Purlin roof system
A timber roof frame in which purlins, carried on posts, support the rafters partway up the slope, shortening their span and allowing wider roofs.
What is a purlin roof system?
A purlin roof system (Slovak: väznicová sústava) is a timber roof frame in which the rafters do not have to span the whole distance from the wall plate to the ridge on their own. Horizontal purlins run along the length of the roof and support the rafters partway up the slope. The purlins are carried by posts, which pass the load down to walls or beams below. The roof is therefore a stack of short supports rather than one long span.
This is the classic roof form of many Slovak family houses. Its logic is simple: the roof covering rests on the rafters, the rafters rest on the purlins, and the purlins rest on the posts. Each link in that chain carries a shorter distance than the whole roof would demand.
Which parts make up a purlin roof, and how do they work together?
Each member has one job, and the frame only works when all of them are present and properly connected.
| Member | Position | Job in the system |
|---|---|---|
| Rafter (krokva) | Sloping, from the wall plate up to the ridge | Carries the roof covering and passes its load to the purlins |
| Wall plate (pomúrnica) | Along the top of the external wall | Receives the lower ends of the rafters and spreads their load into the wall |
| Intermediate purlin | Horizontal, between the wall plate and the ridge | Shortens the unsupported length of each rafter |
| Ridge purlin (hrebeňová väznica) | Along the apex of the roof | Receives the upper rafter ends and carries them to the posts |
| Post (stolica) | Vertical or inclined, under the purlins | Carries the purlins down to the walls or beams below |
| Strut or brace | Diagonal, between a post and a purlin or tie beam | Stops the posts from leaning and stiffens the frame |
The purlins do most of the structural work, because they turn one long rafter into several short ones. Their types and sizing are covered in the purlin entry, so this article treats them as one part of the whole.
How do king-post and queen-post arrangements differ?
The posts can be arranged in two classic ways. In a king-post arrangement (Slovak: stojatá stolica), a single vertical post stands on the tie beam (väzný trám) and carries the ridge area, with braces holding it steady. In a queen-post arrangement (ležatá stolica), two inclined posts rise from the tie beam towards each other and carry the purlins, which leaves a more open space under the ridge than a single central post does.
The sibling articles on the king-post truss and the queen-post truss cover the geometry and detailing of each. The choice between them depends on the room the owner wants under the ridge and on the walls below, and it should be settled by the designer, not on site.
What are the advantages of a purlin roof?
The main advantage is span. Because the purlins split each rafter into shorter lengths, the same timber can cover a wider building than a rafter-only frame would. The purlin roof is also usually gentler on the external walls. Its loads travel mostly down through posts to interior or structural walls, so the top of the external walls is pushed apart less than it would be by a roof that depends on rafters alone. Whether this holds in a particular house depends on the design, which an engineer must check.
Two more advantages matter for a family house. The frame is flexible: the posts and purlins can be adapted for a vikier (dormer window), for a valbová strecha (hip) at one end, or for a change of roof plane. It is also built on site from individual members, so the carpentry can respond to the actual walls and openings rather than to a factory drawing.
What are the drawbacks of a purlin roof in a family house?
The biggest drawback is the attic. The posts and braces stand in the space under the roof, so a purlin roof rarely gives a clear loft. Storage can be arranged along the sides, but a usable room in the roof has to be planned around the posts. Other drawbacks are practical:
- Posts and braces take up attic space and limit usable headroom near the centre of the roof.
- Posts must land on a structural wall or beam, which constrains the plan of the floor below.
- The frame has more members and joints than a prefabricated truss, so it demands careful carpentry and detailing.
How does a purlin roof compare with a prefabricated roof truss?
A prefabricated roof truss is a triangulated frame made in a factory and lifted into place as a unit. It spans from wall to wall without internal posts, which keeps the attic clear. It does need lifting equipment, it has a fixed geometry, and it is harder to change on site. A purlin roof is built on site from individual members. It takes longer to assemble but adapts more easily. The table summarises the trade-off.
| Aspect | Purlin roof system | Prefabricated roof truss |
|---|---|---|
| Attic space | Obstructed by posts and braces | Clear, with no internal posts |
| Adaptation for dormers and hips | Flexible, detailed on site | Limited, changes usually need new or extra framing |
| Assembly | Built on site from individual members | Factory-made units, lifted into place |
| Load path | Rafters, purlins, posts, then walls | Whole truss from wall to wall |
| Role of the wall plate | Receives rafter feet and anchors the frame | Bears the truss ends and is fixed to the ring beam |
Neither system is better in every case. The right choice depends on how the roof space will be used, on the plan below, and on the experience of the designer and the carpenters.
Where does a purlin roof fit in a Slovak family house?
In a typical family house, the purlin roof sits on a ring beam (veniec) or a load-bearing masonry top, with the wall plate fixed to it. Designers usually align the posts with an interior load-bearing wall, so the floor plan and the roof frame have to be drawn together. This matters most when the owner plans a room in the roof, because the posts decide where the usable floor can go. For a house with a hip at one end or dormers along the slope, the purlin frame is often a practical base, provided the engineer details the openings and the changes of direction.
What are common misconceptions about purlin roofs?
- A purlin roof is always cheaper. Cost depends on timber, labour and the specific design, so it cannot be assumed from the name of the system.
- Posts can be removed to open the attic. Removing a post changes the load path, so the change needs an engineer's design, not a site decision.
- A single purlin makes a purlin roof. A purlin is one member; the system is the combination of members, posts, braces and bearings.
Frequently asked questions
- Is a purlin roof the same as a rafter roof?
- No. A rafter roof relies on the rafters to span the full distance, sometimes with a collar tie, while a purlin roof adds intermediate purlins and posts that shorten the span each rafter has to cover.
- Can a purlin roof have dormers?
- Yes, but the frame must be designed around the opening. A dormer interrupts rafters and purlins, so the engineer has to detail how the loads are carried around it.
- Does a purlin roof need an interior wall?
- Not always, but the posts need solid bearing, usually an interior load-bearing wall or a beam, so the design must include something that can carry them.
- Is a purlin roof a good choice for a loft conversion?
- Often not, if the room should feel open, because the posts and braces intrude into the space. A roof with a clear span, such as a prefabricated truss, is usually easier to convert, though the final choice depends on the project.
- Who should check the design of a purlin roof?
- A qualified structural engineer should size the purlins, posts, braces and bearings, because the loads depend on the site, the roof geometry and the roof covering.