Building components · Frame Members
Jack Rafter
Explains the jack rafter as the member created by hip and valley geometry, and why its bearing at the diagonal matters.
Educational reference entry. What this element has to do, and whether any particular arrangement of it is adequate, depends on the whole assembly, the loading, the exposure, local requirements, the manufacturer's instructions and qualified professional design.
Overview
What a jack rafter is
A jack rafter is a rafter that does not reach the ridge. Where a hip or valley crosses a roof slope, the rafters on that part of the slope are cut short and frame into the diagonal member instead, each one a different length from its neighbour.
They exist entirely because of roof geometry. A simple gable roof has none; every hipped or valleyed roof has many.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a common rafter. A common rafter runs the full slope from plate to ridge; a jack rafter is interrupted by a hip or valley.
- It is not a hip or valley rafter. Those are the diagonal members; jack rafters are what frames into them.
- It is not a noggin or a trimmer, both of which run between members rather than up the slope.
Terminology
Common names and aliases
These names describe the same element. The library keeps one entry per component so that a trade term or a regional name never becomes a second, thinner page.
- jack rafters
- cripple rafter
- shortened rafter
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- The connection into the hip or valley, which is where its upper end is supported.
- The hip or valley member itself, since every jack rafter delivers its load there.
- Its bearing at the wall plate, which is the same as any rafter's.
- Roof geometry, which determines the compound angle at which it meets the diagonal.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Every jack rafter loads the hip or valley it frames into, which is why those diagonal members carry so much more than a common rafter.
Each jack rafter is a different length with compound-angled cuts, which is what makes hipped roofs more demanding to cut than gabled ones.
A poor fit at the diagonal means the load is not delivered as assumed, and the fit is hard to inspect once the roof is covered.
Materials
Commonly formed from
Material families this element is commonly realised in. This states common practice only — it is not a statement that any of these materials suits this component in a particular building, which depends on the whole assembly and on qualified review.
Assemblies
Building systems this element is met in
The role this component fills inside each assembly. A role describes what the element does there — it is not a statement that this component suits that system in any particular building.
Junctions
Interfaces this element takes part in
Junctions where the condition between this element and another governs how the construction behaves. These name the junction and the subjects it raises; they are not details, arrangements or approved ways of building.
Jack Rafter to Hip
Where shortened rafters frame into a hip, delivering the load of two roof slopes onto one diagonal member.
Raises: Structural transfer · Buildability · Weathering
Jack Rafter to Valley
Where shortened rafters frame into a valley, beneath the line at which two roof slopes concentrate their water.
Raises: Structural transfer · Moisture · Weathering · Buildability
Where it is met
Application contexts
Building contexts where this element is commonly encountered. Association is not recommendation: appearing here does not mean the component suits every instance of that context, or any particular one.
- Pitched Roof
Related elements
Neighbouring components
Typed relationships to other elements. There is deliberately no generic “related” category: an edge that cannot be classified does not exist.
Clarifications
Commonly misunderstood
- That jack rafters are minor members. They are the means by which most of a hipped roof's load reaches the hip.
- That they can be omitted or spaced differently from the common rafters. They continue the same spacing and carry the same loading.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- How are the jack rafters connected into the hip or valley?
- Does the hip or valley member account for the load arriving from every jack rafter?
- Is the spacing continuous with the common rafters across the slope?
- Where openings interrupt jack rafters, how is that trimmed?
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this element involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this element comes up.
What this entry does not do
- No section, spacing, angle or connection is stated here.
- Roof carpentry geometry is a design and workmanship matter; this entry describes what the member is, not how to cut one.
Structural Frame Members
The columns, beams, joists, rafters, plates and bracing that make up a load-carrying frame, identified by the position each occupies.
Browse all frame members entries →