Truss alterations



Alterations to a roof truss

If you plan on making any alterations to a roof truss, then this work should be properly designed before you start.

There are many different types of roof truss, including King and Queen Post trusses, trussed rafters and collar trusses.

King and Queen post trusses

King and Queen post trusses are normally made from substantial timber sections . A typical modification to this type of truss, is cutting through the bottom truss member to allow a door to be fitted.   One of the main function of the bottom chord of the truss is to tie the truss together. Without it, the integrity of the truss is lost and the truss will spread out and push the walls out with it.

Our analysis of a King or Queen Post determines the forces in the truss. This then enables 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.

Trussed rafters

Most modern roofs are built with trussed rafters. These are a series of timber trusses which are normally spaced at around 600mm centres. They generally span between the outside walls of the building. If normal trusses have been used (ie not attic trusses) then the roof space will be full of bracing timbers.

The most common alteration for this type of roof is when a loft conversion is planned. It is possible to strengthen the roof to allow the bracing timbers to be removed so that the space can be used. However, the alterations need to be properly designed. If this type of alteration is not done properly it will lead to all sorts of difficulties later on. At best it will be picked up by a Surveyor when you eventually sell the house. At worst it will lead to excessive deflection or the collapse of the roof.

Our basic design is for a ridged trussed rafter roof. If your roof is complicated with hips and valleys then please get in touch and we will provide you with a custom quote.

The price for our structural calculations for alterations to a trussed rafter includes carrying out a detailed structural analysis of the existing truss. Our design also includes preparing the calculations for the necessary strengthening work.

We need the following information to design the alterations to a roof truss

the span and centres of the trusses

the height of the trusses

the timber sizes of all the truss members

the position and number of any purlins

The length of the building (at right angles to the trusses)

the type of roof covering (eg slate or concrete tiles)

Design information

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