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by the NFCC

Engineered timber


Solid timber joists are being replaced by engineered products such as timber I-beams and metal web joists which are being used as floor joists. These are lightweight and rigid and can span much larger distances without the need for intermediate structural support.

General considerations

Engineered timber is reliant on fire protection measures. It will provide fire resistance for a defined period of time as a complete system under standard fire testing. This differs from traditional timber which has inherent fire resistance relative to the amount of excess or sacrificial timber that is present.

Engineered joists usually have a smaller cross section than traditional timber joists. As these joists have a relatively open structure firefighters need to be aware that this creates a continuous floor void unlike traditional timber floor joists. Also this open structure allows for services to be easily passed through the floor voids.

Holes can be created in timber I-beams to pass building services through which may weaken the beams however; these holes can be pre-drilled and part of the design.

Services are easily passed through steel web beams due to their open structure.

Inherent benefits

  • Can often be identified from building type and age with reasonable probability (for example, domestic buildings constructed since 2005)

Inherent hazards

  • Prone to rapid failure once fire protection is breached
  • Usually hidden behind passive fire protection and therefore not readily visible for identification
  • Concealed firespread in floor void

Further information

Modern methods of house construction: a surveyor's guide, BRE Trust

A guide to modern methods of construction (NF1), NHBC Foundation