Ask an Expert

Ask an Expert

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 just seeking some info about how to exchange between natural timber and LVL in terms of strength.

Span tables for LVL and other engineered wood products are published by the manufacturers, since their structural properties are product specific. There is no direct equivalence between the properties of LVL and a particular stress grade of timber. However, the two can be compared by designing, say, a floor joist in LVL by reference to the manufacturer's span tables and comparing it with the size for a particular stress grade of timber, derived from the supplements to Australian Standard 1684, "Residential timber-framed construction".

Q. Just wondering if you are allowed to substitute MGP10 pine Framing with F8 Grade?

To use an 'F' grade instead of MGP10 you really need to go to F11 to match the stiffness properties. MGP10 has a stiffness value of 10,000 MPa (that's why it's called MGP10), but F8 has a stiffness value of only 9,100 MPa. But the stiffness value for F11 is 10,500 MPa so it's actually a little better than MGP10.

Q. I am currently reviewing the structural capacity of a single storey brick veneer house which will be undergoing a first storey addition. I understand it was built in the late 1970's/early 1980's and have seen the internal timber framing which appears to be a hardwood timber. What is the structural equivalent grade of the timber likely to be?

We can't give you a definitive answer without knowing the type of timber. However, we can say it's unlikely to be less than F11. The timber would now be classed as "seasoned" even if it was originally unseasoned when the house was built. Green hardwood was commonly used for house framing in Victoria and the eastern States at the time, although now nearly all house framing is pine. If you can identify the species of timber we could give you a more positive answer.

Q. I am replacing an internal stud frame wall with a beam. The wall carries a skillion roof, clad in zincalume. The span is 5.4 metres. Client has a jarrah beam (recycled) dimensions 300 x 100mm. Is this sufficient for the span?

Your question really needs some engineering input to determine the loads on the beam. Presumably there will be some ceiling load as well as the weight of the roof. Also recycled timber needs special consideration. Your engineer will find guidance on designing with recycled timber via this link: Span tables are available dealing with sizes up to 290 x 90, and there may be one that applies to your situation.

Q. Building a deck at the moment and the engineers report says I need to use f7 treated pine for joists. Can I substitute f7 for mgp10? Which one is better?

A lot of treated pine is actually graded F5, but if you can obtain treated pine that is correctly graded and branded MGP10 it will have greater stiffness than F7, but slightly lower strength. Timber design is generally governed by stiffness (resistance to deflection) rather than strength. However, your engineer should be able to advise you. It's just a simple matter of looking up the span tables in Australian Standard 1684 to see what sizes are required for the deck in MGP10 compared with F7.

Q. What size and grade of pine studs would be needed to build walls 3300mm high. Span of trusses is 8445mm, pitch 30.0, Iron roof covering, overhang 150mm. Walls are to be weatherboard cladded.

Your "Roof Load Width" (ie. the width of roof contributing load to the studs each side) will be 5026 mm. The tables in Australian Standard 1684 show that double studs of 2/90 x 35 MGP10 spaced at 600 mm are suitable for a Roof Load Width up to 6200 mm and a height up to 3600 mm, so this stud size is well within the safe range. However, note that AS 1684 only considers wall heights up to 3000 mm floor to ceiling with respect to racking forces, wall bracing capacities, uplift forces and shear forces. Some engineering input will therefore be needed to allow for your stud height of 3300 mm. Studs at the sides of openings and studs supporting concentrated loads may also need to be a larger size.

