Last Updated on September 26, 2026 by Umar Farooq
Cleaning a shed is not a timber job or a metal job — it is a building job, and the building is twelve millimetres thick. There is no cavity behind the cladding, no drainage layer, and nothing on the inside face of the board except your lawnmower. So the method is chemistry, a soft brush and a rinse that always travels downward, with the machine kept off the roof entirely and kept away from the bottom six inches of wall.
Every guide to this job is a fence guide with the nouns changed. That is understandable, because a shed wall looks exactly like a fence panel and the species question is genuinely the same one. It is also the wrong frame: a fence has weather on both sides and nothing behind it, and a shed has a room behind it with your things in it. Think of it as a structure rather than a surface and three problems appear that no timber page on this site covers. The most useful of the three is underneath your feet.
Four Numbers Are the Entire Building
Manufacturers publish the whole specification, and it is shorter than you would like. Tiger Sheds’ technical data sheet for its TigerFlex shiplap apex gives the cladding as “Superior Grade 12mm Finish Shiplap T&G Cladding” on “28x44mm Finish Rounded Four Corner Framing”, with “12mm Thick T&G Roof and Floor — NO OSB / Solid Sheet Board”, and the roof covered in “Black Mineral Felt” (Tiger Sheds, TigerFlex Shiplap Apex Shed technical data sheet).
Four numbers, and they describe the building completely. A 12mm wall board on a 28mm by 44mm frame, a 12mm floor board, a 12mm roof board with felt on it. There is no fifth number because there is no fifth layer.
Put that next to the wall of the house behind it. A modern clad wall is a stack: cladding, breather membrane, sheathing, a deliberately drained and vented cavity, insulation, a vapour control layer, plasterboard. Seven layers, of which the outer one is expected to leak occasionally and the second exists specifically to catch it. A shed has one layer, and it is also the structural bracing, and it is also the interior decoration. Twelve millimetres — half an inch, near enough — is the entire distance between the weather and your bag of tulip bulbs.
The metal version says so out loud. Arrow’s owner’s manual for a galvanised steel building asks you to “periodically clean and wax the exterior surface”, touch up scratches the moment you see them, and keep the roof clear with a long-handled soft-bristled broom. Then, under a heading called Other Tips, in the same short list as an instruction to wash the ink part numbers off the panels, it offers this: “Silicone caulking may be used for watertight seals throughout the building” (Arrow Storage Products, Owner’s Manual & Assembly Instructions, model WH1014-D1). Watertight is a tip. It is optional, it is your job, and it costs about four pounds in a tube. Condensation gets the same treatment — “a plastic sheet (vapor barrier) placed under the entire floor area with good ventilation”, supplied by you, after purchase.
None of that makes a shed badly made. It makes a shed exactly what it is sold as: a dry box, in normal weather, assembled by an amateur, with no redundancy anywhere.
Nothing Behind the Cladding Is Waiting to Catch Water
Here is the opinion I will defend on this page, and it is the whole reason a shed needs its own article. Every other wall on your property is designed to leak a little, and that is precisely what makes it safe to wash. A shed is not designed to leak at all, and that is what makes it not.
The standard warning on a lapped wall is that a jet aimed upward drives water behind the laps. That warning assumes a *behind*. On a house there is one, and it is a designed space: water past the cladding lands on a membrane, runs down and comes out at the bottom, and nobody ever knows. On a shed the reverse face of the board you are spraying is the wall of the room. There is no second event. Whatever goes through arrives.
Three paths through, in rough order of how often they are the culprit.
The joint itself. Shiplap is a machined profile — Tiger describes its board as having “scalloping to help prevent water ingress”, which is a drip groove formed into the underside of the lap. It works by giving falling water a corner to let go of. Aim a stream up at it and the same groove becomes a channel with a direction, and the direction is inward. A cheaper overlap or featheredge shed has no rebate at all, only thin boards nailed along their laps, so the lap flexes away from the board below when something pushes on it. Put a hand flat on an overlap wall and push.
The fixings. Every cladding board is pinned through its face into a 28mm by 44mm frame, and the panels are then screwed to each other at the corners, through both sets of boards, from outside. Each of those is a hole with a fastener in it and nothing else.
