The house design is finished, the walls are up, and the architect's drawings say just one sentence: "roof truss per structural drawing." The investor is left alone with a question we hear at JB Tartak literally every week: fine, but what types of roof trusses even exist, and how different are they, in a way that actually matters for my wallet and my roof? Rafter, purlin, trussed rafter — the names sound similar, and choosing the wrong construction can cost several thousand zloty more, or rule out plans for a usable attic altogether. Over 35 years working at the sawmill, I've seen hundreds of conversations like this and I know exactly where investors tend to get lost. If you're looking for the basics first — what a roof truss even is and what it's made of — check out our article Roof truss — what it is, what it's made of, and how it's built →. Here we go one step further: I break down the three main types of roof trusses in detail — with specific spans, angles, costs, and the situations where each type works, and where it doesn't.
Three questions that decide which roof truss to choose
Before comparing the construction types, it's worth setting the right criteria first. When an investor asks me which type of roof truss to choose, I always answer with three questions — and only the answers to those lead to a specific type:
- What is the roof span (the distance between load-bearing walls without support)? This is the most important factor — it determines whether the rafters can manage on their own or need additional support.
- What roof pitch angle does the design call for? Very flat roofs (below 20°) and very steep roofs (above 45°) favour different construction solutions.
- Will the attic be habitable, or just a technical space for storage? Some truss types physically prevent free arrangement of the attic interior.
Span, angle, and the attic's intended use — these three variables point to the right type of construction in 90% of cases, even before the structural engineer runs the first calculation. So let's go through the three most commonly used types of roof trusses, one by one.
Rafter roof — the simplest and cheapest option
Structure and how it works
A rafter roof is the most minimalist wooden roof structure you can build. It consists solely of wall plates (beams resting on the walls) and rafters — pairs of sloped beams meeting at the ridge and forming an A-shape. No purlins, no struts, no intermediate supports. Each pair of rafters works independently, transferring the load directly onto the wall.
This solution only works because, at short spans, rafters are able to carry the roof's weight on their own without deflecting and without excessively pushing the walls outward. Beyond a certain length, physics stops being forgiving — and that's when you need a collar tie, a purlin, or a different type of construction.
Spans and pitch angles
A rafter roof works well for spans of up to around 7 metres (without a collar tie) and a roof pitch of 30–50°. At greater spans the rafters would need such a large cross-section that the construction stops being economical — it works out cheaper to switch truss type than to keep adding timber.
Pros and cons
The biggest advantage is cost and simplicity — less material, fewer joints, shorter assembly time. A carpentry crew can put up a simple rafter roof over a garage or a small extension in literally a few days. The downside is a lack of flexibility: at this span and layout, there's no question of a usable attic — the space under a rafter roof is practically always a technical loft, not a room.
When to choose a rafter roof
Garages, carports, utility extensions, small single-family homes with no plans for a habitable attic. If the load-bearing walls in your design are less than 7 metres apart and you don't need an extra room under the slope, this is most likely the most economical choice.
Purlin roof (purlin-and-strut roof) — for large and complex roofs
Structure and how it works
Once the span goes past 7 meters, rafters alone aren't enough — they need support partway along their length. That's where the purlin comes in: a horizontal beam running along the slope, resting on posts, which in turn stand on sole plates laid across the floor structure or internal walls. Cleats — pairs of boards clamped to both sides of a rafter and post and bolted together — stiffen the whole structure. Hence the full name: purlin-and-strut roof.
The construction distinguishes two types of elements: main trusses (heavier, spaced 3–5 metres apart, resting on posts) and intermediate trusses (lighter rafters resting directly on the purlins). This distinction allows the load to be distributed more efficiently than in a simple rafter roof.
Spans and applications
A purlin roof handles spans where a rafter roof has long since stopped being economical — up to roughly 11–16 metres, depending on cross-sections and support spacing. The roof pitch here is exceptionally flexible: from almost flat roofs (6°) to very steep ones (70°). That makes it the most universal type of roof truss — it works just as well in a classic single-family home with an extensive usable attic as it does in halls or buildings with an unusual roof shape (hip, multi-pitch, with dormers).
