Wooden treads on a metal stair frame

Stairs that combine steel and wood usually follow one of a few types. Two side stringers carry the treads at both ends, with a bracket or plate welded on for each step. A mono-stringer is a single beam in the middle or off-centre, straight or zigzag, with a plate under each tread; it looks light, but the tread overhangs on both sides, so the stiffness of the beam matters. On a cantilever stair the treads seem to grow out of the wall: steel brackets anchored in a load-bearing wall hold them, hidden inside or under the wood.

The frame type shapes the treads directly: their thickness, the way they are fixed and how much steel stays in view.

The frame and the treads are two different trades. A metalwork shop works out the connections, welds the frame, protects it with primer, paint or powder coating and installs it. A joinery makes the treads, any risers and other wooden parts, and fits them. Each answers for its own work, but the quality of the stair is decided where the two meet.

That is why one party should coordinate both: agree the drawings, the plate positions and hole patterns, the gap to the wall, the tread thickness, and who checks what before the next step. Without a coordinator, a millimetre problem in the steel turns into a dispute about the wood.

What Else makes the wooden part. The metal frames are made by our long-standing subcontractors; we have no metal production of our own, but we can coordinate the frame and the treads as one order.

Ordering treads from the drawing before the frame is in place saves a few weeks but risks treads that do not fit: welded frames rarely match the drawing to the millimetre, and walls even less. The usual sequence:

• The stair is designed together with the tread thickness and the fixing details.
• The frame is made, installed, aligned and fixed, and its coating is finished.
• The installed frame is measured plate by plate, with templates taken where the walls are uneven.
• The treads are made to those measurements, finished and left to acclimatise.
• The treads are fitted once the wet works are complete and the room is heated, then protected until the end of the project.

Treads are usually screwed to the steel plates from below, so no fixings show on top. The screws go into pre-drilled holes or into threaded inserts set in the wood, which also lets a tread be removed later. The holes in the plate are slotted or oversized across the grain, because a solid tread changes its width slightly with the seasons and must be able to move without cracking or pulling out its fixings.

Between steel and wood there should be an isolating layer: rubber or felt pads, or a bed of elastic adhesive. It removes the hard contact that drums and creaks underfoot, takes up small unevenness and keeps the wood off bare steel. That matters for oak in particular: its tannins can leave dark stains where they meet iron and moisture, so the plates are primed or painted and the fixings have a corrosion-resistant coating.

A welded frame is accurate to a few millimetres, not to tenths. Plates may sit slightly out of level or out of line, and the joinery deals with that through thin shims, the isolating pads and measuring every plate separately. For the person on the stair, what counts is that every tread is level, every rise is the same height and the nosings form a straight line. A small gap between the tread and the wall leaves room for movement; a skirting or a shadow gap can hide it.

Finished treads usually go in near the end of the work, but painters, electricians and cleaners still pass by. Cover each tread with protective board or heavy card, fixed with tape that does not lift the finish, and never stick tape straight onto fresh lacquer. Keep the stair dry, do not drag tools or materials over it, and remove the protection only for the final clean.