Seasonal movement. Timber shrinks in a dry summer and a tongue-and-groove joint accommodates that by opening. The gap that lets water through in August is not there in March, which is why people wash a shed successfully one year and flood it the next with the same technique.
So the first job is not cleaning. It is finding out where your particular shed is open, and a shed lets you do that in a way a house never will, because you can get inside it and shut the door. On a bright day, empty enough of it to stand in the middle, close the door, and let your eyes adjust for a full minute. Every pinhole of daylight is a path, and paths that pass light pass water once the water has momentum behind it. It feels ridiculous, sitting in a dark shed counting stars. It is the most useful two minutes in the job.
Then take the advantage the shape gives you. A shed’s eaves sit at roughly six and a half to seven feet, which makes it the only building on the property you can stand beside and aim *downward* at along its whole height, ridge to floor line, with no ladder and without ever pointing the stream level or up. The angle is free and worth more than any setting on the machine.
Getting it wrong does not produce a leak you mop up. It produces a closed timber box with wet contents and one door, which is a separate problem with its own page: how to remove mildew odour covers what a damp shed does next, why it is worst at ankle height, and why ventilation rather than sealing is the fix.
The Wettest Wood in a Shed Is Two Inches Off the Ground
This is the part I would keep if I could only keep one.
A shed floor is 12mm of tongue-and-groove board sitting on bearers, and the bearers sit on the base. That puts the underside of your floor two to four inches above ground level, in the least ventilated cubic foot in the garden, permanently in shade, with no airflow beneath it unless somebody deliberately provided some. The bottom cladding board then runs down past the floor line to within an inch or two of the base, with no flashing, no drip edge and no starter strip — the details a house wall gets at exactly this junction and a shed wall does not.
That height is already marginal before you arrive with a hose. Rain hitting the base splashes back onto the bottom board, and where the slab or paving is bigger than the shed, water stands in the strip outside the wall and soaks the timber above it. That is why shed makers’ base guidance keeps repeating the same three instructions: make the base the same size as the floor or a fraction smaller so the floor oversails it, put a drainage layer of gravel underneath, and where the base is larger, lift the bearers on a damp-proof course so puddles cannot reach the floor timbers. The guidance exists because this junction is the one that fails.
Now add the wash. A homeowner electric machine runs 1.2 to 1.6 GPM. Call it 1.4, and call the job forty minutes of trigger time, which is realistic for a six by eight shed with detergent and two rinses. That is about 56 US gallons — and the next part is my arithmetic from a published flow rate, not anybody’s published figure. Spread evenly over a six-by-eight footprint, 56 gallons is close to two inches of rainfall. It does not spread evenly. Almost all of it drains down the walls into a strip of base perhaps six inches wide around the perimeter, roughly fourteen square feet, where the same volume works out at over six inches of rain — in forty minutes, against the one part of the building with no drainage detail and no air movement.
That is the mechanism nobody mentions. Washing the outside of a shed makes the inside damp from the bottom, not from the walls, which is also why sheds often smell worse in the month after a clean than they did before it.
- Pick the weather for the base, not for the walls. A dry morning with two or three dry days behind it and ahead of it. The walls dry in an hour. The bearers do not.
- Do not let the runoff pond. Brush water away from the perimeter as you work, and do the elevation that drains away from the shed first.
- Leave the greenest stripe alone. The bottom six inches of any shed wall is the dirtiest part of it because it takes the splash, which makes it both the place your wrist wants to go and the place a jet does the most harm. That band gets cleaner, a soft brush, dwell time and a rinse from above — never a stream aimed into the gap between the board and the base.
- Look underneath when you are done. A phone camera pushed under the bearers on a stick tells you in ten seconds whether water is sitting there, and whether it is still sitting there tomorrow.
Felt, Sheet and Shingle All Lift From Below
Three coverings account for nearly every shed roof: mineral felt, corrugated bitumen or metal sheet, and asphalt shingle. All three fail from below, because all three are lapped, all three are nailed through the lap, and all three sit on the shallowest pitch the manufacturer allows.
