Notes
What size steel beam do I need, and what does it sit on?
It depends on what the beam carries, not just the width of the opening, and it is sized by calculation. Here is what decides it, and what each end needs under it.
What decides the size of a steel beam?
Most people call it an RSJ. Whatever the name, these set its size:
- Span. Bending grows faster than the span, and sag faster still: the span lesson shows why.
- What it carries. Floors, walls, the roof, and anything landing on it at one point: a post, a roof strut, another beam.
- How far it may bend. A beam strong enough to be safe can still sag enough to crack a ceiling. That limit often decides the size.
- Restraint. A beam held sideways along its top, by a floor fixed to it, carries more than one left free to twist.
- Depth. A beam hidden in the floor can be no deeper than the floor, so it may be heavier instead.
- Grade. A stronger steel carries more, but bends just as much, so where sag decides, it does not help.
- What it sits on. How far each end bears, and what is under it.
Drawings made for this site. True proportion, not a design.
Key to the drawings
- Existing masonry A flat tone: everything already there, cut or seen.
- Concrete Aggregate triangles and fine dots, scattered at random: foundations, slabs, padstones, lean-mix cavity fill, encasement.
- Timber, cut A box with a cross. Seen along its length, an outline only.
- Steel, cut Solid, at the section's true proportion.
- Load path Where the load goes, from the roof to the ground; dashed where it runs beyond the cut.
- Load marks Arrows at the symbol weight: loads down onto a member, bearing up under a foundation.
- Hidden, beyond or removed Out of the cut, beyond it, or taken out.
- Break line The drawing stops; the building does not.
Why can two openings of the same width need different beams?
Because the width is only one of the inputs. Between a kitchen and a dining room, a beam may carry a strip of floor and nothing else. The same width under a wall that runs up to the roof may carry the floor, that wall, part of the roof and a strut landing on it. That is why which way the joists run is the first thing we check.
Can I use a size from a table, or from a similar job?
Not as a design. Published tables cover only the plainest cases, under a list of conditions. A similar job had different loads and different walls under its ends. And the beam is only part of the answer: the calculation also sizes the padstones, checks the walls and foundations under each end, and says how the beam is held sideways and protected from fire.
What does Building Control need?
Taking out a wall that carries load is building work, so Building Control sees it. For a beam they ask for calculations for the beam and its bearings, and a drawing of where it goes and what it sits on. We answer Building Control’s questions on our calculations as part of the job (more in the wall note).
Source: The Building Regulations 2010 (England), regulation 3.
What is a padstone, and what does it do?
A beam gathers the load along its length and hands it down at two small patches, one at each end. Brick and block take load well spread out, and badly concentrated. A padstone is a block of strong material built into the wall under each end, spreading the load over more wall. It sits within the wall’s width, and in a party wall it stops in your half.
Drawings made for this site. True proportion, not a design.
What are padstones made of?
- Precast concrete. The usual choice: made in standard sizes and built in with the courses round it.
- Concrete cast in place, where the size or shape is unusual.
- Engineering bricks or solid concrete blocks, for small ones.
- A steel plate, where there is no depth for a block, designed like the rest.
Whatever it is made of, it is one solid piece.
How big does a padstone need to be?
Big enough to bring the pressure under it down to what the wall can take, so its size comes from the load at the beam end and the strength of the masonry. More load, or weaker blocks, means a longer or deeper padstone; a light beam on good brickwork may need none. It is part of the same calculation as the beam.
Source: NHBC Standards 2025, chapter 6.5, clause 6.5.5 and Tables 6 and 7.
Where do bearings go wrong?
Source: NHBC Standards 2025, clause 6.5.4.2, on openings. Drawings made for this site, not a design.
- Straight on brick or block, where a padstone is needed: the masonry crushes and cracks.
- On a cut brick or loose packing. A padstone works because it is one solid piece; offcuts and slates are not.
- Over a door or window below. The load lands on a lintel, not a wall.
- Too near the end of a wall or an opening, with no wall beside it to spread into.
What we need from you
- Floor plans, or a sketch with the opening marked.
- Photographs of both sides of the wall, and of the ceiling above it.
- What is above: another wall, a bathroom, a water tank, the loft.
- Which way the floorboards run in the room above.