When applied as external cladding, the service lives of various species of timber can range anywhere from 15 to 60 years. That’s a massive, gaping window of time where colors can fluctuate, cracks can occur and extreme weathering can take place. And so, while cost and aesthetic style will be driving factors in your material selection, the key consideration should always be how a species’ performance factors will level up to your design needs. What’s the use in affordability and beauty if they don’t last?
The first distinction that is useful to wrap your head around is the difference between hardwood and softwood, as the dichotomy is sometimes a little bit misunderstood. Oftentimes, it is assumed that hardwood is hard and softwood is soft, and while that is an apt place to begin your investigation, it is also an oversimplification.
Hardwood features a more porous and noticeable grain than softwood; image via Diffen.
Hardwood vs. Softwood
Hardwood comes from angiosperm trees that are not monocots, which are typically broad-leaved or flowering. In these species, vessels transport water throughout the wood, which give it a more porous and noticeable grain. Typically, hardwood has a higher density and slower growth and replenishment rate than softwood and is thus both stronger and more expensive. While common examples such as maple, mahogany and walnut are more historically reminiscent of small-scale craftsmanship and high-quality furniture, hardwoods such as oak and chestnut are popular selections for wood cladding, especially in the U.K.
Softwood — the overwhelming majority of timber that you see in building components — comes from gymnosperm trees, which typically have needles and cones. Cheaper, faster-growing and easier to manipulate, cedar, Douglas fir, pine and redwood are all widespread species of softwood cladding. Primed with this distinction, here’s what you need to know about a few of the most popular types of wood cladding before specifying timber for your next project:
In reference to the surrounding nature and agrarian landscape, House in Oxfordshire by Peter Feeny Architects is entirely clad in oak. Boards extend all the way over the roof, giving the structure a naturally unified appearance.
Oak, which has a density of 36 to 56 pounds per cubic foot, is an extremely hard-wearing wood that can provide a natural and attractive appearance for anywhere between 40 and 60 years. Providing that only heartwood is used and any sapwood is excluded, oak can be used as a cladding material without any treatment. Yet while oak is durable and naturally resistant to rot, most manufacturers still recommend applying a protective treatment in order to avoid damage caused by uncontrolled exposure to ultraviolet radiation and moisture.
Sapwood — the outer layer of the tree trunk — is susceptible to fungus, contains a lot of moisture and will considerably shrink when dried. Heartwood contains less moisture and is the stronger more stable spine of the tree; image via Virginia Wood Flows.
With a slow replenishing cycle of 50 to 100 years, oak is an expensive choice for cladding an entire building. Additionally, the fixing methods associated with hardwood are typically more expensive than softwood. However, depending on where you live, sourcing locally grown timber can make the process more affordable.
Cedar, which has a lower density of 23 pounds per cubic foot, is the most popular choice for wood cladding, and it’s no secret why. As reflected in its density, cedar is soft and lightweight, yet it possesses impressive durability and stability. Not only does cedar provide excellent acoustic and thermal insulation, but it also demonstrates a low shrinkage factor and resistance to warping and twisting.
Like oak, cedar is naturally resistant to decay and insect attack and can be used without preservative treatment as long as sapwood is excluded. Good quality heartwood will achieve similar levels of weather resistance to oak and has an expected service life of 40 to 60 years.
Western red cedar once weathered and silvered down; image via English Woodlands Timber
However, when working with cedar, there are a few things to remember. Natural oils in the timber have corrosive effects on ferrous metals, which means that while cedar can be nailed and screwed without splitting, galvanized or stainless-steel fixings should always be used. Finally, compared to oak, cedar is less resistant to physical knocks and scrapes, and areas of high pollution will quickly accelerate the rate at which cedar becomes stained.
Located in Dublin, Laneway Wall Garden House by Donaghy & Dimond Architects features both new and already-weathered Douglas fir cladding and joinery. With time, the entire palette will naturally even out.
Douglas Fir (Softwood)
A great all-rounder with a density of 33 pounds per cubic foot, Douglas fir is a softwood with the reputation of being a species that can be used untreated. However, this, again, is only true of heartwood, which will still only offer a service life of 25 to 30 years. One of the most uniquely fascinating things about Douglas fir is that because this species of tree tends to grow tall and straight, wood boards are available in longer continuous lengths than most other species can offer. Depending on the nature of your design, this may prove to be an extremely desirable quality in creating a visually appealing structure.
No matter what species you choose, it is important to remember that, without exception, all wood weathers. And so when speaking with clients about specifying wood cladding, it is essential that they understand that no wood-clad building will have a stagnant appearance from inception to decay. Many manufacturers even warn that after five years, all species will have weathered to a near-matching silver-gray. That said, always make sure that the timber delivered to site meets specification and do not be afraid to reject damaged or visually unacceptable pieces.
Finally, as a natural commodity, lumber products tend to be volatile in both supply and price, which is why fast-growing softwoods are so favorable and common. These details are what make every decision around wood cladding one that is directly tied to the time and place in which you are building.