The pitch is the part worth understanding, because one manufacturer has quantified it and the table is more persuasive than any warning. Onduline’s installation specification for its corrugated bitumen sheets sets out three cases. Above 15 degrees, purlins at 61cm centres and a 17cm sheet end lap. Between 10 and 15 degrees, purlins at 45cm and the end lap grows to 20cm. Between 5 and 10 degrees — which is a great many pent sheds — “a fully supporting roof deck should be laid (20mm minimum)”, the end lap goes to 30cm, and the side lap doubles from one corrugation to two (Onduline, Onduline Classic installation).
Read what that table is telling you. As the pitch falls, water travels further up the underside of a lap before gravity beats it. The manufacturer has measured how much further, and charges you 13cm of extra sheet and a doubled side lap to cover it — for rain, arriving at whatever angle the sky chose. A jet aimed up at an eaves arrives at a pitch of zero or worse, with momentum, from the one direction the lap has no answer to. There is no end lap long enough for that.
The fixing schedule makes the same point from the other end. Onduline’s instruction is “every corrugation fixed at eaves, overlaps and ridges”, with every second corrugation at intermediate purlins, so the laps are the most perforated part of the roof and each perforation is a hole closed only by the head of a nail. The maximum sheet overhang at the eaves is 7cm — under three inches of unsupported material between the drip edge and the first line of support.
Felt is worse, and it is the covering most sheds actually have. A shed felt roof is bitumen-saturated material rolled out in strips, lapped, and fixed with clout nails along the edges and through the laps; the field of the roof has nothing holding it down at all, and UV makes it brittle from the day it goes on. A jet under a felt edge or a lap lifts it, and lifted felt does not lie back down. The repair for a lifted lap is a new roll of felt, which is a Saturday rather than a rinse.
Asphalt shingles on a shed sit below the pitch they were designed for, which leaves the self-seal strip doing most of the work of holding each tab flat. The site’s position on shingles is not a matter of degree and has not changed: they do not get pressure, ever, on a house or on a shed. The alternative that does work is on how to clean roof shingles without pressure washing.
What the manufacturers themselves ask for is modest to the point of being an anticlimax. Onduline’s entire published maintenance instruction is that “the roof should be cleaned regularly of leaves and debris, as leaf mould can reduce the effective life of the product”, plus keeping branches off the surface. Arrow’s is a long-handled soft-bristled broom. Two companies, two very different materials, and neither maintenance section contains a tool with a trigger on it.
Identify the Cladding Before You Mix Anything
Material does not change any of the above; the structure argument holds whatever the shed is made of. It does decide what you are allowed to put on it, and four different products get sold as “a shed”. Two minutes with a fingernail settles which one you own.
| What you have | How to tell | What it needs | Where the detail lives |
| Bare or dip-treated softwood | Visible grain, a thin factory colour that has faded unevenly, boards you can dent with a thumbnail | Oxygen-bleach wood cleaner, dwell, soft brush, low-pressure rinse | Cleaning cedar siding |
| Painted or solid-stained timber | A film you can lift at an edge with a fingernail | Depends entirely on whether the film is sound | Pressure washing painted wood |
| Painted or galvanised thin steel | Sounds hollow, no grain, panel ribs, screws with washers | Mild detergent and a cloth; the coating is the whole product | Cleaning a metal roof |
| Moulded plastic or resin | Flexes under a thumb, moulded grain pattern, hollow double-wall panels | Mild detergent, soft brush, no solvents | — |
Softwood is what most timber sheds are, and its pressure ceiling sits below where any consumer machine idles. The figure, and what furring looks like once you have gone past it, are on the cedar page rather than restated here — the half worth carrying over is that the damage is invisible until the wood dries, so a test patch gets judged hours later rather than minutes. If the shed is grey rather than green, that greyness is sun damage rather than dirt, and how to pressure wash a fence has the brightener argument for it.
Thin steel is where the structure argument is at its sharpest, because on a factory-coated panel the coating is the product and the steel is only what it is stuck to. Arrow’s own remedy for a scratch says how it thinks about that: wire-brush the spot and get touch-up paint on it the moment you notice it. On timber a scratch is cosmetic until you decide otherwise. On a panel this thin it is the start of a maintenance task with a deadline.
