Cut roof or trusses: what is actually being decided?
With a pitched roof, people usually ask first about the covering. The more important decision sits one layer lower. The load-bearing structure decides whether the attic can become living space and how quickly the house is under a roof. It also decides who answers for the structural design and how much freedom remains in the roof shape. The load-bearing structure of a roof must meet the requirements of mechanical resistance and stability (§ 26 ods. 2 of Decree No. 532/2002 Coll.).
In Slovak, the word krov covers the whole load-bearing structure of a pitched roof. In practice it hides two approaches. A carpenter-built cut roof is designed in the project. Carpenters assemble it on the house from separate members: rafters, purlins, posts and collar ties. A prefabricated truss is a triangular frame made in a factory in which the members are joined with pressed metal nail plates. The trusses arrive finished and a crane sets them side by side. Both approaches lead to a sound roof. They differ in what can be done with the space beneath it and in when the decision has to be made.
How does a carpenter-built roof work?
The basis is the rafter, a sloping beam from the eaves to the ridge. Rafters stand at regular spacing, and the way they are supported defines the type of system. The structural engineer sets the spacing and the section of the rafters for the specific roof. No single value is valid for every house.
Rafter and collar-beam systems
In the simplest rafter system, two rafters lean against each other at the ridge. At the bottom they bear on a wall plate on the external wall. As the span grows, the rafters sag and push the walls outwards. That is why a horizontal collar tie goes between them. It stiffens the pair of rafters and shortens their bending length. A collar-beam system leaves the space below the collars open, which suits a habitable attic. The collars often form the ceiling in the highest part of the room.
Purlin system
For larger spans and more complex shapes, a purlin system takes over. A purlin is a longitudinal beam that supports the rafters at mid-length and itself rests on posts or on internal load-bearing walls. It allows freer shapes, dormers and hips, but the attic layout has to respect the posts and struts.
What are trusses and when do they make sense?
A truss works as a flat girder in the shape of a triangle. The top chord carries the roof layers. The bottom chord also forms part of the ceiling structure. The web members between them distribute the forces. A truss usually spans the full width of the house without internal supports.
Trusses are supplied by a specialist manufacturer from the designer's inputs. The web members fill the attic and must not be cut or removed after erection, because the truss would stop working. With standard trusses the attic therefore stays unusable. Trusses with an open room in the middle exist, but they must be ordered from the start.
Trusses suit a house where you know the attic will not be lived in and the plan is simple. A carpenter-built roof suits a house with a habitable attic, now or later. It also suits a roof with dormers and an articulated shape. Make the decision before the timber is ordered.
The prices of the two solutions cannot be compared in general. They depend on the span, the roof shape, the covering and the scope of the quote. Ask for quotes on both solutions for the same project and the same scope, including erection, crane and anchoring.
Gable, hip or mansard roof?
The shape of the roof shows up in the complexity of both the structure and the covering. Every hip and valley means angled cuts, flashings and more work on the covering. The simplest is the gable roof with two planes and two gable walls, which also gives the most usable attic. A hip roof has sloping planes on all sides and a lower silhouette, but the attic narrows from every side. A mansard roof has two pitches on each side: a steep lower one and a flatter upper one. It gives almost full-height attic rooms, but it is the most demanding structurally.
That is why it pays to let the roof shape follow from a simple floor plan. Every projection adds a valley, where water and snow meet.
What roof pitch can I choose?
Roof pitch is not only a matter of taste. Three things constrain it.
- The land-use plan. If the binding part of the plan or the planning authority's binding opinion sets conditions for the roof, the building intent must meet them. The office checks compliance with the binding opinion of the planning authority (§ 57 ods. 2 písm. c of Act No. 25/2025 Coll.). Read the municipality's conditions before the first sketch.
- The covering. The manufacturer of the covering sets the smallest pitch in its documentation. Building works are to follow the instructions of the manufacturers of construction products (§ 27 ods. 2). At a lower pitch, additional measures set by the manufacturer are needed.
- Snow. A pitched roof with planes steeper than 25° must have a snow guard (§ 26 ods. 6 of Decree No. 532/2002 Coll.).
Decree No. 532/2002 Coll. stays in force until the new technical decrees apply. That is at the latest 31 March 2027 (§ 84 ods. 23 of Act No. 25/2025 Coll.).
What makes up a roof from rafter to covering?
