Ventilation and insulation
Rafters or Trusses: How Pitched Roof Structures Are Built
Pitched roofs are built either as cut roofs assembled on site around purlins or as factory-made trussed rafters, and the difference decides what can be altered and what must not be touched. Compare the roof structure against the attributes that actually decide it, then book a survey. What it covers: Cut roof · Trussed rafter · Purlin.
Pitched roofs are built either as cut roofs assembled on site around purlins or as factory-made trussed rafters, and the difference decides what can be altered and what must not be touched. It takes ten seconds to tell.
What you are looking at
- Purlin
- A horizontal member supporting the rafters mid-span in a cut roof.
- Strut
- A member transferring purlin load down to a load-bearing wall.
- Collar
- A horizontal tie between rafters, resisting spread.
- Wall plate
- The timber on the wall head that the rafters bear onto.
Construction date predicts the type accurately enough to set expectations before anyone climbs up, and it decides the cost of a conversion more than any other single fact. Cutting a truss member removes capacity the rest of the frame was never designed to carry.
4 related pages cover this in more depth: roof component terminology, sagging roof ridge, velux window installation birmingham and how to measure roof pitch.
What is the difference between a cut roof and a trussed roof?
A cut roof is assembled on site from rafters, purlins, struts and collars cut to fit. A trussed roof uses factory-made triangulated frames delivered complete. Cut roofs predominate before the 1960s and trusses after.
A purlin is a horizontal member supporting rafters mid-span in a cut roof.
How do you tell which one you have?
Look into the loft. Horizontal beams running along the roof with diagonal props beneath them indicate a cut roof. A series of identical triangular frames with internal webs, leaving little clear space, indicates trussed rafters.
A strut is a member transferring purlin load down to a load-bearing wall. A collar is a horizontal tie between rafters resisting spread. Members share load and must not be cut without engineered design.
Can you cut a truss?
No, not without engineered design. Every member of a truss carries load the others depend on. Removing or notching one transfers force the remaining members were never sized for, which is why unauthorised cutting causes structural failure.
The alteration of the cut roof is can be altered with appropriate design, and offers usable void space. The distinction of the age indicator is cut roofs predominate before the 1960s, trusses after. Trussed roofs have webbed voids with little usable space.
What are purlins, struts and collars for?
Purlins support rafters part-way along their span, struts transfer purlin load down to a load-bearing wall, and collars tie opposing rafters together to resist them spreading apart at the wall plate.
- A wall plate is the timber on the wall head that rafters bear onto.
- Loft conversion implication: a trussed roof requires structural redesign; a cut roof frequently does not
- Truss members share load and cannot be cut without engineered design.
What does it mean for a loft conversion?
A cut roof often converts with modest structural work because the void is already usable. A trussed roof requires the frames to be redesigned and replaced with an engineered alternative, which changes the scope and cost substantially.
- A cut roof can be altered with an appropriate structural design.
- A collar ties rafters together and resists a spread.
- A construction type decides what a loft conversion involves.
The structure decides what the roof can become.
Why alterations start with the frame
A webbed triangular frame is a truss; horizontal purlins with diagonal struts is a cut roof. Anyone can tell from the loft hatch. Cutting a truss member removes capacity the rest of the frame was never designed to carry, and it is done more often than the structural rule allows.
Getting the structure identified
A cut roof frequently converts with modest structural work; a trussed roof needs redesign, which changes the project entirely. Construction date predicts the type accurately enough to set expectations before anyone climbs into the loft.
Cut roof, and 6 more
- cut roof
- trussed rafter
- purlin
- strut
- collar
- wall plate
- webbed void
A cut roof is built on site from rafters carried on a purlin, itself carried on struts, and it leaves usable open space in the middle — which is why a loft conversion is possible on older stock. A trussed rafter is a factory-made frame whose webs fill that space and carry the load through the whole triangle. Cutting a web to make room removes structure the design depends on, so a trussed roof cannot be altered without a structural engineer specifying the replacement.
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