You cannot make wood truly waterproof, and that is a good thing. Wood moves: it takes up moisture and gives it back off again, and a treatment that seals that off completely also seals the moisture in the wood. That is exactly how rot starts. What you do instead is make wood water-repellent and moisture-regulating, and that does not start with a tin. It starts with the construction: a fall so water runs off, end grain sealed, no contact with the ground and an air gap everywhere. Then you pick the product: oil for hardwood and decking, stain or opaque paint for softwood, and varnish only indoors or on stable hardwood. The condition for all of it: the wood has to be dry, with a moisture content of no more than about 18 per cent. Treat wetter wood than that and your coating will flake within a season while the moisture stays locked in.
Hello fellow DIYer,
“I just want to waterproof it.”
I hear it all the time in the builders’ merchant, and I know exactly what people mean. You have a fence, a deck or a bench, you want it to last, and you are looking for the one tin that sorts it out.
Only: that tin does not exist.
I once gave a spruce garden bench two proper coats of yacht varnish. Lovely and glossy. Beautiful. Two years later I put my thumb straight through the seat. Under that sealed varnish the wood had quietly rotted away, and I had not spotted it because the varnish still looked perfectly fine.
That was my lesson: wood you seal up rots from the inside out.
In this article I explain how it really works. What those classes on the label at the timber rack mean, which treatment belongs with which wood, in what order you work, how often you need to keep on top of it, and which eight mistakes will cost your wood years.
Here we go:
Why wood never becomes waterproof
Wood is made of tiny tubes that once carried water up through the tree. Those tubes are still in there. Stand wood out in the rain and it soaks up water like a sponge, especially at the end grain (the face you have sawn, across the grain).
That in itself is not a problem. Wood gets wet and dries out again, it has been doing that for centuries. It only becomes a problem when it stays wet for too long. Above a wood moisture content of roughly 20 per cent, fungi get their chance, and that is when rot begins.
So timber protection comes down to one thing: making sure moisture can get back out quickly.
Which makes the two classic mistakes obvious straight away:
- A sealed, hard coating on softwood outdoors. Sooner or later it cracks (the wood moves, the coating does not), water gets in through the crack, and it can never get back out.
- Treating wood that is still wet. You lock the moisture in. Flaking coating, and underneath it the perfect growing tank.
The classes on the label: what do they mean?
Every piece of garden timber carries a label with a class on it. There are two of them and they get mixed up constantly.
Durability class (BS EN 350): how strong is the wood itself?
This is about the natural resistance of the heartwood to fungi and insects. Five classes, from 1 (very durable) to 5 (not durable).
| Class | Meaning | Indicative service life outdoors, above ground | Examples |
|---|---|---|---|
| 1 | Very durable | 25 years and more | Azobé (ekki), teak, Accoya (modified) |
| 2 | Durable | roughly 15 to 25 years | European oak, western red cedar, bangkirai, thermally modified timber |
| 3 | Moderately durable | roughly 10 to 15 years | Douglas fir, larch, pressure-treated softwood |
| 4 | Slightly durable | roughly 5 to 10 years | Untreated spruce, pine, larch (depending on origin) |
| 5 | Not durable | less than 5 years | Beech, poplar, sapwood of virtually any species |
Two things that are almost never mentioned and that make all the difference:
The class applies only to heartwood. The paler sapwood on the outside of the trunk is not durable in practically any species, oak and Douglas fir included. If there is a lot of sapwood in your board, you will not reach the class. You can spot it by the lighter colour and by the fact that sapwood takes up moisture far more readily.
Origin counts. Slow-grown Douglas fir or larch from a cold climate has tight growth rings and performs noticeably better than fast-grown timber from a warm climate. That is why you sometimes see the same species listed in class 3 in one table and class 4 in another.
Use class (BS EN 335): how hard a life does the wood have?
