Roof Truss Types, Explained

A truss is a triangulated frame that turns a long span into a series of short members in pure tension and compression. Which configuration you need is decided by span, by what the roof space is for, and by the load — not by preference.

Roof Truss Types, Explained — practical guidance from the team running roof installation on the Garden Route. Below are the truss types you will actually encounter on a South African roof, what separates them, why bracing matters more than the truss shape, and what an A19 certificate does and does not cover.

What a truss is doing, and its five parts

A rafter spanning a wide room has to resist bending along its whole length, which is structurally inefficient — the middle of the member is doing very little work while the top and bottom fibres do all of it.

A truss solves that with triangles. Break the span into a triangulated frame and every member carries either pure tension or pure compression along its length, which is what timber and steel are good at. That is why a truss made of relatively slender members outperforms a much heavier single beam over the same span.

The five components, whatever the configuration: the top chords (the rafters, in compression, carrying the roof covering); the bottom chord (the tie, in tension, which is what stops the walls being pushed outward); the webs (the internal members that triangulate the frame); the joints or nodes, in South African practice usually nail plates pressed into the timber; and the bearing points where the truss sits on the wall plate.

The bottom chord is the part people misunderstand. It is not a ceiling joist that happens to be there. It is the tie holding the whole triangle together, and cutting it — to run a duct, to fit a hatch, to make room for a water tank — destroys the truss. We find cut bottom chords in roof spaces more often than we should.

The joints are the other critical part. A nail-plated joint is engineered, and the plate size and position are part of the design. A plate that has been knocked loose in handling, or a joint that has been repaired with a bit of ply and some nails, is not the joint the engineer designed.

In practice almost no roof in South Africa is hand-cut any more. Trusses are designed and manufactured under licence to one of the plated-truss systems — MiTek and its equivalents — using proprietary software that produces the layout drawing, the member sizes and the nail-plate schedule for your specific roof. That output is what an engineer signs, and it is what should reach the site with the delivery. The A19 certificate at the end of the job certifies that the erected roof matches that design, which is why alterations on site are a design matter rather than a carpentry decision.

The truss types you will actually meet

There are dozens of named configurations. Six cover almost everything on a South African residential or light commercial roof.

King post is the simplest — a single vertical member from the apex to the centre of the bottom chord, with two struts. Short spans only: garages, porches, small outbuildings. Its virtue is simplicity and its limit is span.

Queen post replaces the single vertical with two, opening up the centre. It reaches further than a king post and leaves a usable void in the middle, which is where the attic idea starts.

Fink is the workhorse of South African housing, and it is what is on most tiled and sheeted houses you will look at. A W-shaped web pattern, extremely material-efficient, prefabricated in a factory and delivered ready to erect. The web pattern is also what makes the roof space effectively unusable for anything but storage.

Howe uses vertical members in tension and diagonals in compression, and it handles heavier loads over longer spans. More common on commercial and industrial roofs than on houses.

Scissor raises the bottom chord to a slope, producing a vaulted ceiling below without a separate structure. It is the architectural choice, and it costs more in both design and material because it works harder.

Attic truss is designed with a clear rectangular void in the middle so the roof space becomes a room. It carries floor loading as well as roof loading, so the members are substantially heavier — and it has to be designed as an attic truss from the start, because you cannot convert a fink truss roof into an attic by cutting the webs out.

Type	Configuration	Typical use	Roof space	Relative cost
King post	Single central vertical	Garages, porches, outbuildings	None usable	Lowest
Queen post	Two verticals, open centre	Medium spans	Limited void	Low
Fink	W-shaped webs	Most SA houses	Storage only	Lowest per metre of span
Howe	Verticals in tension	Longer spans, heavier loads	None usable	Moderate
Scissor	Sloped bottom chord	Vaulted ceilings	None — it is the ceiling	Higher
Attic	Clear rectangular void	Habitable roof space	A room	Highest

Which is strongest, and why that is the wrong question

“What is the strongest roof truss?” is asked constantly and it does not have a useful answer, because strength in a truss is a property of the design rather than of the shape.

Any of these configurations can be designed to carry more load — by increasing member sizes, by using a higher timber grade, by adding web members, or by reducing the spacing between trusses. A fink truss at close centres in a high grade will outperform a howe truss at wide centres in a low grade. The name tells you the geometry, not the capacity.

What the configurations genuinely differ in is efficiency for a given purpose. Fink is the most material-efficient way to span a typical house. Howe suits longer spans and heavier point loads. Attic trusses carry floor loads as well as roof loads. Scissor trusses buy you a vaulted ceiling and pay for it in material.

The right question is therefore: what is the span, what is the roof covering, what wind load applies at this location, what will the roof space be used for, and are there any point loads — a geyser, a solar array, a water tank? Answer those and the configuration follows.

Point loads deserve a specific mention. A solar PV array, a geyser or a tank in the roof space is a load the original design either allowed for or did not. Adding one to an existing roof without an engineer’s check is one of the more common ways a sound roof structure gets quietly overloaded.

Bracing, and why it matters more than the truss type

A truss is a two-dimensional structure. On its own it is strong in its own plane and has almost no stability out of it — you can push a single erected truss over with one hand.

