A collar roof is a roof where the ceiling level is above eaves level. These are often found in older properties and if the property is being extended sometimes the new roof is formed in a similar way to match the existing. New properties may also be designed like this, particularly if planning restrictions limit the height of the building.

The problem with this type of roof is that there is no tie between the rafter feet and the rafters impose horizontal forces on the supporting walls which tend to push the walls out. There are two solutions with this type of roof. the first is to introduce purlins to support the roof structure and the second solution is to design the roof framing to limit the horizontal deflection of the walls to an acceptable level. If the roof is long and there are no internal loadbearing walls, then purlins are not a practical option. In these circumstances the rafters and ceiling joists have to be designed as a series of plane frames.

In this type of roof, the bending stresses in the rafters can be much higher than in a normal tied roof and the connection between the rafters and the collar needs to be properly designed.

Our collar roof truss design includes carrying out a detailed structural analysis of the truss. Our structural design also includes preparing the calculations for the joint connections and an assessment of the horizontal deflection of the truss.

We need the following information to design a collar roof:.

the span of the roof truss

the height of the ridge above eaves level

the height of the collar above eaves level

the height of the eaves level above floor level

Design information

All design work is carried out in accordance with BS5268 (The structural use of timber).

The calculations provided will be suitable for Building Regulation approval.