This is not about the wood but about where you put it.
| Use class | Situation | Example |
|---|---|---|
| UC 1 | Indoors, dry | Furniture, internal panelling |
| UC 2 | Indoors, risk of condensation | Roof structure, bathroom panelling |
| UC 3 | Outdoors, above ground | Fence, cladding, decking, window frames |
| UC 4 | Contact with ground or fresh water | Posts in the ground, bank revetments, retaining walls |
| UC 5 | Salt water | Jetties, harbour works |
The link between the two is the crux: a board treated for UC 3 (above ground) will not survive if you put it in the soil. And that is exactly what happens in thousands of gardens with “pressure-treated” timber that goes into the ground as a post.
If you want posts in the ground, buy timber that is explicitly treated for ground contact (UC 4), or hardwood in durability class 1 or 2, or set your post in a hot-dip galvanised post anchor so the timber never touches the soil.
Pressure-treated timber: what it is and what it is not
Pressure-treated timber, still widely called tanalised, is softwood (usually spruce) that has had a preservative forced into it under pressure. That usually moves it from class 4 to roughly class 3.
- The old salt formulations are no longer used. The name has stuck, but the products have been replaced by more modern ones.
- Look for a recognised quality mark (in the UK, for example the WPA Benchmark mark for treated timber): it shows the timber was treated to a fixed set of requirements.
- Standard treated spruce is intended for use above ground. Above ground and well ventilated, you will usually get ten to fifteen years out of it.
- The treatment sits in the outer layer. Saw a board through and the end grain is unprotected. Treat every saw cut with a suitable end-grain preserver or preservative basecoat. This is the number one reason treated fences start rotting at the top.
Step 1: constructional timber protection (this does more than any tin)
Before you pick up a single brush: this is where your biggest gain sits. Carpenters call it constructional timber protection, and it often buys more service life than the best coating.
Seven rules:
1. Make sure water runs off instead of standing. Every horizontal surface outdoors is a pond in the making. Give post tops, handrails and sills a fall of at least 10 to 15 degrees. A post top cut on the angle is the cheapest life extension there is.
2. Seal the end grain. End grain soaks up moisture many times faster than the face, and that is where rot starts. Treat every saw cut straight away, and on an important structure do it two or three times until the wood stops taking anything up. There are dedicated end-grain sealers, but several coats of the same preservative basecoat work just as well.
3. No contact with the ground. Wood lying on paving or on sand stays wet. Set your deck joists on slabs or rubber pads with an air gap underneath, and use post anchors rather than timber cast into concrete.
4. A ventilation gap everywhere. Cladding on battens with a cavity behind it. Deck boards with a 5 to 8 millimetre gap between them. A fence held a few centimetres clear of the ground. Air that can pass along the wood dries it.
5. Put the right metal in. Use stainless steel (A2 or A4) or hot-dip galvanised screws. Ordinary zinc-plated screws rust and leave black streaks, and with oak and western red cedar the tannins also react with iron, which gives you permanent black staining.
6. Protect the top. A cap on a fence post, a drip edge above cladding, an overhang above a door. Water that never lands on it is water you never have to hold back.
7. Think about the underside. Treat the underside of boards and the back of cladding before you fit them, so before you screw them down. Afterwards you can never get at it, and that is often exactly where moisture sits longest.
Step 2: when are you allowed to treat?
Three conditions, and all three are hard rules.
The wood has to be dry enough
No more than about 18 per cent wood moisture content. Coating manufacturers usually set a limit of 17 to 19 per cent. Above that value your coating will flake, and on top of that you are sealing moisture in.
How do you measure it? With a moisture meter, available from 20 to 40 euros. Pins into the wood, read it off. That is worth the investment if you have a bigger job ahead of you, because it is the only hard answer to the question “can I start yet”.
No meter? Rules of thumb:
- Freshly delivered garden timber, stacked soaking wet: let it dry stacked and ventilated for a few weeks to a few months first (with battens between the layers).
- After a spell of rain: reckon on at least two to three dry days with a bit of wind.
- The drop test for treated timber: put a drop of water on the wood. If it beads up, the wood is still too oily or too wet. If it soaks in within about a minute, you are good to go.
