Roof basics – A sketch of a cross section of a typical carpenter framed roof

In this case I have drawn something like we used to do before we had a truss plant in our town. To build the same roof today, with all the extra work involved in getting it approved for cyclonic conditions would not be economical. However, I am getting visitors to this site from all over the world, so I have put it here in the hope that somebody finds it useful.

The timber sizes are only average, they are obviously governed by the spans involved.

Mostly the roofs were just nailed together, with some joints strapped or clipped.

The roof batten (75 x 50) and rafter (150 x 50) spacings are for sheet iron roofing.

Under purlins are needed on longer spans to support the rafters. Quite often the rafters are joined at the under purlins. Roof trusses have been around for centuries, and large trusses would span a large roof, then the purlins would sit on them and in turn the rafters sit on the under purlins. In the case above the under purlins are propped off internal walls.

The collar ties join opposite rafters. Usually if the roof was deemed big enough for them, they were put onto every rafter.

The hanging beams are packed up off the tops of the walls and support the ceiling joists, via timber droppers (50 x 50) nailed to both. They vary a lot in size, from 100 x 50 where the wall span is short, to 225 x 50 typically over living rooms.

The wind brace (150 x 35) is used in gable roofs to stiffen them up in the direction at right angles to the view pictured. They are fixed to the ridge and hanging beams. They are hardly used in hip roofs, because the whole hip end is a brace.

Click here for more on roof framing, marking out rafters, marking out the ridge

Nail Plated Timber Trusses.

metal nail plate roof truss

Roof basics – metal nail plate Trusses, the junction of two roofs at right angles. This is the truss version of a scotch valley.

About the late 1950’s early 60’s prefabricated roof trusses came onto the scene. The name that still sticks with me is Gang-Nail. I may be wrong, but I think they were the first company to build roof trusses that rely on a multi nail plate connector to mass produce cheap and easy roof trusses.

They are no longer the cutting edge of roof design, rather they are used for the thousands of simple cost-effective house designs that are mainstay of the house construction industry.

Standard metal nail plate trusses cannot, normally be used for flat pitch roofs. Unless of course provision is made in the design, to provide the depth that the truss system uses.

Gang-nail trusses use lighter weight timber than traditional roofs and get their strength from the fact that when assembled all the small bits of timber in them, become and act as one unit. Because they are thin, they have no strength until they are braced in the position that the design calls for.

Made to order, usually at a franchise outlet of the main company. Each roof is designed from the drawings that the builder or owner provides. The complex calculations are done on proprietary computer programs. There is no need for anyone on site to do any measuring of degrees or calculating lengths.

The end results are cheaper and more accurate roof frames that are quick to build. Fixing with all the various brackets and clips etc. can easily be done by anyone who can swing a hammer and use a drill. With every set of trusses, you get a diagram and list of fixing instructions.

Click here for more on truss fixing details, with plenty truss photographs.

Smaller local type companies are also now doing similar things in steel. With the introduction of cheap mass-produced roll formed steel sections for wall framing, it did not take long for the same materials to be used for lightweight roof trusses.

All Steel Roof Construction

Steel roofs are the way to go if you want something unusual or different. In many cases they can also be very cost efficient, using standard steel roll formed purlin and batten sections.

all steel metal roofs, hip roof