This depends on the style of roof you have. If you have a Pent roof on your building, you should not get a roof truss. Buildings that have an apex roof and a length of 10ft or more will be supplied with an apex roof truss. The length of the roof truss will be equal to the width of the garden building. Apex buildings over 10ft in length will be supplied with one roof truss. However, If the building is 20ft long it will come with three roof trusses.