Pros and cons
The advantage is strength and versatility — this structure copes where rafter and trussed-rafter roofs simply can't. A usable loft? No problem, as long as the post spacing allows for it. A complicated roof shape with dormers and roof windows? Purlin roofs are built for exactly that.
There's one downside, but it's significant: cost. More material, more joints, more carpentry hours — and the structure demands an experienced crew, because a mistake in post spacing or purlin cross-section affects the stability of the whole roof. Assembly takes longer than with simpler types, and the design has to account for internal walls or beams the posts can rest on — which sometimes forces changes to the room layout on the floor below.
When to choose a purlin roof
Houses with spans over 7–8 meters, multi-slope or dormer roofs, buildings with an extensive usable loft, halls and outbuildings with large floor areas. If your architect designed a roof with several slopes at different angles, this is almost certainly the type you're looking at.
Trussed rafter roof (roof trusses) — prefabrication and fast assembly
Structure and how it works
A trussed rafter roof — commonly known as roof trusses or fink trusses — is a completely different way of thinking about roof construction. Instead of joining elements on site, each triangular truss — top chords (the rafter equivalent), a bottom chord (the ceiling joist equivalent), and diagonal webs connecting them — is manufactured in a factory, based on computer calculations. Joints are made with punched metal plates pressed in hydraulically, giving very precise and repeatable connections. Finished trusses arrive on site and are lifted into place by crane, one after another, usually spaced 60–100 cm apart.
Spans and applications
Roof trusses work best for gable roofs with a shallow pitch (14–23°) and spans up to about 12 meters, though reinforced versions are produced for larger industrial spans too. This is the standard in developer-built housing — entire estates of single-family homes are built today on roof trusses, because a crane crew can install a whole roof in a single day.
Pros and cons
The biggest advantage is speed and predictability — trusses arrive ready-made, with structural calculations from the manufacturing plant, and assembly doesn't require an experienced carpentry crew, just a crane operator and a few pairs of hands to position and bolt them in place. Material cost alone can be competitive, especially on large, repeatable projects.
The downside is fundamental and often catches investors off guard after the fact: the internal bracing within the truss physically blocks the attic space. There's no way to fit a room in there, or even move freely between the members. If the design calls for a usable attic, trussed rafters are simply out of the question — unless you go for more expensive, specially designed trusses with a clear central space, which in practice brings the cost close to that of a purlin roof. The second downside is lower flexibility with unusual roof shapes — any change requires redesigning and re-manufacturing, so no changes "on the fly" on site.
When to choose roof trusses
Houses with no plans for a usable loft, tight construction schedules, developer and repeatable projects, roofs with a shallow pitch and a simple gable shape. If speed matters more than an extra floor under the roof, this is the fastest route to a closed shell.
Rafter vs purlin vs trussed rafter — comparison table
| Criterion | Rafter roof | Purlin roof (purlin-and-strut) | Trussed rafter (roof trusses) |
|---|---|---|---|
| Span | up to ~7 m | up to 11–16 m | up to ~12 m (standard) |
| Roof pitch | 30–50° | 6–70° (very versatile) | usually 14–23° |
| Usable loft | no (technical attic) | yes, no restrictions | no (webbing blocks the space) |
| On-site assembly time | shortest | longest | shortest (1–2 days), but + manufacturing lead time |
| Crew required | basic carpentry | experienced carpentry crew | crane operator + installers |
| Shape flexibility | low | high (dormers, multi-slope) | low (changes mean new prefab) |
| Typical use | garages, extensions, small houses | houses with lofts, complex roofs, halls | developer estates, simple gable roofs |
What does each truss type cost?
A precise costing always depends on roof area, timber species and grade, region, and crew rates — you'll find the full cost breakdown in a separate article dedicated entirely to roof truss pricing: How Much Does a Roof Truss Cost? Material and Labor Prices →. Here we're after something different: showing how the structural type itself affects the budget, for a comparable roof area.
A rafter roof comes out cheapest — less timber per square meter of roof, and the shortest crew time. It's the natural baseline for comparison.
