Flitch beam design
A flitch beam sandwiches a steel plate between timber, giving the strength of steel with the fixability of timber. We design flitch beams and their bolt patterns to the Eurocodes for clients across the UK.
What is a flitch beam and why use one?
It is a steel plate bolted between timber members, acting as one beam. It is chosen where you want the strength of steel but the ability to nail joists directly to timber, a lighter beam that can be handled by hand, or a shallower beam than an all-timber solution.
When a flitch beam is the right answer
A flitch beam is a composite of one or more steel plates bolted between timber members, working together as a single beam. It earns its place where a plain steel beam would be awkward. The timber faces let joists and rafters be nailed or screwed directly to the beam, which a bare steel section will not allow without extra timber packers. The finished beam is lighter than an equivalent steel section, so it can be carried in and lifted by hand into a loft or a restricted space where craning a heavy universal beam is impossible. It is also shallower than an all-timber solution of the same capacity, which helps where headroom is tight.
The trade-off is that a flitch beam is more design work than a single steel section, because it is two materials sharing load. We size it where the practical benefits, hand handling, direct timber fixing, a slimmer depth, outweigh the simplicity of a UB. Typical uses are loft conversion beams that must be manoeuvred up a staircase, ridge and floor beams in period buildings where a steel would be too heavy to place, and openings where joists need to hang off the beam without steel connectors.
Load sharing and the bolt pattern
The design principle is that the steel and timber deflect together, so they share the load in proportion to their stiffness, their modulus times their second moment of area. Steel is far stiffer than timber, so the steel plate carries the large majority of the bending, while the timber stabilises the plate against buckling, provides the fixing surface and takes its share of the load. We calculate the stiffness of each component, distribute the applied moment between them, and check the steel plate and the timber flitches each against their own material limits, the steel to BS EN 1993-1-1 and the timber to BS EN 1995-1-1.
The bolts are what make the sandwich act as one beam, and their pattern is a genuine calculation, not a nominal grid. The bolts transfer the horizontal shear between the timber and the steel so the layers do not slide past one another, and they hold the thin steel plate against buckling out of plane. We set the bolt diameter, spacing and edge distances so the shear flow is carried and the plate is restrained, spacing them more closely near the supports where the shear is highest. The plate is drilled to match, and we give the timber grade, plate thickness and the full bolting schedule so it is built correctly.
Bearings, deflection and buildability
At the supports the reaction comes largely from the steel plate, so the bearing detail has to deliver the plate onto a padstone or bearing that suits, not just the timber faces. We check the bearing and specify the support so the steel is properly seated. Deflection is checked on the combined section under service load and kept within span/360, or tighter where finishes or masonry above are sensitive, because the whole reason for the beam is often a clean, crack-free opening.
Buildability is a real advantage of the flitch beam and we keep it that way. The components are sized so they can be carried to the position and bolted up in place, and we note whether the beam is assembled on the ground and lifted or built up in situ. The output is a clear specification, the plate size and grade, the timber sections and grade, the bolt schedule, bearings and any propping, ready for the builder and Building Control. We deliver these designs across the UK, with site visits arranged by region where the loft or opening needs surveying first.
What we check.
The points our calculations resolve for a project like this.
- Load shared between steel and timber in proportion to their relative stiffness
- Steel plate checked to BS EN 1993-1-1, timber flitches to BS EN 1995-1-1
- Bolt diameter, spacing and edge distance sized for the shear flow between layers
- Steel plate restrained against out-of-plane buckling by the timber and bolts
- Bearing detail that delivers the plate reaction onto a suitable padstone
- Assembly method, hand handling or in situ build, confirmed for buildability
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
What is a flitch beam and why use one?
It is a steel plate bolted between timber members, acting as one beam. It is chosen where you want the strength of steel but the ability to nail joists directly to timber, a lighter beam that can be handled by hand, or a shallower beam than an all-timber solution.
When is a flitch beam better than a steel beam?
When a plain steel beam would be too heavy to carry into a loft or restricted space, when joists need to fix directly to the beam without steel connectors, or when a slimmer beam is needed for headroom. If none of those apply, a single steel section is often simpler.
How much does flitch beam design cost?
Our fee starts for a flitch beam calculation and full specification, fixed before we begin. Multiple beams or an unusual build-up will affect the fee, which we agree up front.
Why does the bolt pattern matter so much?
The bolts make the steel and timber act as a single beam by transferring the shear between them and holding the steel plate against buckling. If the pattern is wrong the layers slide and the beam does not reach its capacity, so we calculate the bolt size and spacing rather than using a nominal grid.
Can a flitch beam be assembled on site?
Yes, and that is often why it is chosen. It can be carried up in parts and bolted together in position, which suits lofts and tight spaces. We confirm on the specification whether it is assembled on the ground and lifted or built up in place.
Related in structural design & elements.
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Send the drawings or describe the project. We confirm scope, deliverables and a realistic timescale.
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