A roof structure must protect the building from the outdoor climate and drain rainwater. It must also hold snow and ice so that they do not endanger pedestrians (§ 26 ods. 1 of Decree No. 532/2002 Coll.). It must meet the thermal requirements of Slovak technical standards (§ 26 ods. 3). Its design must prevent condensation of water vapour. A roof in which vapour condenses may count as acceptable only if this does not endanger its function and service life (§ 26 ods. 4). A typical build-up over a habitable attic looks like this, from outside in:
- The covering sheds most of the water.
- Roof battens carry the covering, and the type of covering sets their spacing.
- Counter-battens run down the slope over the rafters. They form a ventilated gap under the covering, with air intake at the eaves and outlet at the ridge.
- The underlay membrane is a vapour-open sheet that catches wind-driven water under the covering and leads it to the eaves while letting water vapour escape.
- Insulation between the rafters.
- Insulation below the rafters covers the thermal bridges through the rafters and gives room for services.
- A vapour barrier stops humid indoor air from reaching the insulation. It must be taped airtight at the walls and around penetrations.
- The internal lining, usually plasterboard on a batten grid.
The thickness of the thermal insulation is calculated under technical standards. If it exceeds the depth of the rafters, insulation is added below or above them. The design sets the exact order and materials according to the manufacturer's system.
Why does the structural engineer size the timber?
A roof carries more than its own weight and the covering. Snow load is set by Eurocode 1, titled Actions on structures, and its Part 1-3 covers snow (per the ÚNMS SR catalogue). National annexes to the Eurocodes contain country-specific data, such as climatic data (ÚNMS SR). Wind is added, lifting the roof in places, and snow drifts unevenly at valleys and changes of pitch. The sections of rafters, purlins and joints are designed under Eurocode 5 for timber structures (EN 1995). Since April 2010, only the Eurocodes have been used to design building structures in Slovakia (ÚNMS SR).
That is why you should not copy roof dimensions from a neighbour or another project. The same roof shape with a different covering, at a different altitude or with photovoltaics added later needs a different calculation. Leave the dimensions and rafter spacing to the structural engineer. The standards are paid for, so there are no span tables here.
Do I need a permit to repair or replace a roof?
Replacing the covering, gutters and carpentry roof products is maintenance. It needs neither a decision on the building intent nor a notification (§ 18 ods. 5 písm. a bod 1 of Act No. 25/2025 Coll.). Maintenance needs a notification if it could affect stability, fire safety, the appearance in public space or the environment (§ 18 ods. 4 písm. c).
Renewal of the roof envelope is a building alteration (§ 2 ods. 9 písm. a). It needs a notification in the cases listed in § 18 ods. 3. Examples are a cultural monument, a heritage zone, or work that could endanger mechanical resistance and stability (písm. f, g, h). When you replace the roof structure, ask the building office which case applies to you. This text follows the version in force until 31 December 2026. Amendment No. 243/2026 Coll. changes § 18 and § 2 from 1 January 2027.
Can the attic be converted to living space later?
Yes, but the preparation has to be done now. An attic conversion needs four things that are hard to add afterwards:
- A structure that leaves the space open. A collar-beam or purlin system, not standard trusses.
- A floor designed for residential loads. The structural engineer decides whether a later change needs assessment and strengthening.
- Room for a staircase. A used loft space needs at least one main staircase (§ 27 ods. 1 of Decree No. 532/2002 Coll.). Upper limits for the slope of the flights are in § 27 ods. 4.
- Enough height. A Slovak technical standard sets the clear height of a habitable room. The decree allows 2,300 mm as the lowest clear height in the attic. The other microclimate parameters must then still meet the basic requirements for a healthy environment (§ 43 ods. 5).
Creating living rooms in an unused attic may be a vstavba. A vstavba inserts a new structure into the existing volume and keeps the outer dimensions (§ 5 ods. 2 of Act No. 25/2025 Coll.). Raising a building by an attic is a nadstavba (§ 5 ods. 4). Both are changes of a completed building. They need a decision on the building intent unless the act says otherwise (§ 18 ods. 2). Changing the use of individual rooms is not a change in the use of the building. The original function must stay and fire safety must not get worse (§ 68 ods. 1). Check with the building office what applies to your house.
Collar ties can carry the ceiling in the highest part of the room. Do not cut or move them without an engineer, because they hold the shape of the whole roof. For the dormer and for renovation, see the article on insulating the roof and attic.
How do I decide?
If you know the attic will never be lived in and the plan is simple, consider trusses. If the attic is to be lived in now or later, choose a carpenter-built roof, most often a collar-beam roof. Size the floor below it for residential loads from the start. The roof structure also relates to what the rest of the house is built from. See the article on what to build a family house from. The question "cut roof or trusses" is really about what you want to do with the attic in ten years. Answer it before the timber is ordered.