Cut roof or trussed roof: how to choose

The question is asked as though it is a matter of taste. It is not. In most cases the building decides, and the only real choice is whether you find that out at design stage or after the trusses have been fabricated.

A trussed rafter roof is a set of engineered components made in a factory. Each truss is a triangulated frame of stress graded timber joined with punched metal nail plates, designed for the span, pitch, loading and spacing of your particular roof. They arrive on a lorry, get slung into position, braced, strapped and squared, and the roof is up.

A hand cut roof is built timber by timber on site. Wall plate, ridge board, common rafters, hip and jack rafters, valley sets, purlins, struts, collars, binders and ceiling ties, all set out and cut to the building that is actually in front of you rather than to a fabricator's layout.

Both are perfectly good roofs. Both are built to the same structural design intent. What separates them is what happens when the roof stops being a simple rectangle, and what you want the space underneath it to do.

The trade-offs, side by side

Where a trussed roof wins

On repeat plots, comfortably. If a developer is building forty houses from six house types, the trussed roof is the right tool: the design is done once, the sets arrive plot by plot, and the gang gets faster with every roof. Output on that kind of work is measured in plots per day rather than days per plot. On the Quadring development, seven plots were roofed in on a single site day. On the Long Sutton estate, six attic trussed roofs went on in one day.

Trusses also carry their loads more efficiently than cut timber, which is why they can span further on smaller sections and why they need no internal load bearing walls under them. That gives an open plan ground floor without a spine wall running through the middle of it, which is often the real reason the architect specified them.

What they demand in return is discipline. Trusses have to be stored upright and supported, not stacked flat in the mud. They have to be braced exactly as the design says, with longitudinal, diagonal and chevron bracing, because until the bracing is in an unbraced set of trusses is a row of dominoes. Wall plates have to be strapped down. Girder trusses have to be built up and bolted exactly as specified, because everything hanging off them relies on it. And no truss gets cut, notched or drilled on site without the truss designer's written approval. Openings for stairs, chimneys and rooflights are trimmed with girder trusses and hangers, designed in before fabrication.

See roof trusses supplied and fitted for how AJB Group handles delivery, storage, slinging and bracing.

Where a cut roof wins

Everywhere the drawing stops being repeatable. The set out is the job: get the roof plan, the section and the pitch right on the ridge, and every hip and valley after it lines through. Get it wrong and none of them do.

Cut roofs are what irregular buildings need. Steel framed structures, barn and shell conversions, extensions that must tie into an existing roof, listed and heritage work, squint blocks, and anything with a ceiling that has to follow the pitch. They also suit sites where a crane simply cannot get in, because loose timber can be walked through a building that a lorry and a lift cannot reach.

The other advantage is that a cut roof does not have a lead time attached to it. If a job is already behind and the blockwork is up, a cut roof can start on the Monday. A truss order cannot.

More detail on the method is on the hand cut roofs page, and the wider service sits under roofing carpentry.

Lead time and the design freeze

This is the single most common reason a roof decision goes wrong. A trussed roof needs the design settled before it can be fabricated: spans, pitches, overhangs, openings, wall positions, loadings and any room in the roof. Once the set is made, changing the layout means changing the order, and by then the bricklayer is usually on the last lift.

So the practical rule is to fix the roof design earlier than feels necessary. If there is any chance of a room in the roof, an extra rooflight, a chimney, a stair position moving or an internal wall coming out, resolve it before the truss order goes in. Lead times move with demand and season, and no carpenter can quote you a reliable figure months in advance, so ask the fabricator for their current lead time and add the delivery slot to your programme rather than assuming the trusses arrive when the blockwork finishes.

A cut roof carries none of that risk, which is worth real money on a job where the design is still moving.

Crane day and access

Craning is the weather critical part of a trussed roof. A set on the ground is a liability: it takes up the site, it can be damaged, and it holds up the bricklayer. A set in the air is a roof. The lift is normally planned for the morning, with the gang bracing behind it through the day.

Access decides more roofs than people expect. A tight town centre plot, a narrow lane, overhead cables, a soft yard in February, or a building sitting behind another building, any of these can make a lift impractical. AJB Group owns its crane and slings its own sets, so there is no separate hire booking and no waiting on an operator, but a crane still needs somewhere to stand and something to reach over.

What actually drives the cost

No two roofs price the same, and anyone quoting a rate per square metre without seeing the drawing is guessing. These are the things that move the number, in roughly the order they matter:

• Geometry. Hips, valleys, dormers, changes of pitch and non-square blocks all add setting out, cutting and time. A simple gable roof is the cheapest roof there is.
• Room in the roof. An attic set costs more than a standard set because it carries a floor. It is still almost always the cheapest square metres in the build, because it needs no extra footprint or foundation.
• Span and loading. Longer spans and heavier coverings mean deeper sections, more girder trusses and more steel interface.
• Access and plant. Whether a crane can reach, how far the timber has to be carried, and how much scaffold is in the way.
• Storey height. A roof six storeys up is a different job from the same roof on a bungalow.
• Scope. Structure only, or structure through to felt, batten and tile. A roof on and roofed in package removes a trade from your programme but adds to the carpentry price.

What we will not do is give you a headline rate on a web page. Send the roof plan, the sections and the truss layout or timber schedule and you get a real price against your actual drawing.

The middle option: attic trusses

An attic truss carries a clear rectangular void inside it, so the roof space arrives as a usable floor built into the structure. It is the compromise that gets forgotten: factory speed, but with a room in the roof. The catch is that the decision has to be taken before the order, because retro-fitting the same room later means stripping a finished roof.

Hipped attic sets need girder trusses and jack rafters at the hip end, which adds time but keeps the roof line clean. Where the geometry will not take an attic truss at all, a hand cut roof with purlins and dormers gives the same result by a different route. See attic trusses and room in roof and dormers and Velux openings.