King and queen post trusses are normally made from substantial timber sections and they should not be modified without the alterations being properly designed. A typical modification to a king post or a queen post truss, is cutting through the bottom truss member, to enable a passageway to be added or a door to be fitted. We have seen such alterations carried out where the ends of the newly cut bottom chord have just been supported on timber posts from the floor below ! One of the main function of the bottom chord of the truss is actually to tie the truss together. Without it, the integrity of the truss is lost and gradually the truss will spread out and push the walls with it.

Our design starts with a detailed structural analysis of the forces in the truss, which then enable us to design a ‘U’ shaped steel frame which is capable of resisting the tie force as a bending moment around the new opening. The frame is bolted to the bottom chord of the truss to allow the door opening to be created.



We need the following information to design an alteration to a king post truss: .

the span of the truss (the distance between the supporting walls)

the height of the truss.

the centre to centre distance of the trusses (if there are more than one) or the distance between the centre line of the truss and the supporting walls on either side of it, this is needed to work out the roof loading on the truss

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.