Structure and support · Frames & Load-Bearing Walls
Pitched Roof Carcass System
This entry treats the pitched roof as a triangulated structure whose members restrain each other, so that the outward push at the eaves, the bracing that ties the array together and the usability of the roof space can be understood as one arrangement rather than as separate topics.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What roof carcass is
A pitched roof carcass is a set of inclined members carrying a covering down to bearings on the walls. Because those members lean, the load on them does not travel straight down: part of it appears at the feet as an outward push against the wall head. How that push is dealt with is the thing that separates one roof carcass from another, and it is decided before any question of covering or insulation arises.
Two arrangements account for most roofs. A trussed rafter array is made in a factory, triangulated within each unit and delivered complete, with the bottom member acting as both ceiling support and tie; a cut roof is framed in place from rafters, purlins, struts, binders and a ridge, and the space beneath can be shaped as the design wants. A third arrangement supports the rafters at the top on a beam, so that load is taken vertically at the ridge and the outward push does not arise in the first place.
That third arrangement is why the carcass, not the covering, decides whether a roof space can ever be occupied. A roof with a tie at ceiling level has a triangle whose bottom member cannot simply be removed for headroom; a roof carried on a ridge beam does not have one to remove. The question of what the space can become is settled in the framework, long before anyone thinks about a stair or a rooflight.
The nearest sibling is the timber platform frame system, which is a wall structure built storey by storey, each floor forming the platform for the walls above. The carcass is what spans across the top of those walls. The testable difference is the direction of the load leaving the structure: a platform frame passes weight straight down through walls in compression, while the carcass pushes the wall head outward as well as down, and something has to hold that push. The two are not a choice between arrangements: the carcass lands on the frame.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- roof structure
- trussed rafter roof
- cut roof
- traditional cut and pitch roof
- roof framing
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Give a roof its shape and carry the covering, the loads on it and its own weight down to the walls.
- Resolve the outward push produced at the feet of inclined members, by tying it or by removing it at the ridge.
- Hold the array of frames upright and square along the length of the roof so that it cannot lean over.
- Tie the roof to the walls beneath so that the two act together rather than one merely sitting on the other.
- Determine, by its arrangement, whether the space beneath the slopes can be used or only reached.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is not meaningful in this system. These roles interact as parts of one whole rather than stacking up in a sequence, so the list below is not a build-up and nothing should be read into the order it appears in.
- Primary supportRafters or trussed rafters
The inclined members following the slope from wall head to ridge. In a trussed array each is part of a triangulated unit with its own internal members; in a cut roof each is an individual member supported partway along by purlins.
- Primary supportTies, collars and ridge beams
The members that deal with the outward push, whether by tying the feet together at ceiling level, by restraining the slopes higher up, or by carrying the rafters at the ridge so that the push does not arise.
- Primary supportBracing in the plane of the roof
Diagonal and longitudinal members tying the frames into an array. Each frame is stiff within its own plane and has nothing to stop it leaning along the roof, which is what this bracing is there to resist. How far it runs along the roof, and what it is fixed to at each end, belongs to the design of the array.
- AttachmentWall plate, straps and connectors
The plate spreading the bearing along the wall head, the straps tying the roof down and back to the walls, and the connectors making up each frame. They turn a set of members resting on masonry into a roof joined to a building.
- SubstrateBattens or sarking deck
The plane fixed across the rafters to carry the covering. Running across the slope it also ties the rafters to one another, so what appears to be a support for tiles is doing work within the carcass as well.
- Cavity or voidRoof space
The volume enclosed between the slopes and the ceiling below. Its shape is a by-product of the framing arrangement, and whether it can be occupied, used for storage or only inspected follows from the members that pass through it.
- Edge and terminationEaves, verge and ridge lines
Where the carcass stops. The eaves carry the bearing and the overhang, the verge closes the end of the array, and the ridge is where opposing slopes meet, each of them a place where several members and the covering have to be resolved together.
