RSJ cost: steel beams for extensions

RSJ, UB and UC: what you are buying

RSJ stands for rolled steel joist, an older section with tapered flanges. Builders still use the name for any steel beam, but what arrives on site today is almost always a Universal Beam, or UB: an I-shape, taller than it is wide, with parallel flanges.

Where depth is tight, for example when the beam has to hide inside a floor, the engineer may choose a Universal Column, or UC, laid flat as a beam. It is squarer and heavier for the same strength.

Sections are named by depth, width and weight per metre. A 203 x 133 x 25 UB is about 203 mm deep, 133 mm wide and weighs 25 kg for every metre of length. Since 2014 fabricated structural steel has had to carry CE marking, now UKCA or CE marking, under BS EN 1090-1, so ask the fabricator for the declaration of performance with the delivery note.

What an RSJ costs

These are our national guide ranges, not quotes. Building control charge their own fee for a structural alteration, set by each council. When the beam is part of a whole extension, its cost is normally inside the builder’s price for the job.

Why fitting costs more than the steel

A 5 m length of 254 x 146 x 43 UB weighs 215 kg. Getting that through a house and up to ceiling height is a job for four people or a lifting machine, and it is the smallest part of the work.

Before any brick comes out, the wall above is held on temporary props with steel needles through the masonry. Padstones are cast or bedded at each end to spread the load into the wall below. The beam is lifted in, packed tight to the underside of the brickwork with steel shims and dry mortar, and left to take the load before the props come down. Then come fire protection, plastering and decoration.

That sequence, and the skill in it, is what you are paying for.

Beam, goalpost or box frame

A plain beam needs wall left at each end to sit on. Engineers like to see a return or pier of masonry beside the opening, both to carry the load down and because the back wall helps to stop the house racking sideways.

Take the opening almost the full width of the house and those piers disappear. The answer is a goalpost: a beam welded or bolted to two steel columns, standing on new concrete pads. If the house also needs sideways stiffness put back, a bottom beam joins the two columns below floor level to make a box frame, sometimes called a picture frame.

This is where most of the cost difference lies. A goalpost commonly costs two to three times as much as a simple beam, because of the extra steel, the rigid connections and the digging.

What the rules ask for

Any beam that holds up part of a house is building work, and needs building regulations approval. You, or your builder, submit the engineer’s calculations, and the inspector will want to see the beam, its bearings and its fire protection before they are covered up.

Fire protection is needed wherever the beam carries a floor or a wall: 30 minutes in a typical two-storey house. Two layers of plasterboard or a coat of intumescent paint are the usual ways of providing it.

If the beam bears into a wall shared with a neighbour, the Party Wall etc. Act 1996 applies in England and Wales, and you must serve notice at least two months before the work begins.

Planning permission is not needed for the beam itself. In Scotland the structural work needs a building warrant before it starts.

Keeping the cost down

• Bring the engineer in while the layout is still on paper. Moving an opening 300 mm to leave a pier can turn a goalpost into a plain beam.
• Ask whether a standard section will do. Fabricators stock common sizes, and odd ones cost more and take longer.
• If there is no clear route through the house, have the beam made in two lengths with a bolted splice. It is cheaper than a crane over the roof.
• Have the steel priced with the rest of the extension. A firm already on site with props and labour will fit it for less than a separate visit.