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Steel frame

a combination of steel and timber frame

Using steel frame for the whole of a house structure is very uncommon in the UK. Usually the clear spans required for a house can be achieved using timber with the incorporation of occasional steel members such as RSJs where masonry spans a window or door. There are published tables of timber spans for standard roof joists, floor joists etc. which comply with the Building Regulations. (See Structural Timber)   It can be very difficult to prevent cold bridging where steel members are incorporated into the external structure especially where Passivhaus or zero carbon standards are to be met

It is becoming increasingly popular to incorporate elements of steel framework into timber frame and kit houses (see RSJs in the center of the picture) in order to achieve larger spans than are normally available with timber. This is often down to the preferences of the engineer doing the calculations rather than being an inherent necessity. Very often laminated timber would do the job better in several ways

  • lower embodied energy
  • less thermal conductivity. As the insulation standards for housing become more stringent, cold bridging becomes more of a problem and steel is a particularly poor insulator. This may be important in outside walls or at the bottom and top of columns. With Passivhaus or code 6 standards it may be necessary to position the steel elements on the inside of the main envelope to avoid thermal bridging.
  • better fire performance (steel usually needs cladding to protect it from fire whereas timber is usually simply increased in width by 25mm per exposed face to give fire protection.)
  • steel erector not required (though for small jobs a general contractor can usually handle the erection work)
  • It’s easier to adapt or modify timber beams and it’s easier to fasten to them.
  • timber columns can usually go straight onto the foundation slab whereas steel requires fixing plates set in the concrete. The point here is that timber columns can be made slightly over length and trimmed on site whereas steel columns usually need nuts and studs or bolts with shims to make final adjustments. See the section on the Segal method.

Building regulations


The Building Regulations part A covers the structure of a building. This Approved Document goes into a lot of detail for traditional masonry buildings but almost none for timber frame, steel frame, earth building SIPs etc. For these you will need to consult a structural engineer (while SIPs structures are usually handled by the manufacturer)

Fire safety

With most forms of construction there will be implications concerning fire safety. These are covered in the Building Regulations and you can see examples of how to conform with these in Part B (Fire Safety)

Site preparation and resistance to contaminants

This section, Part C, covers site remediation along with protection from nasties which might affect the construction and occupants such as damp, rain, radon etc.

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