Cover Image
close this bookCarpentry for Vocational Schools - A Teacher's handbook (GTZ, 252 p.)
close this folder13. ROOFING
View the document(introduction...)
View the document13.1. Different roof shapes
View the document13.2. Technical terms of a roof
View the document13.3. Rafter and ridge roof
View the document13.4. Roof truss
View the document13.5. Diagonal roof bracing
View the document13.6. Roofing iron
View the document13.7. Laying of roofing iron
View the document13.8. Finishing a roof

13.4. Roof truss

Where very large spans are required or where low ceilings are designed it is normal to use trusses which are also easier to erect. The roof trusses are made up by a series of triangles. At the bottom cord allow a camber of 12 mm. The camber allows for settlement of the truss when loaded. Trusses are placed at about 3000 mm intervals.


Figure

a) Different truss types according to the overhangs


Sloped overhang:


Straight overhang;

b) Different Truss types according to the shape:


Standard Truss


Kingpost Truss:

c) Procedure for making a truss:

To make the trusses it is easier to draw a pattern of a truss on a straight surface like a concrete slab or above floor joists or a layer of boards. So you make sure each truss get the same size and shape. Nail all the joints with nail plates, turn the truss and drive in the nail plates on the other side.


Figure

d) How to set the braces: - Kingpost Truss,


Span up to 6 m.


Span up to 9 m.


Span up to 12 m.

e) How to set the braces: - Standard truss,


Span up to 6 m.


Span up to 9 m.


Span up to 12 m.

f) Erecting roof trusses:

Mark the truss position on the top of the wall plates. Erect gable-end truss, ensuring trusses are flush and plumb with the wall plate. If it is necessary tack the roof trusses into position. Braces are fixed to the ground outside the building. If these braces are too long, they will proof ineffective. In this case, attach braces internally to the internal walls. Stretch a string line to the apex of one gable truss to the apex of the other. All intermediate trusses are positioned exactly to the string line. Position the second truss temporarily, and brace back to the first truss. Continue erecting trusses in this way. Secure trusses to the top plate with ripple grips, straps or specified anchorage hardware. Do not leave roof trusses overnight without anchorage. After erecting all trusses you start to nail the battens, making sure the truss is exactly plumb and straight. If that procedure is finished attach permanent diagonal braces.

NOTE: Before erecting roof trusses, you have to straighten out the top plates. Brace it well, especially at that part, where a roof truss is placed.


Figure