The weather has to play along
- Application temperature is usually between about 8 and 25 degrees, check the label
- Not in full sun. The coating then dries at the surface faster than it can soak in. Work on the shaded side, or early in the morning and late in the afternoon.
- No rain within the drying time. Do not look at “touch dry in 2 hours” but at the recoating time on the tin, and leave yourself a margin.
- Not in high humidity or just before a night frost. Late autumn evenings with dew settling on the wood are notorious coating-wreckers.
The wood has to be clean
Green growth, grease, dust and old loose coatings all have to come off. Treat greened wood with a mould and algae remover first, let it work, scrub it and rinse well. Then let it dry out completely (reckon on several days).
Never take a pressure washer at full power to softwood: you blast away the soft spring growth rings and are left with a rough, fibrous surface that takes up moisture even faster. If you do want to use one, use a fan nozzle at low pressure and keep your distance.
Step 3: which treatment do you choose?
Only now the tin. And here is the comparison you need.
| Product | How it works | Suitable for | Maintenance | Watch out for |
|---|---|---|---|---|
| Preservative basecoat (primer stain) | Soaks in deep, fungicidal, no film | Softwood, as the first coat under stain or paint | Basecoat only | Rarely a finish coat; gives little UV protection |
| Translucent stain | Soaks in, moisture-regulating, wood stays visible | Softwood, garden timber, cladding | Every 2 to 3 years | Little pigment means little UV protection, so it greys faster |
| Opaque stain | Partly soaks in, leaves a thin film | Softwood outdoors, fences, garden sheds | Every 4 to 6 years | Best all-rounder for softwood outdoors |
| Opaque paint (water-based) | Film coating with plenty of pigment | Window frames, doors, cladding | Every 5 to 8 years | Always over a proper primer system; end grain gets extra |
| Wood oil | Soaks in, feeds the wood, no film | Hardwood, decking, garden furniture, worktops | Yearly, sometimes twice a year | Protects against drying out and staining, not against everything; the wood still greys |
| Varnish / lacquer | Hard, sealed film | Indoors, and on stable hardwood under a canopy | Every 2 to 4 years outdoors | On softwood outdoors: it will do what my bench did |
| Water repellent | Makes the surface water-repellent, stays vapour-open | Masonry facades, sometimes wood | Every 5 to 10 years | No UV protection, no colour, no scratch or dirt resistance |
The choice in one line per situation
- Fence in treated spruce: give the end grain and saw cuts an extra coat, then opaque stain. Or simply let it grey, which is perfectly fine too, but do the end grain either way.
- Hardwood deck: hardwood oil, or nothing. Never varnish. A deck lies flat, holds standing water and gets walked on: any film coating will break down.
- Deck in treated spruce: a dedicated decking oil or an opaque stain approved for walked-on surfaces. And above all: good ventilation underneath.
- Cladding in Douglas fir or larch: translucent or opaque stain, or let it grey on purpose. Greying is purely cosmetic and does not damage the wood, as long as you have built it soundly.
- Window frames and external doors: an opaque paint system, primer plus two coats. This is not the place to experiment, because a leaking frame sill costs thousands.
- Garden furniture in teak or iroko: oil for the colour, or nothing for the silver-grey look. Both are fine.
- Scaffold-board furniture that lives outdoors: opaque stain or a water-based exterior paint. Scaffold board is spruce, so class 4, and it genuinely needs protection.
- Play equipment or a sandpit: choose a product declared suitable for children’s toys, or leave it untreated and use timber with a higher durability class.
Why dark colours and translucent coatings behave differently
Two things the builders’ merchant does not tell you:
Pigment is UV protection. The more translucent the coating, the less it protects the wood from sunlight. Clear greys the fastest, semi-translucent holds out longer, opaque longest of all. Which is exactly why a clear stain needs doing every two years and an opaque paint lasts five.
Dark colours get hotter. A dark brown or anthracite fence facing south can climb a long way in temperature over the summer. More heat means more movement in the wood and so more stress on the coating. On large sunlit surfaces a mid tone is often wiser than near-black.
