Rafter roof system

What is a rafter roof system?

A rafter roof system (Slovak krokvová sústava) is the simplest traditional roof frame. It is built from pairs of rafters that lean against each other at the ridge and rest on the top of the walls, with no purlins and no collar beams in the basic form. Each pair forms an A-shaped frame that carries the roof covering, the sheathing, and the snow and wind loads down into the building.

Why does a rafter roof push the walls outwards?

Gravity and snow press down on the slope, and the two rafters of a pair cannot stand up on their own. Their load runs along each rafter, and at the foot the rafter pushes down and outward onto the wall plate. That outward push is called thrust. In a bare rafter roof the thrust is greatest at the feet, because nothing above the walls holds the two rafters apart.

If the feet are left free, the thrust spreads the walls outwards at the top. Masonry has little resistance to this, so the walls can lean, crack, or move away from the roof. For this reason the foot of a rafter roof must be tied, so that the two sides of each pair cannot spread.

How must the rafter feet be tied?

The tie can take three common forms. The choice depends on the building, the wall type, and what has to stay open or closed below the roof, so the table compares them side by side.

Tie type	How it works	Where it suits	What to watch for
Ceiling joists (stropné trámy)	Joists span across the building at the foot of the rafters and connect opposite wall plates, so each pair is held by a horizontal member.	Most houses with a flat ceiling below the roof, where the joists also carry the ceiling.	The joists must be fixed to the rafter feet and the wall plates. They cannot be cut or removed later without a replacement tie.
Ring beam (stužujúci veniec)	A continuous reinforced concrete band at the top of the walls ties every foot to the same level.	Masonry houses where a ring beam is cast anyway.	Each rafter must be fixed to the ring beam, usually through the wall plate, so that uplift is resisted as well as thrust.
Steel tie or strap	Metal bars or straps join opposite feet, or fix each foot to the wall structure.	Renovations and roofs where a timber tie cannot be placed.	Size and fixings come from the structural design, not from a catalogue.

How does a rafter roof compare with purlin and collar-beam roofs?

The three traditional frames differ mainly in where the load is collected. The purlin roof system (väznicová sústava) adds horizontal purlins under the rafters. The collar-beam roof system (hambálková sústava) ties each pair with collar beams higher up the slope. The table compares how each one behaves.

Aspect	Rafter (krokvová)	Collar-beam (hambálková)	Purlin (väznicová)
How loads travel	Load runs down each rafter pair straight to the wall plate.	Rafters carry load to collar beams near the top, which help the pair work together before the load reaches the walls.	Covering to rafters, rafters to purlins, purlins to walls or internal supports.
Span suitability	Short spans between walls, where bare rafters stay within their limits.	Medium spans, where the collar beams shorten the effective span of the rafters.	Longer spans, or buildings where internal supports can carry the purlins.
Free attic space	Greatest, because no members sit at mid-height.	Reduced at collar level, which limits headroom.	Reduced by purlins and posts, which often sit inside the rooms below.
Thrust on outer walls	Full outward thrust at the feet unless the feet are tied.	Collar beams limit how far the pair can spread, but the feet still need a tie.	Often lower on outer walls, because purlins can bear on internal walls or posts.

In short, the rafter roof gives the most free space and needs a firm foot tie. The collar-beam roof trades some headroom for a stiffer upper frame. The purlin roof moves much of the load onto internal supports, so the outer walls usually carry less thrust. None of the three removes the need for a properly designed structure.

When is a rafter roof system a suitable choice?

A rafter roof suits a building with walls close enough that a single pair of rafters spans between them comfortably, and a simple gable form. It is light, quick to set out, and leaves the attic open, which matters if the space will later become living accommodation.

The limit is span. As the distance between the walls grows, each rafter becomes longer and must be deeper or stronger to avoid bending, and the foot tie has to carry more thrust. At that point the designer usually adds purlins or collar beams rather than only enlarging the timber. Snow load also matters, because the design must follow the snow load zone of the site, which varies across Slovakia by altitude and region.

How are the rafter pairs joined and fixed?

At the ridge the two rafters of each pair meet at the top, either butted against each other or set against a ridge board that keeps the pairs in line. At the foot each rafter is notched so it sits over the wall plate and cannot slide down the slope, then fixed with nails, bolts, or metal connectors. The pairs are tied at the foot as described above and held against sideways movement by the roof sheathing and bracing.

Connector type, number of fixings, and timber grade are design decisions for the structural engineer. A rafter that looks right on site can still be undersized for the snow zone or the wind exposure, so dimensions are never copied from another house.

What mistakes are most common with a rafter roof?

The most serious mistake is leaving the feet untied, either by omission or because a tie was removed later. The second is cutting or removing a ceiling joist during a loft conversion or renovation without checking what it was holding. A third is spanning too far with rafters that are too light for the load. Poor fixing of the rafter feet to the wall plate is also a common fault, because it leaves the roof free to lift in wind.

How is a rafter roof system different from a roof truss?

A rafter roof is built from individual members, cut and assembled on site or pre-cut and then erected. A roof truss is a complete, pre-engineered unit in which members form a triangulated frame, usually made in a factory. The rafter roof gives freedom to shape the attic, while the truss gives a tested, repeatable frame. The internal web members of a truss occupy attic space that a bare rafter roof leaves free.