Naturally Waterproof Wood - Choosing the right species

 Collection: Field Notes — Preserving Natural Materials at Sea 

Series Hub: Preserving Wood 

Subject: Which species resist decay by nature, and which need help to survive the sea

The durability classifications you find in timber reference books — Class 1 through Class 5, from very durable to not durable — look more authoritative than they are. They are averages. They describe what a species tends to do, not what any particular plank will do, and they apply to heartwood. Use them as a starting point and you will find yourself surprised, occasionally badly. I have had larch that lasted and larch that failed, from the same supplier, in similar conditions, for reasons that were not immediately obvious. Getting clearer on what durability actually rests on — the chemistry, the cell structure, the distinction between heart and sap — made the classification numbers feel less like answers and more like compressed questions.

This note is about those underlying mechanisms, and about the specific species I work with at VAKA: white and European oak, western red cedar, larch, spruce, ash, and red grandis. None of them are simple. The more useful ones turn out to be useful for reasons that are worth understanding rather than just accepting.

The treatment notes in the Preserving Wood series cover what to do once you have chosen a species. This note is the step before that — what you are starting with, and what it means for the maintenance commitment you are taking on. The VAKA field notes hub has the broader context for why these choices matter beyond the immediate practical question.

What Natural Resistance Actually Is

The first thing worth clarifying is that natural durability is not a single property. It is the combined effect of several different mechanisms, and a species can be strong on one and weak on another. A timber might have excellent resistance to fungal decay but be vulnerable to UV degradation. It might resist moisture uptake through the face but have open end grain that wicks freely. It might have good heartwood durability but negligible sapwood resistance. Understanding which mechanism is doing the work in a given species tells you where treatment is genuinely necessary and where you are adding it out of habit.

The primary source of biological resistance in most durable species is heartwood extractives — compounds deposited in the cell lumens as sapwood converts to heartwood over the life of the tree. These are phenols, tannins, terpenoids, flavonoids, and related compounds, and they are toxic or unpalatable to the fungi and bacteria that would otherwise consume the wood. The extractive content varies between species, between trees of the same species, and between the inner and outer heartwood of a single tree. It is not uniform, which is why durability claims are always averages.

The second mechanism is cell structure. Some species develop physical features in the heartwood that make it relatively impermeable to liquid penetration independent of extractive chemistry. White oak is the clearest example: it develops tyloses, bubble-like protrusions that grow into and block the vessel lumens in the heartwood. The result is that white oak heartwood does not transmit liquid readily — which is why it has been used for barrel staves for centuries, and why it performs so well in wet conditions even in sections where the extractive content might be variable. Red oak lacks tyloses despite having a similar density and a similar extractive profile, which makes it considerably more permeable and significantly less durable. The two look nearly identical in plank form. Confusing them is an expensive mistake, and I mention it because it is a common one.

The Sapwood Question

This matters more than the species tables suggest, and it is where I have seen the most significant practical failures in timber selection.

The durability classifications apply to heartwood. Sapwood in most species has little or no biological resistance regardless of what the heartwood does. Oak sapwood will fail in wet conditions roughly as fast as untreated pine. Cedar sapwood is not cedar in the sense that makes cedar useful. The pale outer band on a plank — sometimes narrow, sometimes startlingly wide depending on the log — is a different material from the heartwood beside it, and treating it as equivalent because it came from the same tree is the route to unexpected early failures.

Specifying heartwood, checking material on arrival, and rejecting sections with wide sapwood margins is basic quality control for any build intended to last. It sounds obvious until you are standing at a timber yard looking at a stack of planks and the sapwood-heavy ones are the same price as the others.

Oak — White and European

Oak is the primary structural species in VAKA builds: frames, keels, knees, inwales, the hardworking internal structure that everything else depends on. Its track record in marine construction is long enough to constitute a form of evidence in itself. But the mechanism behind its durability is worth understanding rather than just accepting.

European oak and white oak both develop the tyloses described above, making the heartwood effectively impermeable to liquid penetration. This is a structural property that operates independently of the extractive chemistry — even in sections of heartwood where the tannin content might be lower, the tyloses provide a baseline water resistance that most other temperate hardwoods cannot match. It is a belt-and-braces design: two different resistance mechanisms operating simultaneously.

The tannin content creates its own complication. Oak heartwood is rich in condensed tannins, which react with iron to produce the familiar black staining around iron fastenings. This is worth knowing before specifying fastening materials — the staining is a diagnostic indicator that iron and tannin are in contact, which means moisture is also present at that contact point, which means corrosion is occurring. The Natural Sealants and Adhesives series covers fastening choices in more detail. The relevant note here is that the same tannin that makes oak durable makes the choice of fastenings consequential in a way it is not with low-tannin species.

Using sapwood in wet or structural locations is the one thing that will reliably undermine everything else.

Western Red Cedar

Cedar is not inherently durable in the way oak is — its Class 2 to 3 rating reflects real-world performance that is better than density would suggest, but not in the same league as old-growth tropical hardwoods or even larch. What makes it interesting is why it performs as well as it does given how light it is.

The answer is thujaplicins — a group of tropolone compounds in the heartwood that have significant antimicrobial activity. They are not tannins; they are a different class of compound entirely, and their biocidal mechanism is distinct from the protein-precipitating action of tannins. Cedar also contains natural oils that contribute to moisture resistance — the distinctive smell of cedar is partly these oils, and a freshly sawn plank of western red cedar in good condition has a noticeable aromatic quality that diminishes as the oils weather from the surface.