- Edge and terminationTrimming around openings
Members framing the holes made for chimneys, rooflights and dormers. What passes through the roof interrupts rafters, and the load they were carrying has to travel around the opening on members arranged to receive it.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
The push at the feet is the organising fact of the whole arrangement. Load on a leaning rafter arrives at the wall head partly as a horizontal shove, and unless something resists it the feet spread, the ridge drops and the wall is pushed out at the top. The ceiling tie, the collar or the truss's own bottom member is what holds that shove, which is why taking out a ceiling to open up a room is a structural act and not a decorative one.
A trussed rafter is a complete triangulated unit and it is complete only while every member is intact. Cutting one to make room for a tank, a duct or a walkway does not weaken it slightly; it removes the triangulation the unit depends on, and the neighbouring frames can only take up the slack if the bracing connects them well enough to do so. The array is stronger than the sum of its frames precisely because that bracing exists.
Where a beam at the ridge carries the rafters, the logic inverts. The load goes up to the beam and down through whatever supports it at each end rather than out to the feet, so the outward push does not arise rather than being resisted. Whether an existing tied roof can be converted to that arrangement, and its ties then taken out, is a determination for a structural engineer for that roof, along with what carries the beam, what its supports bear on, and what afterwards restrains the wall heads and holds the roof down against uplift.
The wall plate and the straps make the roof and the wall one structure rather than two stacked things. The plate spreads the bearing so that the load is not concentrated on a few points of masonry, and the straps hold the roof down against uplift and tie the wall head back to the roof. The relationship runs both ways: the roof restrains the top of the wall, so a roof taken off for repair leaves a wall standing in a condition it was not designed to be in.
The battens and the covering are part of the carcass's behaviour, not merely a load on it. Running across the slope, the battens tie the rafters together and share load between them, and the weight of the covering itself is part of what the framing was arranged for and part of what holds the roof down. Changing from one covering family to another is therefore a question for the carcass rather than a change of appearance.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Primary support
These families are commonly encountered as rafters, ties, purlins and beams. Which member does what, and what it has to be, follows from the arrangement chosen for that roof and is a matter for the designer and the truss manufacturer's information.
Attachment
Punched plates, straps, hangers and gussets are commonly encountered at the joints and at the wall head. Their arrangement is part of the structural design, and a connector is not interchangeable with another because it looks similar.
Substrate
Board families are commonly encountered where the slope is decked rather than battened. A deck of this kind changes how the rafters act together, so its presence or absence is a structural matter as much as a covering one.
Protection
Treated and modified timber families are commonly encountered where members are exposed at the eaves or verge. What treatment achieves depends on the exposure and the manufacturer's documentation, and no durability outcome is stated here.
Control layer
Layers of this kind are commonly encountered draped over or fixed to the carcass, although they belong to the roof assembly rather than to the framework. How they sit relative to the members and to any void is assessed for the whole build-up by a qualified professional.
Junctions
Where this system meets others
Interfaces are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves a junction is a detail designed for the specific building — not a rule of thumb.
Bearing and restraint at the wall head
The plate spreads the roof's bearing along the masonry and the straps tie the two together. The relationship works in both directions, so the wall carries the roof while the roof restrains the top of the wall against leaning.
Read about Load-Bearing Masonry →Assembly built onto the carcass
Insulation, underlay and any ventilated void are arranged around members whose positions are already fixed. Where insulation goes relative to those members decides whether the roof space is inside or outside the insulated envelope, which is a design question rather than a framing one.
Read about Cold Pitched Roof →Insulation at rafter level
Taking insulation up the slope is what makes a roof space habitable, and it puts insulation into or around members that were sized for structure. Whether the depth available is enough for both, and what happens to any void, is settled by a professional.
Read about Warm Pitched Roof →Covering carried on the battens
The covering is fixed to battens that also tie the rafters together, and its weight is part of what the carcass was arranged around. A change of covering family is therefore referred back to the structure before it is treated as a decision about appearance.
Read about Tile and Slate Covering →Eaves and overhang
The eaves is where the members bear, where the overhang is formed and where the covering, the gutter and the wall head all arrive at once. It is the most crowded junction in the roof and the one most often resolved by more than one trade.
