Learning Outcome Lesson Objective Understand the techniques used in the construction of superstructures for low-rise domestic and commercial buildings Lesson Objective To be able to explain the techniques used for construction of roofs in domestic and commercial construction

Recap What is the purpose of site investigation? What are its key stages? What is the difference between a trenchfill foundation and a traditional strip foundation? When is a wide strip foundation required? In what situations are strip foundations not suitable? Why is piling becoming more common? What is the function of the ground beam which sits on top of the piles? When are rafts sometimes used?

Roof construction What is the function of a roof? Weatherproof Provide shelter Strong Durable Fire resistant Sound resistant Retain heat Aesthetically pleasing

Roof construction In the UK most domestic roofs are pitched. Although this is partly due to aesthetic and structural factors the main concern is the need to shed rainwater. Tiles and slates overlap and require varying degrees of slope, or pitch, to ensure that water running off each tile cannot run under the one below. These modern concrete tiles can be laid to a minimum pitch of about 25 degrees. Traditional tiles and slates usually require steeper pitches, sometimes as much as 45 degrees, to be effective

Roof Construction

Roof construction Traditional roofs Traditional pitched roofs consist of rafters and purlins alone, with the purlins supported on posts, masonry walls or primary trusses. New traditional roofs can still be found, particularly on one-off housing developments

Roof construction Trussed Rafters The vast majority of roofs are made from factory made trussed rafters. These are specifically designed for each job and offer a number of advantages over ‘cut’ roofs. What are they? they are cheaper and lighter they do not require skilled labour they do not require internal loadbearing walls they are fast to erect they can be designed to very shallow pitches to suit modern roof coverings

Roof construction Trussed rafters are designed to be both light and strong. The triangulation of the timbers prevents them from excess deflection under load and unlike ‘cut’ roofs they do not need additional ridge boards, purlins or struts. The individual truss members are held together by nail plates which are pressed into position by heavy rollers or hydraulic presses. They are supported on timber wall plates just like ‘cut’ roofs. They can be secured by nailing through the metal plates but are best secured by using special truss clips as shown.

Roof construction

Roof construction Insulation at Ceiling Level The insulation is usually placed at ceiling level in simple domestic pitched roofs. Why? To minimise both the heated volume and the quantity of insulation material needed. Insulation can be place at ceiling level in two ways. A single layer between the joists or a double layer, with the first layer between the joists and second layer across them.  Most roofs are of the ventilated type but, more recently, breathing roofs offer an alternative option. 

Roof construction Ventilated roof

Roof construction Ventilated roof –what’s wrong with these details?

Roof construction Ventilated roof

Roof construction Ventilated roof

Roof construction Breathing roof A breathing roof offers a warmer, cleaner and drier loft space. It normally requires: a vapour control layer at ceiling level to limit the amount of water vapour entering the loft (and preferably a well sealed ceiling) a permeable underlay (ie 'breathing') to allow dissipation of water vapour from the loft space, 25mm deep counter battens to provide an airspace above the breather membrane for dissipation of moisture vapour, ceiling level insulation that is pushed up tight against the breather membrane to prevent air leakage into the loft at eaves level.

Roof construction Breathing roof

Roof construction Breathing roof

Roof construction Insulation at Rafters Level Placing insulation at rafter level results in the roof space being part of the heated volume - the 'warm attic' solution. It's commonly used in houses where the roof space is used to provide habitable accommodation. The roof can be designed as a traditional vented roof or as a breathing roof. In many cases insulation at rafter level is in board form - these can often be thinner than mineral wool or glass fibre quilts because of their better insulating properties. The roof can be a ventilated or breathing design

Roof construction Insulation at Rafters Level –ventilated roof

Roof construction Insulation at Rafters Level –ventilated roof

Roof construction Insulation at Rafters Level –breathing roof

Roof construction Insulation at Rafters Level –breathing roof

Roof construction

Roof construction Insulation at Rafters Level –breathing roof

Roof construction Timber framed construction