What makes a roof stable is the bracing: the members that tie the trusses to each other and to the structure below, turning a row of flat frames into a three-dimensional box. Diagonal bracing in the plane of the rafters, longitudinal bracing along the length, web bracing where the design calls for it, and the connections tying the whole assembly down to the wall plate.

On the Garden Route this is not academic. Wind uplift is a live load, and it acts upward — the coastal southeaster tries to lift the roof off the building. The tie-down connections at the wall plate are what stop it, and they are the single most skimped element in South African roofing.

Bracing is also the thing most commonly removed later. Somebody needs to get a water tank through the roof space, and a brace is in the way. It gets cut. Nobody notices anything for years, and then a big front comes through.

If you look at one thing in your own roof space, look at whether the diagonal bracing is continuous and whether anything has been cut. It is the highest-value five minutes available to a homeowner with a torch.

Design, the A19, and what it actually certifies

Trusses in South Africa are engineered products. They are designed by the truss manufacturer’s software to the specific roof geometry, covering, span and wind zone, and the design is issued as a set of drawings and a certificate.

The A19 certificate is the certification of the erected roof structure by a competent person — that the trusses as built and installed correspond to the engineered design, are correctly braced and correctly fixed. It is not a certificate for the timber, and it is not the manufacturer’s design certificate. It certifies the thing that is actually standing on your building.

You want it in your hands at handover. Banks, insurers and conveyancers all ask for it eventually, and reconstructing it after the ceilings are up is considerably harder and more expensive than obtaining it at the time.

What invalidates a truss roof, in rough order of how often we see it: cutting or notching a member, especially the bottom chord; removing bracing; adding an unplanned point load such as a geyser, tank or solar array; changing the roof covering to something heavier than the design allowed; and damaged or displaced nail plates that were never picked up at erection.

Changing the covering deserves emphasis. Going from tile to sheeting reduces the dead load substantially and is normally the easier direction, but the load pattern still changes and it still needs an engineer’s check. Going the other way — sheeting to tile — adds weight the structure was very likely never designed for, and it is not a change to make on assumption.

Frequently asked questions

What are the main types of roof truss?

On South African residential and light commercial roofs: king post for short spans, queen post for medium spans with a small central void, fink as the standard house truss, howe for longer spans and heavier loads, scissor where a vaulted ceiling is wanted, and attic trusses where the roof space is to become a habitable room. Fink covers the majority of houses because it is the most material-efficient way to span a typical dwelling.

What are the five main components of a roof truss?

The top chords, which act as the rafters in compression and carry the covering; the bottom chord, in tension, which ties the triangle together and stops the walls being pushed outward; the web members, which triangulate the frame; the joints or nodes, usually pressed nail plates in South African practice; and the bearing points where the truss sits on the wall plate. The bottom chord is the one most often mistaken for a ceiling joist and cut — which destroys the truss.

What is the strongest roof truss type?

There isn’t one, because strength is a property of the design rather than the shape. Any configuration can be designed to carry more by increasing member sizes, raising the timber grade, adding webs or reducing the spacing between trusses. A fink truss at close centres in a high grade will outperform a howe truss at wide centres in a low grade. The useful question is the span, the covering, the wind load, what the roof space is for, and whether there are point loads.

What is the cheapest roof truss?

For a typical house span, a fink truss is the most economical, which is exactly why it dominates South African residential roofing — the W-shaped web pattern uses less timber to span a given distance than any of the alternatives. King post trusses are cheaper individually but only work over short spans. Attic and scissor trusses are the most expensive because they are doing more: carrying floor loads, or holding a vaulted ceiling.

Can I cut or modify a roof truss?

No, not without an engineer’s design. Every member is carrying a calculated load, and the bottom chord in particular is the tie holding the whole triangle together. Cutting, notching or drilling a member — to run a duct, fit a hatch or make room for a tank — compromises the truss, and removing bracing compromises the whole roof’s stability out of plane. If something has to change, it needs an engineered modification, not a saw.

What is an A19 certificate?

It is the certification of the erected roof structure by a competent person, confirming that the trusses as built and installed match the engineered design and are correctly braced and fixed. It certifies what is standing on your building, which is different from the manufacturer’s design certificate. Get it at handover: banks, insurers and conveyancers all ask for it, and obtaining it after the ceilings are closed is much harder.

Can I put solar panels or a geyser in my roof space?

Only after an engineer has checked it. Both are point loads, and the truss design either allowed for them or it did not. Adding a PV array, a geyser or a water tank to a roof designed without them is one of the more common ways a sound structure gets quietly overloaded — and the mounting for a PV array also puts fixings through the covering, which is a frequent cause of new leaks on previously sound roofs. Both parts need doing properly.

Can I convert my roof space into a room?

Only if the roof was built with attic trusses, or if the structure is redesigned. A standard fink truss roof cannot be converted by cutting the web members out — the webs are what make the truss work. Creating a habitable roof space means either attic trusses designed for floor loading from the outset, or an engineered replacement of the roof structure. It is a real project, not a conversion.

Planning a roof, or worried about the one you have?

We build to the engineered design and hand over the A19. And if you have found something cut in your roof space, send us a photo — that is a conversation worth having before the next big front.

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