Steel beams and padstones explained
A steel beam carries the load from the structure above across a new opening, and a padstone at each end spreads that load safely into the wall below. Both have to be designed together, because a strong beam on a weak bearing still fails.
What do a steel beam and padstone actually do?
When you remove a load-bearing wall or open up the back of a house, the floors, walls and roof that the wall used to support still need holding up. A steel beam picks up that load along its length and delivers it to two points, one at each end. That is where the padstones come in.
The load arriving at the end of a beam is concentrated on a small area. Old London brickwork, often soft stock bricks in lime mortar, can crush or crack under that kind of point load. A padstone is a block of concrete or dense engineering brick that sits under the beam end and spreads the load over a larger area of wall, so the stress in the brickwork stays within what it can safely carry.
Common steel sections in London homes
Builders often call any steel beam an RSJ (rolled steel joist), but that term strictly refers to an older section shape. Today the engineer will specify one of a handful of standard sections, chosen for the load, the span and the space available.
- Universal beam (UB): the most common choice, deeper than it is wide and efficient in bending, typical for knock-throughs and rear openings
- Universal column (UC): squarer and stockier, used as a column or as a beam where headroom is tight
- Parallel flange channel (PFC): a C-shaped section often used in pairs, or alongside timber joists in lofts
- Flitch beam: a steel plate bolted between timber joists, useful for lighter loads where a full steel section is not needed
- Goalpost or box frame: beams and columns connected together, used where the walls either side cannot take the load or the structure needs extra stiffness
How the engineer sizes the beam
Sizing starts with a load take-down. The engineer works out everything the beam supports, from the floors and walls above to the roof and sometimes a chimney stack, and adds the imposed loads from people and furniture set out in the Eurocodes.
The beam is then checked for bending, shear and deflection. Deflection often governs in houses, because a beam that is strong enough can still sag enough to crack plaster or jam doors upstairs. The engineer also checks that the beam is restrained against buckling sideways, which usually comes from the floor joists bearing onto it. Our guide to structural calculations explains how these checks are set out for Building Control.
The engineer also specifies the steel grade, usually S275 or S355 on domestic work, and the beam size is only valid for that grade. If a supplier offers a substitute section or a lower grade because of stock, the builder should check with the engineer first. A beam that looks similar on site can have noticeably less capacity.
Padstones: why the ends matter most
The bearings are where many problems with steel beams start. A beam can be perfectly sized and still cause cracking if it sits on a single brick or a crumbling pier. The engineer checks the bearing stress on the masonry and specifies a padstone large enough to bring it within safe limits.
Padstones can be precast concrete blocks, cast in place on site, or built from engineering bricks. The engineer sets the size, the minimum length the beam must bear onto it, and how close it can sit to the edge of a wall or opening. Where the end of a beam lands on a narrow pier, the pier itself and the foundation beneath it may need checking too, especially in a terrace where the pier carries loads from several floors.
When the walls cannot take the load
Sometimes there is no suitable masonry for the beam to sit on. A wide rear opening in a terrace can leave only a short return of brickwork, and a side return extension may remove the wall entirely. In those cases the engineer designs steel columns that carry the beam ends straight down to new pad foundations.
Where most of the back wall of a house is removed, the structure also loses some of its resistance to sideways forces such as wind. The engineer may then specify a goalpost frame with rigid connections so the steel holds itself square. These connections are designed to suit the frame, and our steel connection design service covers them where a fabricator or builder needs the detail.
Installation, packing and fire protection
Getting steel into a London terrace is often harder than designing it. Long beams may have to be spliced into shorter lengths and bolted together inside the house, and any splice must be designed by the engineer rather than improvised on the day. The order of work, including temporary props and needles, should follow the engineer's propping guidance.
Once the beam is on its padstones, any gap between the top flange and the masonry above must be packed tight with steel or slate packers and dry-pack mortar. If it is left loose, the wall above drops onto the beam over time and cracks appear. Steel inside a house is normally fire protected with fire-rated board or intumescent paint, and steel in an external wall or cavity needs corrosion protection. Whether the beam should be a downstand or sit flush in the ceiling is covered in our guide on removing a load-bearing wall.
Common mistakes to avoid
Most faults with steelwork are visible before it is boxed in, which is why Building Control wants to see the beam at that stage. These are the issues that come up most often on London jobs.
- Beams bearing directly onto brickwork with no padstone, or onto a padstone smaller than specified
- Gaps above the beam left unpacked, so the wall settles onto it later
- Flanges notched or holes cut on site for services without the engineer's agreement
- Steel sizes chosen by rule of thumb rather than calculation
- External or cavity steel left without any corrosion protection
- Props removed before the padstone mortar or concrete has gained strength
Frequently asked questions
What is the difference between an RSJ and a universal beam?
An RSJ, or rolled steel joist, is an older steel section with tapered flanges. The term has stuck in everyday use, and builders still say RSJ when they mean any steel beam. Modern designs normally use universal beams, which have parallel flanges and come in a wide range of standard sizes. When the engineer's drawing specifies a section, the builder should order exactly that size and grade.
Can a timber beam be used instead of steel?
Sometimes. For short spans and light loads, such as a doorway in a timber stud wall, an engineered timber beam or a flitch beam can work well and is easier to handle. For wider openings, or where the beam carries walls and floors from storeys above, steel is usually needed because a timber beam of the same strength would be much deeper. The engineer will advise which suits the loads.
How far should a steel beam sit onto the wall at each end?
The engineer sets the minimum bearing length in the calculations, and it depends on the load, the padstone and the masonry. On domestic work it is commonly at least 100 mm and often more for heavier loads. The builder should never shorten a beam or reduce the bearing on site to make installation easier. If the beam will not fit, ask the engineer before cutting anything.
Do steel beams in a house need fire protection?
Usually yes. Building Regulations require the structure to keep its strength for a set period in a fire, and bare steel loses strength quickly when hot. In most houses the beam is boxed in with fire-rated plasterboard or coated with intumescent paint. Building Control will confirm the period needed for your property, which can be longer in flats and conversions than in a single house.
Who checks that the steel has been installed correctly?
Building Control inspects the steel before it is covered, checking it matches the approved design. An engineer can also visit to confirm the beam size, padstones, bearings and packing are as specified. Our structural steelwork inspection report records this in writing, which is useful when Building Control asks for confirmation or when a buyer later wants evidence that the work was done properly.