Purlin
Learn more about building terms in our knowledge base at fredsdiyplans.com.