Personally, I have built a lot out of light steel. That really comes from the fact that I did metalwork at school, and I have never had anyone to learn from when it comes to working with timber in my adult life.
However, in my professional work, I have something of a speciality in timber engineering and indeed I am one of a very few of independent engineers who know something about prefabricated timber construction. In fact, I occasionally lecture in timber engineering to other engineers. Having said that steel design is happening in my office on a daily basis.
So all-together I think I can put together a fairly balanced appraisal of the relative pros and cons of steel versus timber in housing construction:
Structural Safety – there is no doubt steel is significantly stronger for a given section size than timber. Somewhere between roughly 5 and 30 times as strong in fact, depending upon the particular materials compared. However, there is no difference in safety because the timber is sized larger to make up for the difference in strength.
Redundancy – As a whole, a timber structure tends to be stronger than its “design” strength as a ramification of the grading process. The result is that in an overload situation on average a timber structure will be more likely to stay up than a steel one.
Structural Section Properties – a solid lump of timber is intrinsically very stable under loads. One of the major problems designing and constructing in light gauge steel is that the sections themselves can distort. If you grab a steel C-section stud you can just twist it. As an engineer, this makes light steel harder to design than timber.
Eccentric Loads – for the same stability issues noted above, timber is generally better at resisting eccentric loads such as trusses supported on the sides of a bottom cord of a girder.
Shrinkage – although once a real problem, the shift to seasoned and engineered timbers has largely voided that issue.
Size Consistency – There can be some variability in the overall sizing of timbers whereas steel is essentially the same profile all the time.
Warping & Twisting – A significant problem with timber – most sticks of timber are not perfectly straight and true when you buy them. Differential shrinkage across the timber grain causes the timber to distort. Some can be real bananas. Steel can vary a little but it’s a marginal problem at worst.
Splitting, Knots, Gum Veins & Other Timber Defects – from a strength perspective these are accounted for in the timber grading process so the presence of defects in the timber is generally not a problem although these defects need to be kept away from joints.
Aesthetic perspective – Beauty is in the eye of the beholder. Some people like the regularity of steel – some like the natural variance in timber. I am definitely a timber person in this regard.
Design Flexibility – the timber industry is somewhat more advanced and sophisticated in the design software. Meanwhile, timber nail plate connections are readily adaptable to a large range of geometry and uses. Timber wall frames are easily cut and built into odd shapes. The net result is that timber frames and trusses are more readily adaptable to complicated and exotic shapes than steel.
Connections – in steel are much harder to make but for a given bolt, screw or rivet size they are much stronger. Timber connections are prone to splitting but on the other hand, there is a vast array of timber connectors that make life pretty easy.
Transport – light steel frames are generally more vulnerable to transport and erection damage than timber frames and trusses.
Environmental Credentials – the timber industry sells itself as an environmentally friendly product. Timber is a renewable resource and timber construction stores carbon over the long term. Steel makes a case that it is actually better when you include the potential to recycle at the end of the life of the building. At least for the moment, I think that’s somewhat dubious because builders aren’t salvaging the steel when they demolish a steel-framed cottage. Hopefully, that will change in the future.
On-Site – steel is so much harder to adjust and or rectify. Of all the things going for timber, this is the real long-term winner.
Holing –Steel is harder to hole on site. I’ve blunted a hole saw or drill in steel far too many times; it’s a real pain in the bum.
Linings – steel can be oily and so glues don’t take so well to the steel. And any steel greater than about 1.5mm thick becomes quite hard to fix linings to. For instance, a screw can wander around before it bites into the steel and work the hole in the lining too large.
Noise – my parents live in a steel house in Queensland and every time the sun comes out the frame warms up and expands and it sounds like we are in a ship that’s just run aground.
Rot/Rust –either material will ultimately degrade in the presence of water. Timber is better in corrosive environments particularly near the sea. Both materials can be protected at a cost so it’s something of a line-ball issue.
Condensation – occurs where the steelwork temperature is lower than the air’s dew point. Repeated condensation can build up in closed building spaces and wreak havoc with linings, insulation etc. Cold steel behind linings can also cause local condensation on the outside of the linings themselves and ultimately discolouration.
Termites – the steel framing industry trades heavily on the fact that “termites don’t eat steel”. True as that is, the widespread use of H2 treated timber has, to a large degree, resolved that problem for new timber buildings. Regardless of the frame type, termites are still happy to come into the house and munch on your skirtings and joinery. And so, the best approach is to keep the critters out of the house in the first place! In that case, steel has no advantage over timber.
Building Fires – I’ve been to many, many building fires and although steel has the obvious advantage of being non-combustible, that doesn’t really matter so much in real life. It’s the contents: the furniture, plasticware, paperwork, carpet, curtains, joinery etc that burn before the frame.
Bushfire – there is no doubt that steel has an advantage here although it’s generally only a major difference at the higher bushfire attack levels.
Availability – As I write, at what is hopefully the end of the pandemic, the material supply is highly constrained, and builders are really struggling to get any material. It started in timber, people morphed to steel, and that ran into supply constraints as well.
Cost – Don’t ask me – see the question of availability above and that is affecting the cost radically. Get quotes for both is my only advice.
So what material is best? Steel or Timber? Clearly from above there are a huge number of factors to consider.
I think it’s interesting to look at what the market has decided. Developers and builders are in it for the money, so they are synthesising all the above factors and using what works for them. And that has come firmly down on the timber-construction side of the fence. I guess it’s a timber fence.