Building Accessories
Roof Extensions for Metal Buildings: Eave and Gable
Definition: A roof extension is an overhang that continues the main roof sheeting and slope beyond the sidewall or endwall of a pre-engineered building. Eave extensions are framed off the main rafters, while gable extensions use extended purlins and edge members to project past the endwall.
Key facts
- Eave extensions keep the same roof slope as the main roof.
- Gable extensions are usually supported by extended purlins and an edge angle or channel.
- Wind uplift on the underside often governs the design.
- Gutters move to the outer edge of an eave extension.
- Soffit liners improve appearance and can protect against birds.
Role in the structure
The roof extension shelters walls, doors and windows from sun and rain and protects wall panels from water running down the face. It adds load to the main rafters or purlins, and its open underside makes it sensitive to wind uplift. Extensions also change the building appearance and are often combined with soffit liners and fascias to create a finished architectural edge.
Eave roof extensions
An eave extension is framed with short extension rafters bolted to the main frame at the eave connection, or with the main rafter itself continued beyond the column. Purlins run over the extension and the roof sheeting continues to the outer edge. The gutter and eave trim are relocated to the edge of the extension.
Because the extension continues the roof plane, the roof slope and the eave height determine the clearance at the outer edge. The project engineer includes the extension load and moment in the main frame design.
Gable roof extensions
A gable extension projects the roof beyond the endwall. Purlins are extended past the endwall rafter and tied with an edge member, and the roof sheeting and rake trim continue to the new edge. Projections at the gable are generally kept smaller than at the eave because the purlins act as cantilevers.
Wider gable projections may require outriggers or heavier purlins. The connection of the purlins to the endwall rafter, and bracing of the edge member, must resist both downward loads and uplift.
Soffits, drainage and appearance
Soffit liner panels can be fixed under the extension to conceal purlins and framing. A soffit trim at the wall line closes the junction with the wall panels. Vents in the soffit can contribute to roof space ventilation where required.
Roof extensions on all four sides create a strong horizontal line and deep shade, which suits warm climates and commercial buildings. They should be coordinated with gutters, downspouts and any canopy so that drainage paths remain clear.
Materials & coatings
- Built-up or hot-rolled extension rafters and connection plates.
- Cold-formed purlins and edge channels or angles, galvanized or primed.
- Profiled roof sheeting continuing from the main roof.
- Soffit liner panels, eave and rake trims, and relocated gutters.
Design considerations
- Projection width at the eave and at the gable.
- Wind uplift on the underside and pressure on the sheeting edge.
- Snow loads and drift at the extension edge in cold regions.
- Effect on main rafter and purlin design.
- Clearance below the extension and gutter position.
- Soffit finish and ventilation requirements.
Fabrication
- Extension rafters are fabricated with end plates to match the main frame.
- Purlins are cut longer at the gable and punched for edge members.
- Soffit liners and trims are formed to suit the extension width.
Erection
- Connect extension rafters to the frame and align them with the main roof plane.
- Install extended purlins and edge members before sheeting.
- Fix roof sheeting with the fastener pattern specified for edge zones.
- Install soffit liner, trims and gutters last.
Inspection & quality
- Check the extension edge is straight and in plane with the roof.
- Verify fasteners at edge zones match the design pattern.
- Inspect trims and soffits for secure fixing.
- Confirm gutters at the outer edge drain freely.
What to include in your RFQ
- Location and width of eave and gable extensions.
- Whether soffit liners are required and their profile and color.
- Gutter and downspout arrangement at the extended edge.
- Any fascia system to be combined with the extension.
- Wind and snow exposure of the site.
3D model & IFC of this component
Rotate the model, isolate the component and download its IFC4 file for your BIM coordination model.
IFC entity: IfcBeam + IfcMember + IfcCovering. Illustrative geometry for coordination and learning. Final member sizes come from the project design.
Frequently Asked Questions
Why are roof extensions used on steel buildings?
They shade walls and openings, keep rain off the wall face and improve appearance. They are popular on showrooms, offices and buildings in hot or wet climates.
Do roof extensions increase the frame size?
They add load and moment to the frame, which may increase member sizes slightly. The engineer accounts for this in the design.
Can roof extensions have a soffit?
Yes. A soffit liner fixed under the purlins gives a clean finish and conceals framing.
Engineering content is general guidance. Every World Steel building is designed by qualified engineers to the code and loads of the project.
