1. 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

2. 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?

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

4. 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

5. Roof Construction

6. 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

7. 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

8. 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.

9. Roof construction

10. 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. 

11. Roof construction
Ventilated roof

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

13. Roof construction
Ventilated roof

14. Roof construction
Ventilated roof

15. 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.

16. Roof construction
Breathing roof

17. Roof construction
Breathing roof

18. 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

19. Roof construction
Insulation at Rafters Level –ventilated roof

20. Roof construction
Insulation at Rafters Level –ventilated roof

21. Roof construction
Insulation at Rafters Level –breathing roof

22. Roof construction
Insulation at Rafters Level –breathing roof

23. Roof construction

24. Roof construction
Insulation at Rafters Level –breathing roof

25. Roof construction
Timber framed construction