Steel Beam Sizing Explained for Homeowners

Daniel Nikolla

Oct 20

2 min read

Updated: 7 days ago

Steel beam sizes are not guesswork and they are not interchangeable. Each beam is designed for its own span, loading and support conditions, which is why two openings that look the same can need different steel.

Here is how an engineer arrives at a beam size and why copying one from a neighbour or an online table is risky.

What the engineer considers

The engineer works out the span, the weight of floors, walls and roof above, and how that load reaches the beam. The beam is then checked for strength and for deflection, so floors do not sag and finishes do not crack.

Supports and padstones

A beam is only as good as what it sits on. Padstones spread the load into the wall, and the wall or column below may itself need strengthening. Poor bearing is a common reason for cracks after wall removal.

Fire protection and fixings

Steel usually needs fire protection, often boards or intumescent coatings, to meet the regulations. The engineer specifies connections and fixings so the beam works as designed.

Why copying does not work

A beam that worked next door may have carried a different load over a different span, on different walls. Standard tables only apply to specific, limited situations. Always have your own calculations before ordering steel.

Ordering and installation

With calculations in hand, your builder can order the correct section and plan temporary support. Delivery and craneage, where needed, are easier to plan when sizes are confirmed early.

Talk to Graal

Planning work on your home or a small development? Graal Structural Engineering provides structural design and project management support across North London. Call 07737188754 or request a free quote through our contact form and we will tell you what is needed and what it is likely to involve.