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Lightweight Steel Roof Framing

"It's All A Matter Of Truss"

Probably one of the most fundamental needs we have in life is a roof over our heads but not on our heads.

Roof structure design

Early roofs were quite simple in design, a horizontal beam along the top to form the ridge and sloping beams placed each with the upper end against the ridge beam and the lower end against the top of the walls of the building. These sloping beams are the rafters and the angle of slope is the pitch of the roof. Much lighter beams known as battens or stringers are fastened horizontally across these rafters onto which the roof tiles are nailed.

At each end of the roof are triangular sections of wall, known as gables, built on the top of the buildings walls and these support the ends of the ridge beam. This type of roof is, from an engineering point of view, quite simple. The ridge rests on the top points of the gables, the rafters rest on top of the ridge beam and the walls. The battens rest on top of the walls. The roof tiles rest on top of the battens. Traditionally the roof beams are all made from wood and nails are used to hold everything in place.

If you look at a traditional Balinese roof it looks very similar but it has a fundamental difference, it does not have gables to support the ridge beam. The ridge beam is supported by the rafters. From an engineering point of view this is significantly different, the rafters must be held in place along the top of the building walls which in turn hold up the ridge.

A triangle is formed between the ridge, the top of the wall on one side and the top of the wall on the other. When we add weight in the form of heavy roof tiles to this roof the weight tries to squash the triangle flat so there is a lot of force acting down the rafters and onto the top of the walls. If the weight is too great the walls can be pushed outwards and over.

What is a roof truss?

To stop this happening beams are fastened horizontally across the bottom end of the rafters to hold the triangular shape in position. We now have a complete triangle of wooden beams, a rafter on each side and a horizontal tie beam across the bottom to tie the rafters together. This wooden triangle is what we call a roof truss (named, of course, after the Rev. Jonathon Herbert Truss, the vicar of Upper Codswollop, who had a thing about being "restrained" and tickled on the buttocks with goose feathers).

The use of tie beams to form trusses has meant that we can now make roofs in all sorts of fancy shapes. Instead of gables we can form half pyramids on the ends of the building known as hip or hipped roofs.

Let us consider roof trusses which, in days gone by were made from substantial wooden beams. As engineering design and the mathematics involved became better understood the basic triangle was progressively modified. A vertical beam was placed from the upper point of the triangle down to the horizontal tie beam. Further extra beams were added and each development made the truss stronger and allowed lighter and lighter wooden beams to be used.

Early roof frames were made up of a few large beams while these days they are made up of hundreds of small strips of wood. This has revolutionised roof construction, it has considerably reduced costs and has also simplified the building construction process. These days in many countries roof trusses are very light in weight and mass produced in factories. The trusses are loaded onto a truck, taken to the site, erected onto the walls and finally battens and some strengthening diagonal bracing strips are added to hold everything in place. The standard of carpentry required is a little more than is required to stick a toothpick up a cat's left nostril.

But not in Bali.

Until now most roofs have not used roof trusses. You will note that the Balinese like open ceilings (what we call cathedral ceilings) and so they use rafters, by using some clever carpentry, avoid using gables and tie beams. The majority of roofs in Bali are still built using the old methods of rafters and ridge beams which are supported on the tops of the walls.

Lightweight steel roof support structures

This is all about to change. The high cost of wood now means that lightweight steel roof frames are cheaper than wood. These lightweight steel frames are built using all the design principles that have resulted from the development of roof trusses.

Unfortunately this means a whole new area of expertise is being introduced much of which requires higher standards of design, higher quality control of materials and more stringent standards of construction. (Oh bum I hear you think). There have already been quite a few disasters and from Jakarta comes news of roofs collapsing.

The problem mainly arises from the use of heavy roof tiles on lightweight structures. To keep costs as low as possible the amount of steel used is kept to a minimum, a lightweight steel roof structure is just strong enough to do its job.

But as we know, amongst the building "professionals" the PAG (poverty avoidance gene) kicks in, perhaps the steel is made a bit thinner, or maybe not as many trusses are used or perhaps not as many bolts to hold it all together. Only slight reductions and the strength of the whole is reduced from "a knat's whisker more than just strong enough" to "a flea's earole" less than strong enough" and disaster strikes. As is so often the case in building construction a very expensive building can be jeopardised for the sake of trying to save the price of a bog roll.

Selecting a good roofing contractor

Let us consider how we can look after ourselves when buying a lightweight steel framed roof. Be aware that soon virtually all roofs will be built this way so every man and his dog and even the dog's fleas are getting in on the act. Before you know it in the back streets of major cities across the country old bean cans will be beaten out, pop riveted together and painted to look like high quality steel roof trusses.

There are some very good roofing suppliers so you need to take care in finding one.

The first and most important thing to note is that the quality of the steel is very important. The various beams that should be used will have been carefully designed to provide maximum strength for minimum weight. You will find different shapes are used for different purposes such as rectangular tube, corrugated strips and angle iron.

There is little room for error. The steel must be high tensile steel of the correct thickness, of the correct alloy and heat treatment and finally with an effective galvanised or zincalume plating to stop it rusting (such thin steel sections could rust very quickly if not properly protected). As you can imagine our back street manufacturer is unlikely to even start to understand, never mind be able to duplicate, these stringent requirements. It is advisable to go for a well known brand of steel. Bluesope and Lysaght are well regarded names to look for.

The second thing to do is to find a responsible adult to build your roof for you. Not easy in a modern world where so many people behave like kids let loose in a sweet shop. These days there is an endless stream of people being dragged up before the beak for crimes that can only be described as child like. "I work in the bank, why can't I take the money home?" Even their courtroom behaviour is juvenile "Sorry milud, I can't come to court today, I don't feel well." Sorry I digress.

The next thing to do is to make sure the roof is properly designed. For professional installers this is not difficult. They punch the numbers into a computer that does the design work for them. This considerably reduces the chances of mistakes. Make sure that a computer generated design is produced for your roof.

Next watch the construction process and check the standard of work. The best steel in the world is somewhat useless in the hands of a tukang highly trained in the art of selling tattoos. Note that the trusses will probably made on site. Things to watch out for:

  • Check that the steel is in good condition and its shape is not damaged.
  • Each joint should have 3 bolts to hold it together.
  • Drilled holes for the bolts should not be drilled too close to the edge of the steel.
  • Dynabolts not just screws should be used to fasten the roof frames to the concrete structure of the building.

If this is all too complicated you can always take a local approach and cut yourself a banana leaf.

Copyright © Phil Wilson 2014
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