Flitch Beams Explained: When to Use a Steel and Timber Composite Beam

Flitch beams come into their own in a specific set of circumstances.

The most common is where you need to match an existing floor depth. In older properties, particularly Victorian and Edwardian terraces, floor zones are tight and you cannot drop a steel beam below the existing floor line. A flitch beam can often be built to fit within the depth of the existing floor joists, because the steel plate sits within the timber rather than below it.

They are also useful where timber connections are preferable to steel ones. Joists can be face-fixed or hung from a flitch beam using standard timber joist hangers, which is often simpler and faster on site than drilling and bolting into a steel flange.

Some builders also find flitch beams easier to handle in awkward spaces. A steel plate and two timbers are lighter and easier to manoeuvre up a narrow staircase than a solid RSJ of equivalent length.

Flitch beams are not a universal solution. They have a lower load capacity than a comparable RSJ, so for heavily loaded spans they are not the right choice. They are also more labour-intensive to fabricate and assemble on site, which can add time to a job.

Moisture is another consideration. If a flitch beam is going somewhere that could get wet, you need to think carefully about both the timber species and any protective coating on the steel plate. Untreated timber and bare steel in a damp environment will cause problems over time.

For longer spans or higher loads, your structural engineer will almost certainly specify a standard universal beam or another steel section rather than a flitch beam. Do not try to substitute one for the other without structural sign-off.

Sizing a flitch beam is a job for a structural engineer. The calculation takes into account the span, the load above (which floor, what load), the depth available, and the combined properties of the steel plate and timber sections. It is not something to estimate from a table in the same way you might size a standard RSJ, because the composite nature of the beam adds variables.

Your engineer will specify the plate thickness, the timber section size, and the bolt pattern that holds it all together. Make sure you build it exactly as specified.

One of the most frequent errors builders make with flitch beams is under-bolting the assembly. The bolts are not just holding the three elements together during installation; they are what makes the beam act as a composite unit under load. If the bolt spacing does not match the engineer’s specification, the steel plate and timbers will behave independently rather than together, which significantly reduces the beam’s capacity. Always follow the bolt pattern on the drawing exactly, and do not substitute a different bolt diameter or grade without going back to the engineer first.

Another common issue is using green or unseasoned timber. As the timber dries out and shrinks, it can loosen the bolts and introduce movement into the assembly. Where possible, use kiln-dried timber to the specified section and check that it is within the moisture content range your engineer has allowed for.

Finally, make sure the bearing at each end is adequate. Like any beam, a flitch beam needs sufficient bearing length on its support to transfer the load safely into the wall or post below. This is something your engineer will specify, but it is worth double-checking on site before you pack out and make good.