Roof frame design – rafter, purlin or collar roof?

The roof frame carries the entire roof and determines how you can use the space beneath it. This guide explains the three basic types, typical roof pitches and timber sizes, the most important joints — and the question of when you need to bring in a structural engineer.

The three basic types: rafter, purlin and collar roofs

Almost every traditional roof frame belongs to one of three families. Which one suits your project depends mainly on the span of the building, how you want to use the space and the roof pitch you have in mind.

Rafter roof: the structural triangle

In a rafter roof (a close-couple roof), two opposing rafters form a rigid triangle together with the ceiling joists. The rafters brace against each other at the ridge, and the horizontal thrust at their feet is taken up by the ceiling – you need neither purlins nor internal supports. This gives a completely clear roof space, but it only works on sufficiently steep roofs (sensible from about 30° upwards) and spans up to around 8 m. Large roof windows or dormers are difficult, because every break in a pair of rafters disrupts the load-bearing system.

Collar roof: a rafter roof with added support

The collar roof is a rafter roof with an extra horizontal member, the collar beam. It shortens the unsupported length of the rafters and makes spans of roughly 8 to 14 m possible. The collar usually sits in the middle third of the rafter length – often positioned so that 2.20 to 2.30 m of headroom remains beneath it. A handy side effect: the row of collars can double as the ceiling for the attic space above.

Purlin roof: rafters on beams

In a purlin roof, the rafters rest on horizontal beams called purlins: the wall plate at the eaves, one or two middle (side) purlins depending on the size of the roof, and a ridge purlin. The purlins pass their loads down through posts (a rule of thumb is one every 3.5 to 4.5 m) into load-bearing walls or the floor structure. Advantages: shallow pitches are also possible, and rafters may be interrupted – ideal for dormers and large roof windows. Disadvantages: posts and struts stand in the roof space, and timber use is higher.

The three roof types compared

Roof pitch: from the covering to snow load

Roof pitch is not purely a matter of taste. Every roof covering has a minimum recommended pitch, below which it only stays reliably watertight with extra measures such as a fully waterproof sub-roof. Typical guide values:

• Plain (beaver-tail) tiles: from approx. 30°
• Interlocking clay tiles: from 19–25° depending on the model
• Concrete roof tiles: from approx. 22°
• Bitumen (asphalt) shingles: from approx. 15°
• Trapezoidal metal sheet and standing seam: from approx. 5–8°

Rafter and collar roofs are typically 30° to 50°, while purlin roofs work at much shallower angles too. A quick calculation: with an 8 m building width and a 40° pitch, each side of the roof rises by tan 40° × 4 m ≈ 3.36 m – that is how far the ridge sits above the eaves. Bear in mind the snow load as well: more snow settles on shallow roofs, and in heavy-snow regions or at altitude the design loads can easily be twice as high as in a low-snow area.

Timber sizes: guide values for structural timber

The usual rafter spacing is 60 to 90 cm, with 70 to 80 cm chosen most often. For a rough first estimate of the rafter depth you can allow one twentieth to one twenty-fourth of the span – this is a design rule of thumb, not a substitute for structural calculations. The sizes below are typical guide values for a detached house:

The standard material is structural solid timber (KVH) in strength class C24. As a guide, expect to pay 400 to 650 € per m³; a complete roof frame built by a carpenter costs roughly 60 to 110 € per m² of roof area depending on region and complexity – both figures vary widely by region.

Birdsmouths, tenons and modern connectors

The birdsmouth: the standard rafter-to-purlin joint

The birdsmouth is the classic notch that lets the rafter sit positively over the purlin. As a guide: a birdsmouth depth of 2.5 to 4 cm, and no more than about a quarter of the rafter depth, so that the remaining section still carries the load. The notch angle matches the roof pitch, and the horizontal seat cut should be at least about 5 cm deep. If every birdsmouth is cut identically, the cutting and jointing goes much faster – one reason to plan the purlin heights precisely.

Traditional and modern joints

• Tenon: joins a post to a purlin or sole plate; the tenon is usually about a third of the timber thickness.
• Step (abutment) joint: transfers compression from struts and head braces; as a guide the depth is one sixth to one quarter of the beam depth.
• Lap (half-lap) joint: a traditional connection, for example for collars, today often replaced by metal connectors.
• Rafter-to-purlin anchors and joist hangers: modern galvanised-steel connectors fixed with ring-shank nails – fast, tested and with clearly defined structural values.
• Diagonal bracing strap: a steel strap (e.g. 40 × 2 mm) tensioned diagonally across the rafters to brace the roof plane.

Important against wind uplift: every rafter-to-purlin connection must also be able to resist tension. So the rafter must not simply rest in the birdsmouth – it also needs an anchor or a screw fixing.

When do you need a structural engineer?

In short: almost always. Besides its own weight, a roof frame carries snow, wind and often a solar PV array – mistakes here only show up after years, or during the first heavy-snow winter. A structural engineer is particularly necessary for:

• Dwellings and projects that require planning permission: structural calculations are almost always mandatory here.
• High snow loads: in a low-snow area the design value might be around 0.65 kN/m², in heavy-snow regions over 1.10 kN/m² – and considerably more at altitude.
• Clear spans over about 4.5 to 5 m or extra loads such as a green roof (0.8 to 1.5 kN/m² depending on the build-up).
• Alterations to an existing roof: adding a dormer, removing a collar or moving a post fundamentally changes the load-bearing system.
• Structures exempt from planning permission, such as a carport or garden shed: these may be allowed without approval depending on local rules, but structural safety remains your responsibility.

Your local building regulations and the requirements of your local building authority always take precedence – table values and rules of thumb from guides are no substitute for a proper structural calculation. Always check with your local building authority. Simple structural calculations for a roof frame cost around 500 to 1,500 € as a guide – money well spent given the risk of failure.

Step by step to your own roof frame plan

1. Set the basic dimensions: building width, eaves height, ridge direction and roof overhang (usually 30 to 60 cm).
2. Choose the roof type according to span and how the space is used: rafter, collar or purlin roof.
3. Set the roof pitch to suit the covering you want.
4. Lay out the rafter grid: 60 to 80 cm spacing, allowing for the positions of windows, dormers and the chimney.
5. Pre-size the sections using guide values and place posts over load-bearing walls.
6. Decide on the joints and bracing: birdsmouths, anchors, diagonal bracing straps.
7. Have the structural design checked and finalise the sections.
8. Draw up the cutting list and prepare the cutting and jointing.

Tip: in a 3D planner such as HolzBau 3D you can lay out purlins, rafters and birdsmouths to scale, spot clashes early and finally export a cutting list with all lengths and cutting angles – a solid basis for the conversation with your structural engineer and carpenter.