Roof Truss Details

This page is about nail plated timber roof trusses. There are of course many other types of timber trusses in existence, but these are by far the most common used in residential construction today.

These roof truss details are all of houses being built in a cyclone prone area and as such they have to be built to a strict building code. What with global warming and climate change, this type of construction may be coming to your area soon.

Roof truss - A hip roof ready for custom orb roof sheeting.

Some history.

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.

There were quite a few methods of joining trusses around at the time, but most of them needed some drilling and bolting. The Gang-Nail plate is just a flat galvanised plate that has stamped in a press to produce dozens of pointed spikes on one of it's faces. One is placed on each side of a plain timber butt joint, and then a hydraulic press forces the plates together forming an extremely strong joint.

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. They can not be used for flat pitch roofs.

This type of roof construction is governed by strict Australian Standards. ( AS4440 at time of writing, but without a doubt it will change sooner rather than later:-)

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Advantages of lightweight trusses.

• Cheap, for certain types of work they are very cost effective.
• They are thin in thickness, Say 35mm,and use cheap and small sections of timber. Sometimes plantation grown, which is a definite plus in these days of deforestation and global warming.
• When they are fixed in their correct positions, these floppy weak looking fabrications are transformed into tremendously strong roof structures.
• They are 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 engineering 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. This makes them a distinct possibility for the average owner builder.
• Usually a roof truss order will also contain all the ancillary fittings that are needed to complete the roof.
• These will include metal ceiling battens, roof battens, purpose made and standard hold down brackets, strap bracing, nail plate connectors etc.
• One of the carpenters on the job where these photo were taken, told me that their team of five men, do one of these roofs in 2-1/2 to 3 days.
• There is almost no sawing for them to do. It is all drilling and bolting and most of all nailing the straps, braces and plate connectors with a coil nail gun. Then screwing down the battens with a heavy duty screwdriver gun.

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Some Disadvantages of metal nail plate Roof Trusses

• Lack of design freedom. Invariably this type of roof is built on variations of rectangular plans. No curves and bends.
• Certain maximum spans and small maximum unsupported overhangs, compared to steel.
• Only a certain range of roof pitches.
• Not got the same fire resistance as all steel construction.
• Not the same termite resistance as all steel.

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General Considerations

I many cases the trusses are delivered to the site by a crane truck, which can lift them directly onto the walls.

With every set of trusses you get a diagram and list of fixing instructions. Of particular interest to owner builders, there are small booklets available for free from the manufacturers, that show the various fixing details of all the components.

Most of the roofs that I have done have used the "metal nail plate " roof truss system. They are an Australian company who have franchised outlets in major cities around Australia.

metal nail plate do house trusses up to 45 deg. pitch and up to 16 m span. They are nearly always designed and built to span the external walls. On some of the details you may notice that some internal walls are connected to the roof truss framing.

These are walls that the the structural engineer has designated a shear wall. That is wall that are designed to stiffen the main external walls. The fixing to the roof in these cases is just as much about holding the shear wall in position, as supporting the roof.

Roof truss - Roof trusses ready to fix.

Here's a shot of a main roof truss on top of the walls. In this case the walls are series 200 (190 thick) hollow reinforced concrete blockwork. This type of wall is tremendously strong and stiff. Note the truss hold down bolts sticking out of the concrete, They are galv. M16 with a cog on the end, and are probably cast into the concrete corefill at least 200.

Roof truss - From underneath.

Here is a shot from underneath showing temporary timber props holding the roof trusses level. Also in this shot you can see that there are no masonry internal walls, instead the designer has chosen to use fully MIG weld steel wall frames, a few can be seen lying down on the left.

Truss Layout Sketches

Roof truss - Sketch of the plan layout of a simple hip roof.

Roof truss - Sketch of a hip end.

You may have noticed that unlike a traditional timber roof, when trusses are used, there is no ridge piece. it is not needed. The roofing battens hold the trusses securely on the top and the ceiling battens hold the bottom chords.

The truncated girder truss is there to give extra strength to the hip areas. They are usually double the thickness of the ordinary trusses (two trusses of the same thickness nailed together), and often depending on the span and pitch of the roof there could be two of them at each hip end.

Some truss details

The roofers will lift up these shorter trusses by hand as they are needed. Typically these trusses are set out at 900 centres.
This shot shows a "L" junction between two roof faces, on the right is a standard roof truss spanning between walls and on the left trusses from the other wall that join to the other roof truss. Shorter trusses of reducing size sit in the backs of them to form the valley behind. Note the tie downs and the diagonal bracing on the far roof face.
Here is the same sort of set up from the outside. The blue coloured metal roof battens are tek screwed with 2/14g screws at every truss and to each other. The diagonal ones are spaced to provide fixings for the roof flashings and valley edges.
Here is a stack of metal roofing battens, or purlins, as you can see they have been cropped to standard lengths with a top cut on them already. They overlap at the joins, to make a very strong and quick fixings.
Here's a typical pre fabricated hold down bracket welded from 6mm angle and flat plate. It is hook shaped over the top of the roof truss with the 16mm bolt going though steel both sides. The hold down bolt into the concrete is 16 mm also
Typical purpose made straps. Note that they turn around the underside and are nailed there also. The roofers here are using coil nail guns. The nails for fixing these straps are actually called strap nails. 30mm x 2.0mm. They look a bit like a clout nail. but they have a stronger head. Don't use clouts!
Fascia clips. No sawing off the rafter ends, there is enough play in the clip to get it straight. When fixing the clips I have a piece of ply at each end and run two string lines for top and bottom edges of the clips. The fascias are some times made on the job from a mobile roll former in a trailer. The longest one I ever fixed, without a join was 22 metres.
The diagonal bracing is usually called "speed brace" and it is certainly quick to fix. Two nails in the top to each truss, joins to span two trusses, and most importantly turned around with 4 nails at the ends.
The eaves are lined on the slope of the roof with 6mm cent sheet. PVC joints. There is a requirement in the BCA that eaves have to be "vermin proof". This is an expensive but good looking way to do it. Otherwise in this case of block walls it would be a vermin mesh between the rafters.
Here is an angle bracket roof truss connection to an internal steel framed shear wall. You can plainly see that the ceiling battens run over the walls. You may be able to pick the lap joint in the batten on the right,
Here's a shot from the inside of a roof that has been sheeted. It is looking up at a Dutch gable with a valley on the right. You can see speed brace in there and the silver reflective foil. Ceiling battens at 450 centres.

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Roof truss - A finished roof.

Here is one of the almost finished houses. A Dutch gable over the double garage and hip ends elsewhere. Corrugated iron roof sheeting and two 300 whirlybird type ventilators. On the far side will also be a solar hot water system.

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