Q. I wish to build a new home with custom orb cladding and I have seen customised homes built with a double wall frame so that the downpipes are in the wall, although i am not interested in this design feature. This made me think that I could build a double wall frame that would provide me greater strength and more depth for insulation. If I went down this path, what would you consider to be the most sensible solution out of these 3 combinations: 1 2 off 90x45 frames providing a depth of 180 in the wall frame 2 2 off 70x35 frames providing a depth of 140 in the wall frame 3 1 off 120x45 frame providing a depth of 120 in the wall frame

Assuming you are happy with the depth achieved by all these combinations, the other main consideration is price. Prices quoted on the net are: $6.10/m for 120 x 45, $3.23/m for 90 x 45, and $2.34/m for 70 x 35. So clearly two 70 x 35 frames are the most economical combination regarding cost of timber, but require twice as much labour to build compared with a single frame of 120 x 45. If you are building the frames yourself you only need to consider your time, but if they are to be made in a truss and frame plant, or built on site, labour will be a factor. The only way to find out the true cost is to get some quotes. That should help you to make a decision.

Q. I am wanting to take out 2 windows and a sliding door to create an opening of approximately 8mtrs. I want to install a 6.228m sliding door set-up with a double hung window at each end (850mm approx.) to add up to 8mtrs. I am wanting to know if the "plywood box beam" would suit this application. My ceiling height is 2650mm allowing a decent height beam and my roof is decramastic tiles which are basically tin so certainly not the weight of concrete roof tiles I look forward to hearing from you and any advice would be greatly appreciated.

We have a design guide with span tables for box beams that will answer your question. It's available from the Wood Solutions website via this link: Refer to Guide no. 7 titled "Plywood Box Beam Construction for Detached Housing". Your beam is classed as a lintel beam and you will see from the tables that an 8m span is achievable with various combinations of materials, depending on the Roof Load Width as defined in Fig. 2 of the Guide. Alternatively you could reduce the effective span to 6.3m if the windows at each side had loadbearing mullions capable of supporting the lintel beam.

Q. I am currently designing a public facility with exposed timber columns, beams, fascia's etc in a beach environment (moist and salt-laden). I have been advised that exposed fascia beams will need to be F14 min. kiln dried hardwood with a durability class 1 or 2. Is it possible to get White Cypress Pine that matches the above properties?

Cypress pine has the required durability but won't achieve F14 grade. You can find suitable timbers by writing "seasoned structural hardwood" in your browser. For example, Boral produces kiln-dried blackbutt and spotted gum in F17 grade, either of which would be suitable (Durability Class 1 outdoors, above ground). In a salt-exposed situation make sure all fasteners are highly corrosion-resistant.

Q. I am a 5th (final) year Architecture student in the University of Moratuwa, Sri Lanka. For my final major project I'm designing a fishery harbor that uses timber (glulam) portal frames as the basic module. My advisor told me however, that I can use glulam for interior but not the exterior due to climate. I would be grateful if you can tell me about the possibility of using glulam structures exposed directly to tropical climate i.e. Sri Lanka with seasonal 6-8 month monsoon rains, high variable humidity and a short dry season.

Your supervisor is right in saying that an exterior environment is more challenging for glulam than an indoor one, particularly in the tropics. Variable humidity is more problematic than consistently high levels, since changes in humidity cause swelling and shrinking cycles. Since each laminate is likely to have a different grain orientation, one laminate may be more prone to swelling or shrinking than its neighbour, causing stresses to build up in the beam with possible splitting, or in extreme cases, delamination. Having said that, glulam is used outdoors (at least in non-tropical areas) with reasonable success, as long as some important points are addressed, namely (1) a water-resistant glue is used such as resorcinol, (2) a durable species of timber is used, or it is preservative treated, (3) the glulam is manufactured under a quality control program that includes sampling and testing of glue bond strength. Glulam bridges are common in the US and in Scandinavian countries, and a paper available via this link may be helpful: There is also a useful technical bulletin available on the net that deals with cracking in glulam - paste this link into your browser for a copy: In Australia glulam is manufactured under strict quality control conditions which are specified in Australian/New Zealand Standard number 1328 Parts 1 and 2. Preservative treatment of glulam is specified in Australian/New Zealand Standard number 1604 Part 5. We hope these comments are some help, taking into account the local climate. Perhaps there are some examples of glulam used externally in Sri Lanka that would provide further guidance.


Did you know?

Logs from plantations cannot produce the sawn hardwood timber produced from logs currently harvested from native forests.