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Structural timbers & engineering

Like to know what wood to use building a pergola or framing a barn? How about the difference between tung oil and polyurethane as a floor finish? How to revive decking or deal with merbau stains? Or how to meet the building code or bushfire standards? Or ask about the environmental advantages of wood or forest certification?

Q. I am after some drawings or brochures on connection details for laminated beam rafters 450 x 90 to laminated column stubs 250 x 90 using galvanised connection plates. Any help would be greatly appreciated!

Connection plates for large glulam members will probably have to be specially fabricated, but there are some useful guidelines available on the net. One of the most comprehensive guides, which includes do's and don'ts for connector design, is available via this link:

Q. Beam size required for a pergola. The pergola beam will have 5.0 mt span between posts. Will have 6 rafters(140 x 45mm) attached to an existing wall at one end and laying over the beam at the other end. What size (treated pine?) beam is required?

Assuming the treated pine is graded F5, and the rafters are spanning 4m, you will need a 240 x 45 beam between the posts. This assumes there is little or no weight from any roof covering, but allows for lightweight roof cladding such as shadecloth or polycarbonate sheeting.

Q. I am in the process of having a tradesman replace the (4) 100x100 posts supporting a deck under the roof line of the house. Age and some dry rot has got to them. He is looking at 2 of 5.4m length which support deck and (tiled) roof at the corners and 2 of 4.2m which support the deck and railings on the longest end of the deck. Laminated Merbau (100x100) posts have been suggested. Given they will need to support considerable weight from the tiled roof (especially those on the corners), will they be structurally strong enough? Deck is approx 6m x 3m.

It's not our policy to engage in structural design work since it's difficult to appreciate all the factors involved without seeing detailed drawings. We are also not sure of the structural properties of the proposed laminated posts. You will need to inquire whether they have been "stress graded", and also make sure they have been manufactured with a water-resistant adhesive for exterior use. Then we suggest you look for a consulting engineer in your area who can check the design and certify it for you.

Q. I need to find out the maximum span to 75mm x 50mm unseasoned hardwood. I have used these for the joists on a colorbond horse shelter. With the 75mm in the vertical position, what is the maximum span allowed?

We assume the joists in question are floor joists. We are not familiar with the design of horse shelters and have no idea what kind of loading a horse would impose on the floor. However, we can say that F8 grade 75 x 50 floor joists in a house can span 1.2 m, spaced at 450 mm centres.

Q. What if the boards on the vehicle pit in my previous question were to carry 1.5 tons?

For a 1.5 tonne load we calculate that 140 x 75 boards would be OK, but the stress grade could be reduced to F17 which gives you a wider range of timbers to choose from.

Q. I was just wondering what sort of wood could I use for a vehicle inspection pit. Need it to carry about at max 3 ton on each board and boards need to be about 120 - 140mm wide and 35 - 36mm thick each board is 950mm in length. It needs to carry the weight on the width of the board not the depth.

That's a pretty heavy load - for example a Holden Astra only weighs about 1.3 tonnes and even a large 4-wheel drive only weighs about 2.3 tonnes. Presumably the entire weight of the vehicle wouldn't be carried on one board. Assuming the 3 tonne figure is right we calculate that you would need much thicker boards. If the timber is "green" at the time it is loaded you would need 140 x 100 in F22 grade hardwood to safely carry a 3 tonne point load.

Q. I need to find out what the standard requirements are for timber spans. I have a horse shelter on a property constructed from 75mm x 50mm hardwood and covered in colourbond. Can you please tell me what the maximum roof span is for this timber size. I would also like to know the same for 100mm x 50mm hardwood.

Allowable spans for timber depend on the strength rating or stress grade, and whether the timber is seasoned or "green". Some timber companies publish span tables, eg. Thora Wholesale Timbers whose tables can be found at Various software programs are also available to calculate beam sizes and you can find the major ones on the Wood Solutions website via this link

Q. Just wanted to inquire about reinforcing roof rafters for an attached box hardwood pergola 6m x 2.7 approx. I was going to use stiffeners/hangers to a soffit trimmer attached to noggins between rafters, was wondering if that was adequate or not enough/too much. Any help is appreciated.

It's a bit complicated to go into here, but there is a very informative 12-page data sheet available on the net that shows recommended methods of attaching a pergola or carport to a house. You can access it by writing TQL 19 in your browser - attachment details are shown on page 7.

Q. We are building an arched roof-line above a deck. Our engineer told us to put in a HSS 4"x10"x3/16" rectangular steel tubing and bend it to fit the arch. Are we able to build this out of lvl?

It's not possible to bend LVL, but if it is only a gentle radius of curvature the arch could be cut out of a wider section. Most LVL is made from pine, so it would need to be preservative treated if it is likely to get wet when it rains. LVL is now available made from hardwood, but it might still not be suitable for weather exposure, depending on the type of hardwood. So the answer is that LVL is a suitable material if protected from the weather, or preservative treated, subject to structural approval from your engineer.

Q. I’m wondering what would the maximum width of a 32mm thick board if it was cut to a trapezoid shape for a radial timber deck? Will it be 135mm or does it need to be cut in from the edge? Can you also give me some direction in finding a size table for structural blackbutt?

A typical range of structural hardwood sizes can be found on the Boral website at this address: The unseasoned or "green" range differs from the range of seasoned sizes, as the timber shrinks slightly during kiln drying. Regarding the decking, if it starts off as a 135mm wide rectangle and it's cut to a trapezoid, presumably one face will still be 135mm but the other face will be narrower.


Did you know?

In 2008 seventy percent of known old-growth trees are in nature conservation reserves.