Step 4: applying it without mistakes
Sand lightly where it is needed: 120 to 180 grit on new wood, coarser over old coatings. Dust off with a brush and a slightly damp cloth.
Start with the end grain. Always. Two or three times, until it stops drinking. Only then the rest.
Work each board end to end, do not stop halfway. Otherwise you get lap marks you will never get rid of.
Thin the first coat if the manufacturer says to: a thinner first coat soaks in deeper.
Two thin coats beat one thick one. A thick coat skins over on the outside, stays soft underneath and wrinkles.
Treat every face before you fit it. Underside, back, ends. This is the step most people skip and the one that buys the most service life.
Brush, roller or spray? For stain and oil on rough timber a good brush is best, because you genuinely rub the product in. A roller is quicker on large smooth surfaces, but always go back over it with a brush afterwards (that is called laying off). Spraying is fast, but you get less product into the wood and a lot more mist across the garden.
Wipe off excess oil after 15 to 30 minutes with a cloth. Oil left standing on the surface goes tacky and attracts dirt.
Watch out for spontaneous combustion with oil. Cloths with linseed oil or wood oil on them can heat up on their own and catch fire. Lay used cloths out flat outside to dry, or put them in a sealed jar of water. This is not scaremongering, it really happens.
Maintenance: how often do you need to keep on top of it?
| Application | Treatment | Interval | Sign that it is time |
|---|---|---|---|
| Hardwood deck | Oil | Yearly, sunny side sometimes twice | Water no longer beads, wood goes dull |
| Softwood deck | Oil or opaque stain | 1 to 2 years | Grey patches, rough surface |
| Softwood fence | Opaque stain | 4 to 6 years | Dull colour, wood soaks up water |
| Softwood fence | Translucent stain | 2 to 3 years | Greying on the sunny side |
| Cladding | Stain or paint | 4 to 8 years | Chalking (white powder on your finger) |
| Window frames | Opaque paint system | 5 to 8 years, sills more often | Hairline cracks, loss of gloss, sealant joints letting go |
| Garden furniture | Oil | Yearly, at the start of the season | Dry, dull wood, rings from glasses |
The best maintenance tip there is: do a round with a bucket of water every spring. Throw some water on your wood. If it beads up, you are in good shape. If it soaks straight in and the patch goes dark, your protection is spent, and touching it up now is far cheaper than next year.
And keep an eye on the spots that give up first: the south and west sides (sun and driving rain), sills, end grain, and anything lying horizontally.
Eight mistakes that cost your wood years
1. Varnishing softwood outdoors. The classic, and the most expensive. The varnish cracks, water gets in and never gets out again. Under an intact-looking varnish film the wood can be completely gone.
2. Starting on wet wood. No more than about 18 per cent moisture. Anything above that is trapped moisture and a flaking coating.
3. Leaving saw cuts untreated. Every time you cut a board to length you open up an unprotected mouth that drinks. Treat it immediately, even when you tell yourself you will get to it later.
4. Wood on the ground or against the soil. Ground contact demands use class 4 or hardwood. A board sat on sand stays wet for days after every shower.
5. Skipping the back and the underside. You treat the pretty face and screw it down. Afterwards you can never get back to it, and that is precisely where moisture lingers longest.
6. Too little ventilation. Deck boards butted tight together, cladding without a cavity, a fence sitting on the paving. Anything that gets no air stays wet.
7. Different systems over each other. Oil over stain, or stain over an old varnish film. It will not stick. Pick one system and stay with it, or strip the old one off first.
8. Waiting until it looks bad. Touching up a coating that is still largely intact is an afternoon’s work. Bringing back greyed, roughened wood means sanding, cleaning, drying and building up again, so a full weekend.
Special cases
Wood in a pond or in water. Above the water: hardwood in durability class 1 or 2. In the water: use class 4 or 5. Do not treat wood that comes into contact with pond water with anything that is not expressly pond-safe, because many wood preservatives are toxic to fish and amphibians.