Read about Eaves System →Openings through the slope
A rooflight or a dormer interrupts rafters, and the load they carried has to travel around the hole. The upstand around the opening then sits on trimming members, so the position of an opening is agreed with whoever arranged the framing.
Read about Rooflight Kerb →Air movement in the roof space
The shape of the void, and whether members obstruct it, is set by the framing. Where the arrangement divides the space into compartments, each one raises its own question about the movement of air within it.
Read about Roof Void Ventilation →
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Movement
- Timber changes dimension as it takes up and gives off moisture, and a roof carcass does most of that changing in its early life. Where members are held rigidly at both ends, or where a plaster ceiling is fixed across a joint that moves, the effect shows in the finishes rather than in the structure.
- Durability
- The parts of the carcass most at risk are the ones nearest the outside air and rainwater: the feet of the rafters at the eaves, the ends of members at a verge, and anything bearing into damp masonry. Those are also the least reachable parts of the roof.
- Buildability
- A trussed array is set out and manufactured before it arrives, so what passes through the roof space has to be known in advance. A cut roof leaves those decisions later but places them with the people framing it, which is a different distribution of risk rather than a lesser one.
- Interfaces
- The eaves, the verge, the ridge, the abutment against a taller wall and any opening are where the carcass meets other systems. Almost everything difficult about a pitched roof happens at one of them and almost nothing happens in the middle of a slope.
- Maintenance and access
- A roof space is usually entered rarely and looked at less, so what is stored, walked on or fixed there tends to be decided by whoever is standing in it. Marking which members may be crossed or loaded is more useful than a general instruction to be careful.
- Moisture
- Whether a roof void carries a risk of condensation depends on the whole build-up, the conditions below and the climate, and it is assessed by a qualified professional. What belongs to this entry is that the framing sets the shape of the void being assessed.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- What resists the outward push at the feet: a tie at ceiling level, a collar, or a beam at the ridge?
- Is the roof space intended to be used, and does the framing arrangement allow for that?
- How is the array braced along the length of the roof, and is that bracing continuous?
- How is the roof tied down and tied back to the walls, and where do those fixings land?
- What passes through the roof, and are the openings framed rather than cut into a finished array?
- Which covering family has the carcass been arranged around, and what changes if that alters?
- Where does the ridge beam, if there is one, deliver its load, and what receives it?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- A ceiling is treated as a lining, when in many roofs its joists are the tie holding the feet of the rafters together.
- Trussed rafters are read as a series of separate frames, when the array depends on bracing to act as one.
- Cutting a truss member is thought of as a small alteration, although it removes the triangulation the unit relies on.
- The covering is assumed to be a free choice, when its weight is part of what the framing was arranged around.
- The roof space is treated as spare storage, although only some members are arranged to be crossed or loaded.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What is holding the feet of the rafters, and would it survive the change being considered?
- Can this roof space be used, and what would have to change in the framing for that to happen?
- Where is the bracing, and does it still work after the alteration being proposed?
- How is the roof strapped to the walls, and can those fixings be seen or confirmed?
- If an opening is formed here, what carries the load around it and what does that member bear on?
- Does the carcass suit the covering intended, and what would a heavier or lighter family mean?
- Which members in the roof space may be walked on or loaded, and which must be left alone?
What this page does not do
- No member size, arrangement or capacity is stated here; a roof carcass is designed by a qualified professional for its own building.
- Altering, notching or removing any member of a trussed rafter changes a manufactured structural unit and is not site carpentry.
- Removing a ceiling, forming an opening or changing a covering can each affect how the roof carries load and how the walls are held.
- Nothing in this entry establishes that a roof space is suitable for storage, occupation or access of any kind.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Meets these systems
Systems this one physically meets. The junction is usually where the design problem lives, so these are worth reading together.
Commonly built alongside
Systems routinely present in the same building without necessarily touching this one.
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Primary Structural Frames and Load-Bearing Wall Systems
Complete primary load paths from roof to foundation in masonry, timber, steel and concrete, together with the stability system without which none of them stands up.
Browse all frames & load-bearing walls entries →