How the two differ in kind
A pitched roof generates outward THRUST at its feet unless something stops it. A conventional truss stops it with a horizontal bottom chord, which is also the ceiling. Vault the ceiling and that chord has to go — so something else has to take the thrust, and the two answers on this page take it in opposite places.
A SCISSOR TRUSS keeps the bottom chord but slopes it upward toward the ridge, so the ceiling follows the underside of the truss. The triangle is still closed, so the thrust is still resisted internally — but the triangle is much shallower than a conventional one, which makes the truss more flexible and, crucially, means its heels MOVE APART as it deflects. That horizontal movement at the bearing is real, it is published by the truss designer, and it has to be accommodated by a slip connection at one end. A pair of heels pinned rigidly to two walls will push those walls apart instead.
A STRUCTURAL RIDGE BEAM removes the thrust entirely by holding the top up. Each rafter becomes a simply supported member spanning from wall to beam, so there is nothing to spread and the ceiling can follow the rafters all the way to the apex. The cost is the beam: it carries half the roof along its whole length, down through posts at its ends into a continuous load path to a foundation — and on a retrofit those posts and that path are usually the hard part rather than the beam.
The factors that actually differ
Show
| Scissor trusses | Rafters on a structural ridge beam | |
|---|---|---|
| What resists the thrust | The truss itself — the sloped bottom chord closes the triangle, so the walls carry vertical load only in principle. | The beam, by holding the ridge up. There is no thrust to resist because the rafters do not lean on each other. |
| Horizontal movement at bearings | Real and designed for. The heels spread as the truss deflects, so one end gets a slip connection. Pinning both ends pushes the walls apart. | None. Rafters bear vertically at both ends. |
| Load path below | Straight down the walls, as for any truss. Nothing new is needed underneath. | Posts at the beam's ends carrying half the roof, and a path through floors to a foundation — which on a retrofit is frequently the binding constraint. |
| Shape of the ceiling | A vault with a flatter apex, because the bottom chord's slope is limited by the truss's own geometry and depth. | A true vault to the ridge, with the ceiling following the rafters exactly. |
| How steep a vault is possible | Limited. The ratio between top and bottom chord slopes governs the truss's efficiency, and past a point it becomes uneconomic and then impossible. | Whatever the roof pitch is. The ceiling can follow it entirely. |
| Insulation depth | Generous at the heel where the chords diverge, tighter near the apex where they converge. | Whatever the rafter depth is, uniformly — which often means deeper rafters than the span needs, purely to hold insulation. |
| Services and fixings | Webs cross the space. Ducts, lights and anything that wants a clear plane have to negotiate them. | A clear plane between rafters, uninterrupted from wall to ridge. |
| Programme and procurement | Designed and fabricated off site, delivered and craned in. Fast on site, and a late change is a long lead time. | Cut on site or from a schedule, with the beam as the only long-lead item. |
Which one, and when
Choose scissor trusses when…
- The roof is new and the walls are being built to receive it, so the slip detail is designed in rather than retrofitted.
- The vault wanted is moderate rather than a full cathedral to the ridge.
- There is nowhere to land the posts a ridge beam would need.
- Speed on site matters and a crane is available.
Choose rafters on a structural ridge beam when…
- The vault has to reach the apex — a true cathedral ceiling with no flat at the top.
- There is a wall or a frame beneath the beam's ends that can take the load down to a foundation.
- The roof plane has to stay clear for services, rooflights or an exposed finish.
- The work is a conversion where trusses cannot be craned in and members have to go up in pieces.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- How much does a scissor truss actually spread?
- Enough that it is a detailing requirement rather than a rounding, and the truss designer publishes the figure for the specific truss. Sloping the bottom chord reduces the depth of the triangle, and a shallower triangle deflects more under the same load — so the heels move apart as it settles. The standard answer is a SLIP CONNECTION at one bearing: a slotted plate or a clip that carries vertical and lateral load while allowing the heel to move horizontally. Fixing both heels rigidly converts that movement into a horizontal force on the walls, and the symptom is a crack at the top of the wall or at the wall-ceiling junction appearing in the first year. The figure to ask for is the horizontal movement at the bearing, and it goes on the drawing rather than being left to the fixer.
- Do I really need a ridge beam for a vaulted ceiling?
- If the ceiling ties are being removed and nothing else closes the triangle, yes — and this is the most consequential misunderstanding in domestic roof alterations. A ridge BOARD merely aligns the rafters; it supports nothing. The triangle formed by the rafters and the ceiling joists is what resists the thrust, and taking out the joists to open up a ceiling removes it. The roof does not fall in; the walls move outward slowly and the ridge drops, and the evidence appears as cracking at the tops of walls and doors that stop closing. A structural ridge beam, sized for half the roof load and supported on posts with a real load path, is what replaces that function.
- Which gives more headroom?
- The ridge beam arrangement, usually by a clear margin, because the ceiling follows the rafters all the way to the apex. A scissor truss's ceiling follows its bottom chord, which cannot slope as steeply as the roof — the truss needs depth between the chords to work, and that depth is subtracted from the room at every point. The practical consequence is that a scissor truss produces a vault with a noticeably flatter apex than the roof outside suggests. Where the architectural intent is a dramatic cathedral space, that difference is usually what decides it; where the intent is simply more height than a flat ceiling, the truss is often enough and much simpler to build.
- Can I vault an existing trussed roof?
- Not by cutting the trusses, and this is an absolute rather than a caution. A trussed rafter is a designed structure whose every member and plate was computed for the geometry as built; cutting a bottom chord or a web removes a member the calculation relied on, and the truss is no longer a truss. It is also why manufacturers and standards prohibit field modification entirely. The routes that do work are removing the trusses over the area and replacing them with a new structural arrangement — a ridge beam and rafters, or scissor trusses designed for the space — which is a structural project with temporary support, not an alteration. Anyone offering to 'just take out the bottom chords' is describing a collapse with a delay on it.
- How is the insulation handled in each?
- Both need a decision about whether the roof is warm or cold, and both are constrained by depth. A rafter vault has only the rafter depth available, so either the rafters are deeper than the span requires — timber bought to hold insulation rather than to carry load — or insulation is added above or below the rafters in a warm-roof build-up. If a ventilated cold roof is intended, a continuous air path from eaves to ridge has to be maintained above the insulation, and that path is what baffles exist to keep open. A scissor truss gives more depth at the heel, which is exactly where a rafter roof is tightest and where eaves ventilation competes with insulation — so it solves the hardest part of the rafter version and gives less room near the apex.
- Which is cheaper?
- Scissor trusses, usually, on a new build with crane access — they arrive engineered, they need no ridge support, and the site labour is erection rather than construction. The ridge beam version's cost is rarely the beam: it is the posts, the padstones, and the load path through whatever is below down to a foundation that can take half a roof at two points. On a retrofit that path may not exist, and creating it can mean opening floors and strengthening foundations, which is where the money goes. The comparison flips where trusses cannot be delivered or craned — a conversion, a constrained site, a room in an existing roof — because a beam and rafters can be carried in and assembled in pieces and a truss cannot.