The limitations are well established. Cedar is soft, with low stiffness and modest surface hardness. It bruises easily. It is not a sensible choice for wear surfaces, heavily loaded structural members, or anywhere subject to impact. At VAKA it is used for planking skins and lightly loaded structural elements where the weight advantage justifies its modest mechanical properties — which is, broadly, the application it has been used in across the Pacific Northwest for as long as there have been boats there.

Larch

European larch is the most naturally durable temperate softwood I am aware of, and the one I find myself reaching for when I need something tougher than cedar but lighter than oak. It is the only softwood that converts meaningfully from sapwood to durable heartwood, and the heartwood has documented biological resistance that clearly exceeds spruce, pine, or fir.

Why it is as durable as it is I am less certain about than I would like. The literature points to its resin content and specific extractive compounds, but the mechanism is less clearly documented than for oak or cedar. What I can say from observation is that larch performs considerably better in exposed wet conditions than its species classification strictly requires it to — tarred larch in particular behaves like a timber two durability classes above it, which suggests a good chemical compatibility between the phenolic compounds in Stockholm tar and whatever is in the larch heartwood, though I have not found a clear mechanistic account of this.

The practical considerations are knottiness — commercially available larch almost always has knots, and structural knots in bending members are a real problem — and the importance of heartwood specification. Larch sapwood is pale and not durable. The distinction between a good piece of larch and a bad one is often visible if you know what you are looking at.

Spruce

Spruce has no meaningful natural durability. This is not a criticism of the material — it is a description of what it is. The extractive content in spruce heartwood is low, the biological resistance is negligible, and in any persistently damp application without treatment it will fail. I use spruce for spars because its stiffness-to-weight ratio is exceptional and there is no comparable material at reasonable cost for that application. But I do not use it expecting durability that it cannot provide.

What this means practically is that spruce in marine service needs to be treated as a species that is entirely dependent on its treatment system rather than contributing anything of its own to biological resistance. End grain sealing is not optional — it is the difference between a spar that lasts and one that fails at the partners within a few seasons. Regular maintenance is not a schedule — it is a structural requirement.

I am sometimes asked whether the lack of natural durability in spruce makes it a poor choice for boat construction. I do not think so, provided the maintenance commitment is clear from the start. A well-maintained spruce spar lasts. An ignored one does not, faster than most other materials would fail under the same neglect.

Ash

Ash sits in a similar position to spruce on durability — its Class 5 rating is about as low as it gets — but it earns its place through different properties entirely. The toughness and shock resistance of ash are genuinely exceptional. For oars, paddles, thwarts, and any structural member subject to repeated impact loading, ash is the correct choice in a way that more durable species are not, because durability under impact and durability against biological decay are different things.

The maintenance implication is the same as for spruce: ash in any wet or damp location needs treatment, consistently applied. It does not contribute to its own survival the way oak or larch does.

There is also an availability question that has become increasingly significant. Ash dieback — caused by the fungus Hymenoscyphus fraxineus — is eliminating mature ash trees across much of the UK and Europe with a thoroughness that is becoming difficult to overstate. I am sourcing ash with more care than I was three years ago, and I am increasingly uncertain about long-term availability. This is not a reason to stop using ash for the applications it is uniquely suited to. It is a reason to source responsibly and to think about alternatives for applications where ash is convenient but not essential.

Red Grandis

Red grandis is a Eucalyptus grandis hybrid that I have been using with increasing confidence over the last few builds. It is plantation-grown, fast-growing, and its physical properties — hardness, dimensional stability, moisture resistance — exceed most temperate hardwoods in a way that was not what I expected before I tested it.

I want to be careful here about what I actually know versus what the supplier data says. The outdoor durability of red grandis in decking and cladding applications is well documented and the evidence base is solid. Its behaviour in fully marine construction — submersed fastenings, bilge exposure, continuous salt water contact — is something I am still gathering direct experience on rather than relying entirely on literature. Early indications are good. The material takes oils and finishes well, dimensions predictably, and has shown no signs of the instability I was initially concerned about. But I hold that conclusion with appropriate tentativeness.

What I can say with confidence is that it fills a practical gap — between the weight and cost of tropical hardwoods and the modest natural durability of temperate softwoods — in a way that makes it worth serious consideration for builders who want to work within a natural materials approach without the supply chain and provenance complications of teak or iroko.

How Resistance Relates to Treatment

The broader point this note is trying to make is that species choice and treatment are not separate decisions. They are a system, and the system needs to be designed rather than assembled from whichever materials are available.

A naturally durable species used without treatment will outperform a non-durable species treated well, under most conditions. But a naturally durable species with good treatment will outperform both, and the treatment requirements for a durable species are less intensive than for a non-durable one — which means the maintenance burden is lower over the life of the boat. This sounds obvious stated plainly, and it is obvious, but it is remarkable how often I see spruce treated the same way as larch, or cedar sapwood treated as if it were heartwood, or end grain given the same treatment as face grain.

The treatment notes that follow in this series — linseed oil, tung oil, Stockholm tar, boat soup, shellac, end grain treatment — are organised broadly by approach rather than by species, because the same treatment often applies across multiple species with adjustments for absorption rate and heartwood chemistry. The species you are working with changes the starting point and the maintenance interval. It does not change the underlying logic.

VAKA designs are built from natural materials, built to be maintained rather than disposed of. Plans at VAKA Boatplans, and the full knowledge base is at Field Notes.