Timber trussed rafters are very familiar to the construction industry and are inexpensive and fast to erect. To create a ‘thermal bridge free’ junction, ‘dropped ceiling’ or ‘bob tail trusses’ allow for the continuity of insulation between the roof void and wall. Airtightness is dealt with at ceiling level with OSB board or other robust membranes. Thermal bridging is minimised with only the timber bracing members of the truss penetrating the insulation layer. There should not be any air movement in the roof void, to avoid thermal bypass. This is achieved by using modern high quality ‘vapour open’, underslate membranes taped at laps and carefully detailed at the eaves and verge.