Flitch Beam Calculations

Structural Calculations for Flitch Beams

A timber and steel flitch beam (often just called a flitch beam) is a composite structural beam made by sandwiching a steel plate between two timber beams, then bolting them together so they act as one unit. These are commonly used in domestic structural applications such as loft conversions and roof constructions.

Structural Calculations for Flitch Beams are usually required when these beams are utilised in place of a steel beam or when added strength is required to a standard timber member. A flitch beam is a composite structural member used primarily in domestic building construction, consisting of one or more steel plates (“flitch plates”) sandwiched between timber members and mechanically fastened so the materials act together to resist bending.

Structural Calculations for Flitch Beams Technical Description

Structural Calculations for Flitch Beams. A flitch beam is a hybrid flexural element formed by combining dimensional timber or engineered wood members with a steel plate, typically arranged in a wood–steel–wood configuration. The components are usually fastened using bolts, or structural screws at specified spacing to ensure composite action under load.

The steel plate, possessing a significantly higher modulus of elasticity and yield strength than timber, carries a disproportionate share of the tensile and compressive stresses induced by bending. The surrounding timber provides lateral stability, protects the steel from fire exposure to some extent, and facilitates connections to adjacent structures.

Structural Behaviour

  • Load sharing is governed by strain compatibility, assuming no slip between materials when properly fastened with bolts or structural screws.

  • The beam’s effective stiffness (EI) is calculated using transformed section analysis, converting the steel plate area into an equivalent wood area based on modular ratio.

  • Shear transfer between timber and steel is resisted by the mechanical fasteners, which must be designed for horizontal shear flow.

  • Deflection control is often superior to all-timber beams of similar depth due to the steel plate’s contribution and  higher EI.

Applications

  • Timber beams carrying relatively light loads such as floor and roof trimmers in loft conversions.

  • Situations with depth limitations where a full steel beam would be impractical

  • Retrofit or reinforcement of existing timber beams

Structural Calculations for Flitch Beams Design Considerations

  • Bolt size, spacing, and edge distances per applicable British Standard design codes

  • Differential shrinkage of wood relative to steel

  • Corrosion protection of the steel plate

  • Fire-resistance and encapsulation requirements as most loadbearing elements in domestic situations require 30 minutes.

  • Constructability and inspection access

A flitch beam is a mechanically fastened, timber–steel composite beam designed to act as a single flexural member, leveraging the stiffness and strength of steel with the constructability and connection compatibility of timber. Mainly used in loft conversions and roof structures.

If you need accurate, compliant flitch beam calculations for your project, our approach helps you move forward with confidence. Whether you’re working structural alterations, or loft conversions, we provide clear flitch beam calculations that support approval and construction.

Get in touch today on 07376 805039 or email  [email protected] to discuss your requirements and find out how our structural calculations can support your next project.