1. Roofs
Enclosure and horizontal structure
Royal
Agricultural
College

2. Prefabrication: light, dry,
fast, flexible
Royal
Agricultural
College

3. What goes on top?
?
Royal
Agricultural
College

4. What goes on top?
?
The roof
Royal
Agricultural
College

5. What does a roof do?
• Two fundamental functions
– Enclosure: separation of interior from
exterior
– Structure: span horizontally across
interior spaces
• Several secondary functions
– Insulation: control of heat transfer
– Service distribution: pipes and wires
– Storage space: we all want lofts Royal
Agricultural
College

6. Structure first
Royal
Agricultural
College

7. Structure first
A dome or
arched vault.
Pushes walls
out
Royal
Agricultural
College

8. Structure first
A dome or
arched vault.
Pushes walls
A column out
supported “tent”.
Very difficult to
build
Royal
Agricultural
College

9. Structure first
A dome or
arched vault.
Pushes walls
A column out
supported “tent”.
Very difficult to Compressed air supporting a
build “skin”. Insulation very difficult
Royal
Agricultural
College

10. Most common domestic
solutions
Triangulated truss Self-supporting beam
Timber is the most common structural roofing material
Royal
Agricultural
College

11. Naming of parts of a simple
pitched roofridge
slope
hip verge
valley
eaves
gable
Roof covering
wall plate Royal
facia Agricultural
College
soffit

12. Rafters and ties create a strong
triangular structure
Rafters
Ties (or
ceiling joists)
But they can sag…
Royal
Agricultural
College

13. Rafters and ties create a strong
triangular structure
But they can sag…
Royal
Agricultural
College

14. Basic solution to sag: hangers
and struts
1 Hanger, secured
The maze of hangers at the ridge, holds
and struts is called up centre of ties.
the web Under tension
2 Struts, supported by hanger,
prop up rafters. Under
compression
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Agricultural
College

15. Basic shapes of prefabricated
“Trussed rafters”
These are built from Fink truss is the
small section softwood, most common
typically 100 x 50mm
Room in roof: not fully Royal
Agricultural
triangulated, so timbers have
College
Image source www.tra.org.uk to be larger

16. Joints: Butt joints with gang-nail
plate connectors on each side
Nail plate
hydraulically
squeezed onto both
sides of joint
Plates formed from sheet Royal
Agricultural
galvanised steel College

17. Jointing of trussed rafters
Gang nail plates used on all joints. No other
connection between timbers. Even the main tie
may be in two pieces with a joint Royal
Agricultural
College

18. View of gang-nailed roof structure
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Agricultural
College

19. Trussed rafters prefabricated
and delivered to site
Royal
Agricultural
College

20. Trussed rafters are weak until built
into position and
must be braced
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Agricultural
College
Image source www.tra.org.uk

21. Network of small timbers build into
a 3-dimensional structure
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Agricultural
College

22. Complex roof shapes can be
built using prefab. units
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Agricultural
College
Image source www.tra.org.uk

23. At the eaves
Essential that the roof
structure is physically tied
down to the walls. The roof
will blow off otherwise
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Agricultural
College

24. That’s a trussed rafter roof,
what about a flat roof?
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Agricultural
College

25. Flat roofs: how do they stay up?
• Flat roofs built from beams of
materials which are stiff and can
take both compression and tension
• Steel is very good but most domestic
construction uses timber
150 -
300
mm
50 - Royal
75mm Agricultural
Max length for solid timber approx 4m College

26. Joists at 600
centres
Wall plates
(100X50) bolted or
strapped to the
wall
Roof beams at 600mm Royal
Agricultural
centres are called joists College

27. Trimmer
Doubled joist
Forming openings
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Agricultural
College

28. Royal
Agricultural
College

29. Roof decking
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Agricultural
College

30. Roof decking
Royal
Agricultural
College

31. Roof decking
Decking must be
strong & water
resistant.
Plywood is ideal, Royal
never chipboard Agricultural
College

32. Basics of a flat roof with a fall
Decking forms base for covering
Royal
Firring piece on each Agricultural
joist to create fall College

33. Flat timber roof eaves
Firring piece on each joist
facia Roof deck and finish to create fall to gutter
Roof joists at 600mm centres
soffit
100X75 wall plate
Stainless steel wall
plate straps at 1200
centres Royal
Agricultural
College

34. Deep beams
• There is a limit to the size of a piece of timber.
• Deep wooden beams can be built up by
laminating (gluing together multiple thin layers)
small pieces of timber
• Laminated timber is actually stronger than
natural timber of the same size.
Royal
Agricultural
College

35. Covering the roof
The structure is complete. Now you need to
keep the rain out and the heat in Royal
Agricultural
College

36. References
• CHUDLEY, R. GREENO, R. (2005).
Construction technology. 4th ed. Pearson
Education.
CHUDLEY, R. GREENO, R. (2006).
Building construction handbook. 6th ed.
Butterworth-Heinemann.
• Trussed rafter association
http://www.tra.org.uk
• Glue laminated timber association
http://www.glulam.co.uk
Royal
Agricultural
College