If you need a trussed rafter roof then the best way to get it designed is to go to a supplier. They will provide all the necessary calculations as part of their contract and it will be included in the price.

However we have just completed a job where a new staircase was being constructed and this involved removing part of an existing trussed rafter. Most trussed rafters are designed with little if any spare capacity and so it wasn't possible to trim the trussed rafter back to the adjoining trusses. It would have been difficult to install steel beams to trim the new opening because of the site conditions. We therefore designed two site fabricated trussed rafters using small timber sections and plywood gussets. These were very easy to install, with minimal disruption.

We need the following information to design this type of alteration:

The span and centres of the trussed rafters

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).

The calculations provided will be suitable for Building Regulation approval.