A purlin roof is noticeably more expensive — more structural elements (purlins, posts, sole plates, cleats), more joints to make with traditional carpentry connections or steel fixings, longer crew time on site. That's the premium you pay for strength and versatility on larger, more complex roofs.
Roof trusses can be price-competitive on material alone, especially for simple, repeatable shapes — but you need to add transport of the finished elements and crane hire, which on a single build (not an estate) can eat into the material savings. The real cost of buying and installing trusses for a typical single-family home usually runs into the tens of thousands of złoty, depending on roof area and region.
The rule I apply when advising investors: don't choose the truss type based on material price alone. A construction that forces you to give up a usable attic, or that can't handle your span, will take back the money you "saved" at the next renovation.
What timber fits each truss type?
Regardless of the construction type chosen, the base material stays the same: softwood timber in C24 grade (pine or spruce), kiln-dried to a maximum moisture content of 18–20% and treated with preservative before installation. The differences show up in cross-sections and lengths — a purlin roof needs sturdier purlins and struts than a rafter roof, and trussed rafters are usually made from narrower but more precisely dimensioned members, because the structural calculations are done by computer, not by an engineer building in a margin "just in case."
For a full rundown of species, strength grades, and timber acceptance rules for roof structures, see our base article on roof trusses → — we cover that topic from the ground up.
Planning a roof and looking for timber — for rafters, purlins, or trusses? At JB Tartak in Studzionka, we'll help you figure out which type fits your project and prepare a quote for C24-grade timber in 15 minutes. Check our roof truss offer → or get in touch directly →
Common mistakes when choosing a roof truss type
Choosing roof trusses without checking loft plans first. This is the mistake I see most often. Investors buy ready-made trusses because they're fast and cheap, then discover after installation that the internal webbing rules out a room under the slope. It's the costliest mistake to fix — the only remedy is replacing the whole structure.
Ignoring snow load zones when sizing cross-sections. In Silesia, where snow loads can run higher than in central Poland, cutting corners on rafter or purlin cross-sections "because the neighbor did it that way" is a direct route to sagging roof slopes after the first heavy winter.
Changing the roof shape after ordering trusses. A prefabricated truss is engineered for one specific design. Adding a dormer or changing the pitch "along the way" means ordering from scratch — not an on-site modification, as with a traditional truss.
Skipping the structural engineer on a purlin roof. Post spacing and purlin cross-section need to be calculated — "by eye," even by an experienced carpenter, isn't enough once spans go past 10 meters.
FAQ — frequently asked questions
Which roof truss type is the cheapest?
A rafter roof — but only up to the 7-meter span where it's technically viable in the first place. Beyond that, the real cost comparison is between a purlin roof and roof trusses, since a rafter roof simply stops being an option.
Can you fit a usable loft under roof trusses?
Not in the standard version — the internal webbing blocks the space. Special trusses with a clear central span exist, designed specifically for usable lofts, but their price gets close to a purlin roof, which erases the main cost advantage of this solution.
Which truss type works best for a roof with dormers and roof windows?
A purlin roof. Its structure — with purlins, posts, and the freedom to place trimmers around openings — handles complex roof shapes the best. Roof trusses and rafter roofs have very limited flexibility here.
Can you combine different truss types in one building?
Yes, it's common practice on complicated building shapes — for example, part of a house with a simple gable roof goes on trusses, while a section with a dormer or bay window is built as a purlin roof on site. The structural engineer always makes this call at the design stage, based on the layout of load-bearing walls.
How long does assembly take for each truss type?
A rafter roof over a small building — a few days. Roof trusses for a typical single-family home — crane assembly usually takes 1–2 days, though you need to add the manufacturing lead time. A purlin roof built traditionally on site is the longest process — a crew of 3–5 carpenters usually needs 2–4 weeks, depending on how complex the roof is.
Not sure which type of roof truss fits your project? At JB Tartak we've been in business since 1990 — 35 years of experience in structural timber and hundreds of roof trusses built across Silesia and the surrounding region. Bring us your plans, or just tell us what you're building — we'll advise you and prepare a quote →
