When do you need a structural calculation?
As soon as an intervention affects the main load-bearing structure of a building, the municipality asks for a structural calculation. This applies to visible expansions, but equally to internal changes that may seem harmless at first glance. An extension with its own foundation, the removal of a load-bearing wall in a terraced house, the installation of a dormer with a load-bearing structure or the building of a roof structure all require a separate calculation in which loads, force distribution and dimensioning are recorded.
Common situations in which a structural calculation is required:
- An extension or annex with a new foundation and steel lintels.
- The removal of a load-bearing wall on the ground floor or upper floor.
- A roof structure or ridge increase that creates a complete new storey.
- A dormer with a load-bearing wall or large span on an existing roof.
What is in the calculation?
A complete structural calculation begins with a starting-points memorandum in which the materials used, load classes, consequence class (CC1, CC2 or CC3) and reliability class are recorded. This is followed by the load assumptions: self-weight of the structure, imposed load, snow and wind load per location. The force distribution shows how these loads are transferred via floors, beams and columns to the foundation.
For each component, a dimensioning follows: cross-section check, buckling check, deformation check and deflection check. Steel lintels are selected from standard profiles (HEA, HEB, IPE) based on span and load. Timber joist floors follow from a strength calculation on the relevant type of wood. Connections between steel-steel, steel-timber and timber connections are worked out separately. The result is a report that can be reviewed by the municipality and used directly by the contractor when ordering material.
Eurocodes and NEN standards (NEN-EN 1990-1999)
Structural calculations in the Netherlands follow the Eurocodes, published as NEN-EN standards with a Dutch national annex. NEN-EN 1990 describes the basis of structural design. NEN-EN 1991 covers loads on structures, including self-weight, imposed loads, snow and wind. NEN-EN 1992 concerns concrete structures, NEN-EN 1993 steel, NEN-EN 1994 steel-concrete composites, NEN-EN 1995 timber and NEN-EN 1996 masonry. NEN-EN 1997 covers geotechnics and foundations, NEN-EN 1998 seismic load and NEN-EN 1999 aluminium.
For most residential construction, NEN-EN 1990, 1991, 1992, 1993 and 1995 are leading. The municipality explicitly checks whether the correct standards have been applied and whether the national annex has been correctly incorporated. The Buildings (Living Environment) Decree refers directly to these Eurocodes as the implementation of the structural-safety requirement. A calculation that meets these prevents additional questions during permit processing and gives the contractor a workable basis for execution.
What does a structural calculation cost?
A structural calculation is available from €499 including VAT. The price depends on the complexity of the intervention: the number of components to be dimensioned, the nature of the structure (steel, timber, concrete or combinations) and the extent to which existing data is available. A simple calculation for a steel lintel above an opening in a terraced house is quicker than a complete roof structure with multiple spans, a new floor and an adapted foundation.
In the quote, the scope, lead time and any additional items are clearly stated in advance. For contractors with multiple projects, annual agreements are possible, with fixed rates and short lead times so the permit application does not become a bottleneck in the planning.
Difference from a structural drawing
A structural calculation and a structural drawing (also called a construction drawing) belong together but are not the same. The calculation is the mathematical proof that the structure meets the Eurocodes, with load assumptions, force distribution and dimensioning. The structural drawing is the graphic result: a drawing on which beams, columns, lintels and reinforcement are drawn with their dimensions, positions and connection details.
The municipality often asks for both documents at the same time. On site, the contractor mainly works with the drawing, but falls back on the calculation for substantiation when in doubt. In practice, the drawing and calculation are worked out in parallel so that any changes are processed in both documents immediately and no discrepancies arise between design and execution.