Wood in the vegetable garden and around edible crops. Choose untreated durable timber (Douglas fir, larch, oak) or put a membrane between the wood and the soil. Treated timber around edible crops is an argument without end; with a simple pond liner or dimpled membrane on the inside of your bed you sidestep the whole question.
Worktops and chopping boards. Only products suitable for food contact. Think boiled linseed oil for outdoors or a dedicated worktop oil for indoors. Never ordinary wood oil with driers in it on something you cut on.
Thermally modified timber. Wood heated to around 200 degrees, which makes it take up far less moisture and puts it in durability class 2. Lovely material, but it is more brittle than ordinary timber: pilot holes are compulsory and it can splinter.
Accoya and other modified species. Chemically modified softwood with very high durability and very little movement. Expensive to buy, but on window frames it can work out cheaper over 25 years than three rounds of repainting. Always use stainless steel fixings and the coating systems the supplier specifies.
Frequently asked questions
Can you really make wood waterproof?
No. Every treatment makes wood water-repellent, not waterproof. And that is exactly what you want: a coating that keeps moisture out from the outside but still lets the wood breathe. A fully sealed coating (like varnish or epoxy on softwood outdoors) traps moisture the moment a crack appears anywhere, and then the wood rots away from within while the outside still looks fine.
What is better outdoors: oil, stain or paint?
That depends on the wood. Oil for hardwood, decking and garden furniture: it soaks in, keeps the wood supple and you never have to sand, but it has to go back on every year. Opaque stain for softwood such as fences and garden sheds: a good balance between protection and upkeep, roughly every four to six years. Opaque paint for window frames and cladding: the most protection and the longest intervals, but more preparation, and you have to stay consistent with the system.
Does treated timber still need treating?
Not compulsory, but sensible on two counts. The treatment protects against fungi and insects, but not against UV and moisture cycling, so without a coating the wood greys and starts to crack. And every saw cut is untreated: you always have to touch those up, otherwise rot starts exactly there. If you want to paint or stain, wait until the wood is dry enough and the water drop soaks in instead of beading off.
How dry does wood need to be before you treat it?
No more than about 18 per cent wood moisture content; coating manufacturers usually work to 17 to 19 per cent. Measure it with a moisture meter (from about 20 to 40 euros). Without a meter: let freshly delivered garden timber dry stacked with battens between the layers for a few weeks first, and after rain wait at least two to three dry days before treating.
How often should I re-treat my fence or deck?
Rules of thumb: hardwood deck with oil every year, fence with opaque stain every four to six years, fence with translucent stain every two to three years, and window frames with opaque paint every five to eight years (sills more often). Just test it: throw water on the wood. If it beads up, you are in good shape. If it soaks straight in and the patch goes dark, it is time.
Why does my fence always rot at the top of the posts?
Because the top is a horizontal surface where water sits, and because it is often a saw cut nobody ever treated. Two solutions that work together: cut the post top on an angle so water runs off, or fit a cap on it, and treat that end grain two or three times with preservative basecoat until it stops taking anything up.
Can I put oil over stain, or the other way round?
No, not just like that. Oil will not bond to a sealed stain film, and stain bonds badly to an oiled, greasy surface. Pick one system and stick with it. If you do want to switch, the old coating has to come off completely by sanding or blasting, and then you build back up from bare wood.
To finish
The question is never “which tin makes my wood waterproof”.
The question is: can the water get off, can the air get past, and is the end grain sealed.
Sort those three, then pick a product that suits your wood, and put one round with a bucket of water in your diary every spring.
Then your garden timber will last twice as long as the neighbour’s, the one with the finest yacht varnish in the street.
Want to get the jobs in and around your house right first time? At Fred’s DIY Plans you will find clear building plans with material lists and step-by-step instructions, from preparation right through to the final coat. Do yourself a favour. Have a look here.
Best regards from Fred Schouten, builder and DIYer through and through
PS: Buy that moisture meter. Thirty euros, fits in your coat pocket, and it stops you at exactly the moment you get impatient and want to start anyway. I have prevented more damage with it than with any tin.