In short
A flitch beam is a steel plate sandwiched between two timber joists and bolted through, so the three act as one. It carries far more than timber of the same depth, and it can be built in place from pieces light enough to carry upstairs.
How a flitch beam is made
Take two lengths of structural timber, say 47 x 220 mm C24 joists. Put a steel plate between them, typically 8 to 12 mm thick and a little shallower than the timber. Drill through all three and bolt them together, usually with M12 bolts in a staggered pattern every 400 to 600 mm along the length.
The plate does most of the work. Steel is roughly twenty times stiffer than softwood, so even a thin plate takes the larger share of the load. The timbers do two things in return. They stop the slender plate buckling sideways, and they give carpenters something to nail and screw into.
Bigger loads call for more layers: three timbers and two plates is common over a wide opening.
Where a flitch beam earns its place
A flitch beam is rarely the strongest choice. For a given depth a Universal Beam carries more, and it is often cheaper. A flitch wins on practicality, in four situations.
Loft conversions. The beam that holds up the new floor has to get into the roof. A steel beam goes in through a hole in the gable or over the scaffold with a crane. A flitch goes up the stairs as two joists and a plate, and is bolted together where it will sit.
Timber floors. Joists hang from a flitch beam on ordinary hangers nailed to its timber faces. Fixing joists to steel means bolting timber into the web first, which is the same thing done the slow way.
Shallow depth. When a beam must hide within a 220 mm floor, a flitch can do a job that plain timber cannot, without the ceiling coming down to make room for steel.
Trimming openings. Around a stairwell or a large rooflight, a flitch is a neat way to stiffen the trimmer joists.
Where steel is the better answer
Once the span passes about five metres, or the beam carries a masonry wall, the plate has to become so thick that a rolled steel section is lighter, stiffer and cheaper. That is the usual case at the back of a house, which is why the opening into a steel frame extension is formed with a Universal Beam, a goalpost or a box frame, not a flitch.
A flitch is also the wrong choice anywhere damp. The steel sits in permanent contact with timber, and moisture trapped between them corrodes the plate and rots the wood where nobody can see.
Does a flitch beam need an engineer?
Yes. There are no flitch beams in the span tables that cover ordinary floor joists, so building control will ask for calculations. The engineer specifies the timber grade and size, the plate thickness and steel grade, the bolt size and spacing, and the bearing at each end.
Those bolts matter more than they look. They pass load between the timber and the steel, and if they are too few, or the holes are oversized, the pieces slip against each other and the beam sags. Good practice is to drill the timbers using the plate as a template so the holes line up tightly.
Fire
Building control will want the beam to last 30 minutes in a fire if it supports a floor in a typical two-storey house. The timber on each side chars slowly and shields the plate for a time, but the top and bottom edges of the steel are exposed. Most engineers simply specify fire-rated plasterboard below, as for any floor. If the beam is to be left on show, raise that at the design stage, because the timbers may need to be oversized to allow for charring.
Buying one
Steel stockholders and fabricators cut and drill flitch plates to order from the engineer’s drawing, usually within a few days. Some timber merchants sell the whole beam assembled. Give the supplier the drawing itself, not a description: the bolt pattern is part of the design.
On site, check three things before the beam is covered. The plate is the thickness specified. Every bolt has a washer under both head and nut. And the beam sits on the padstone or hanger the drawing shows, with the full bearing length.