A kitchen opening, roof opening and loft opening with the same width may carry very different loads. “Light”, “typical” and “heavy” labels do not define those loads. Our former span-band recommendations have therefore been replaced with a numeric deflection comparison tool.
The span entered in the tool
The tool uses the exact distance between idealised support centres. It does not round a 3.7 m input to a nearby 3.5 m or 4 m table row. Ask the engineer to establish the appropriate model span rather than treating the clear opening as interchangeable with it.
Why a small span change can matter
For the same uniform line load, stiffness and simply supported model, moment varies with L² and elastic deflection with L⁴. Moving from 3 m to 4 m increases moment by about 1.78 times and deflection by about 3.16 times. This is a sensitivity comparison, not a recommendation to use a particular beam.
The formulas and unit conversions are set out in our methodology, with the uniform-load deflection expression referenced to SCI P183.
Describe a domestic opening before comparing sections
Sketch the joist directions, widths of floor supported, walls above, roof support locations and any isolated reactions. Mark both support positions and describe the supporting masonry. Note whether the beam will be exposed or enclosed and whether a flush ceiling is required.
The tool can compare selected UB sections under an assumed uniform service load, adding each section’s self-weight. It does not interpret that sketch, determine the loads or account for point forces. Its result table is a record of calculated effects, not a schedule of permissible spans.
Structural work requires a chartered structural engineer and the applicable Building Control approval. This educational article has not received an independent engineering review.