The load-bearing structure of a roof is the structural frame system that receives all loads applied to the roof and transfers them through the walls and foundations to the ground. Load-bearing structures include roof trusses, roof beams, rafters, and related bracing structures. The dimensioning of the load-bearing structure is the responsibility of the structural engineer and is based on Eurocodes and the Finnish National Annex. The dimensioning of the roof's load-bearing structure takes into account several loads: the self-weight of the structure, the weight of the roofing material, snow load, wind load, and possible imposed loads (for example, solar panels or walkways). In Finland, snow load is often the governing factor — its characteristic value ranges from 2.0 kN/m² in Southern Finland to 3.5 kN/m² in Northern Finland and Lapland. Structural capacity problems in roof structures generally manifest after an exceptionally heavy snow winter or when the roof is loaded with unplanned additional loads. In the worst case, exceeding the structural capacity leads to collapse, examples of which exist in Finland particularly in large hall buildings.
Roof trusses and rafter types
The roof truss is the most common roof structure in detached houses and small buildings. It is a factory-manufactured triangular lattice structure consisting of upper and lower chords and diagonal web members. The advantage of a roof truss is its large span relative to the amount of material used — a typical roof truss spans 8–12 metres without intermediate supports.
Nail plate trusses are the most common type. They are manufactured from sawn timber (usually C24 strength class spruce) and joints are made with galvanised nail plates. Design is carried out using computer software that optimises material usage according to the loads. The typical truss spacing is 600–900 mm centre to centre.
In large buildings, glulam beams, steel trusses, or concrete elements are used as the load-bearing structure. Glulam beams enable spans of up to 30 metres and are particularly suitable for hall buildings and church interiors. Steel trusses are common in industrial halls where large open spaces without intermediate columns are required.
Load dimensioning and snow load
The roof's load-bearing structure is dimensioned in accordance with Eurocode EN 1991 and the Finnish National Annex. Partial safety factors are used in the dimensioning, increasing loads by a safety margin. The ultimate limit state verification ensures the structure does not fail, and the serviceability limit state verification limits deflection to an acceptable level.
Snow load is the most significant variable load in Finland. The characteristic value of ground snow varies regionally from 2.0 to 3.5 kN/m², and the snow load on the roof also depends on the roof shape and slope. A low-pitched roof accumulates more snow than a steep one. Roof shape coefficients also account for snow drifting — in roof valleys and alongside walls, the snow load can be several times the uniform load.
Wind load acts on the roof as both pressure and suction. In particular, roof eaves and corners are subject to high wind suction forces that tend to lift off the roofing material and even the load-bearing structure. In coastal areas and at elevated locations, wind load is a significant design factor.
Load-bearing structure problems and strengthening
The most common problems with load-bearing structures are moisture damage, overloading, and design errors. Moisture weakens the strength of timber structures over time — a decayed roof truss can lose its load-bearing capacity significantly. Overloading occurs when loads not accounted for in the original design are added to the roof, such as heavy tile roofing replacing metal sheeting or large solar panel systems.
The condition of the load-bearing structure is assessed by a structural engineer. The assessment includes a visual inspection, moisture survey, and load calculations if necessary. In timber structures, the tap test reveals the worst decay damage — decayed wood sounds hollow. The condition of nail plate connections is checked visually for nail plate embedment and timber splitting.
Strengthening of the structure is possible but always requires a structural engineer's calculations. The most common strengthening methods are adding supplementary members to the roof truss, adding trusses at closer spacing, and strengthening connections. During a roof renovation, a structural engineer should always verify the load-bearing capacity of the existing structure for the new roofing material.
Related terms
Content reviewed and verified
Updated: April 2026
