Primary Framing
Rafters in Pre-Engineered Building Frames
Definition: A rafter is the sloping beam of a rigid frame that spans from the column knee to the ridge or to an interior column. It supports the purlins and roof, and resists bending, shear and axial force from gravity, wind and seismic loads.
Key facts
- Rafters are usually built-up I-sections, deepest at the knee and shallower toward mid-span.
- Long rafters are shipped in segments and joined with bolted end-plate splices.
- Roof slope commonly ranges from shallow, about 1:20, to steeper pitches such as 1:10 or more.
- Purlins bear on the top flange and brace it; flange braces restrain the bottom flange.
- Rafters may support collateral loads such as ducts, sprinklers, lighting and solar arrays.
Role in the structure
Purlins deliver roof loads to the rafter at regular points. The rafter carries those loads in bending to the knee connection, where they pass into the column, and to the ridge, where the opposite rafter balances them. Under wind uplift the bending reverses, so the bottom flange can be in compression and must be braced through flange braces from the purlins.
How does a rafter carry roof loads?
Gravity loads from roofing, purlins, collateral services and snow or roof live load bend the rafter between its supports. Moments are highest at the knee and lower toward the ridge, so the rafter is tapered to suit.
Wind uplift and internal pressure reverse the loading. The engineer checks both cases, including the compression in the lower flange, and sets flange brace positions to keep that flange stable.
Why are rafters spliced?
Rafter lengths for wide buildings exceed what can be shipped as single pieces. Splices are located where moments are manageable and where segment lengths fit standard containers or trailers.
Splices are normally bolted end-plate connections, so assembly on site requires only bolting. The splice location, plate thickness and bolt pattern are part of the frame design.
What affects rafter depth and weight?
Span, bay spacing, roof loads, deflection limits and roof slope all influence the rafter. Steeper slopes can reduce rafter depth because the frame acts more like an arch, while very flat slopes increase bending demand.
Interior columns in a multi-span frame shorten the rafter spans and reduce depth considerably, at the cost of columns inside the building. Owners should discuss which trade-off suits the intended use.
Materials & coatings
- Built-up plate members in structural steel grades selected to the project code.
- Hot-rolled beams for short spans, canopies or endwall rafters.
- Primer, paint systems or galvanizing to suit the environment.
Design considerations
- Roof live, snow, drift, ponding and collateral loads.
- Wind uplift and stability of the bottom flange.
- Vertical deflection limits suited to roofing and ceilings.
- Splice locations matched to transport length.
- Roof slope and drainage requirements.
- Point loads from hung equipment or solar systems.
Fabrication
- Web plates are cut to the tapered profile and welded to flanges.
- End plates at the knee, splices and ridge are welded square to the design geometry.
- Purlin clips or holes for purlins and flange braces are added to the shop drawings.
- Segments are trial-checked for fit where the quality plan requires it.
Erection
- Assemble rafter segments on the ground where space allows, then lift as one piece.
- Use spreader bars or multiple pick points to avoid bending the rafter during lifting.
- Connect knee and ridge plates and install purlins promptly for stability.
- Fit flange braces before removing temporary supports.
Inspection & quality
- Check splice plate contact and bolt installation.
- Confirm ridge alignment and roof slope after erection.
- Verify purlin clip spacing and flange brace locations.
- Inspect for lifting damage or distortion.
What to include in your RFQ
- Building width, roof slope and ridge location.
- Collateral and suspended loads, including future services.
- Snow and drift conditions at roof steps or parapets.
- Required clear height under the rafter.
- Roof system type, as it affects purlin spacing and loads.
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. Illustrative geometry for coordination and learning. Final member sizes come from the project design.
Frequently Asked Questions
What is the most common roof slope for PEB rafters?
Shallow slopes such as 1:10 or 1:20 are common because they reduce wall height and cladding area. The required slope also depends on the roof system and drainage, so the engineer confirms it.
Can equipment be hung from rafters?
Yes, if the loads are declared at design stage. Unplanned loads added later should be checked by an engineer before installation.
Why does my building have rafter splices in different places?
Splice locations depend on moments, shipping limits and erection plans. They are optimized for each frame and need not be symmetric.
Engineering content is general guidance. Every World Steel building is designed by qualified engineers to the code and loads of the project.