Plastic sheds are the ones people assume are bulletproof, and the plastic mostly is. The hollow double-wall panel behind it is not, and neither is the thin steel frame inside that you cannot see and cannot dry.
Cleaner is the same answer for three of the four rows: sodium percarbonate — oxygen bleach — shifts the grey-black film and the green on shaded elevations without the surface damage chlorine bleach causes on softwood, and it is kinder to whatever is planted along the shed wall. Why it behaves differently, and its real dwell limits, are on oxygen bleach for cleaning.
Cleaning a Shed in the Order That Keeps Water Out
The sequence matters more than the kit. The kit is a pump sprayer, a soft long-handled brush, a garden hose, and the machine standing by for two jobs only.
1. Empty it, or empty enough of it. Fertiliser, seed, cardboard, paint, anything electrical, anything upholstered. A shed is small enough to empty completely, which is the one thing it has over every other structure on this list.
2. Do the daylight test with the door shut and mark the joints that pass light. Those are the joints your wrist has to respect for the next hour.
3. Repair before you wash. A loose board, a lifted felt lap, a popped nail, a split in the bottom cladding board. Washing a building through its own faults is filling it.
4. Clear the roof dry. Long-handled soft brush, working down the slope from the verge. Debris comes off better dry than wet, and without adding a gallon of water to a lapped roof.
5. Wet the plants and the base strip with clean water, then apply cleaner to the walls bottom to top with the pump sprayer. Bottom-up is the house rule here for the usual reason: cleaner running down dry timber dries into stripes you can read from the fence.
6. Give it ten to fifteen minutes and remist anything going matte. This step is the cleaning. Everything else is delivery.
7. Brush along the grain with the pole brush, gently, then rinse top to bottom — every pass travelling down and across, never level and never up. That is the site’s normal rinse direction and on a shed it is the only one available, because a top-down rinse keeps water moving away from every lap on the building while a bottom-up rinse aims into all of them at once. The roof, brushed dry at step four, gets no rinse at all.
8. Move the standing water away from the perimeter, then leave the doors and any vents open to dry.
The machine’s two honest jobs are applying cleaner through the soap nozzle at low pressure, and the rinse with the widest tip you own from further back than feels useful. On a building this size a pump sprayer does the first about as well, which is worth knowing before you drag a petrol engine across the lawn for a shed the size of a wardrobe.
Leave the treatment for a separate day. Every timber shed maker ties its guarantee to a maintenance coat, and a coat over timber still carrying wash water is a coat that peels. Twelve millimetres of softwood holds a surprising amount of water and gives it up slowly, particularly at the bottom.
FAQ
Can you pressure wash a shed?
Partly, and less of it than you would expect. The walls take a low-pressure rinse with the widest tip, from well back, every pass travelling downward. The roof takes none at all, whatever it is covered in. The bottom six inches of wall and the junction between cladding and base want a brush rather than a stream. Treat the machine as an applicator and a rinse tool rather than the cleaning force and the job goes fine.
What PSI is safe on a wooden shed?
The same softwood ceiling that applies to cedar siding and fence boards, which sits below the starting output of the cheapest electric machine sold — the figure is on the cedar siding page. The more useful control on a shed is not pressure at all. It is angle and distance, and a shed is short enough to keep every pass pointing down without a ladder.
How do you get green algae off a shed without a pressure washer?
An oxygen-bleach wood cleaner in a pump sprayer, applied bottom to top, kept wet ten to fifteen minutes, worked with a soft long-handled brush, then rinsed with a garden hose from above. On a building this size that is faster than setting the machine up, and the green comes off either way, because the chemistry is doing it.
Can you pressure wash a shed roof?
No, and the reason is the pitch rather than the material. Shed roofs sit at or near the minimum pitch their covering allows, and manufacturers already demand a longer lap at shallow pitches because rain travels further up a lap when gravity has less help. Onduline’s specification goes from a 17cm end lap above 15 degrees to a 30cm end lap and a doubled side lap between 5 and 10 degrees. A jet arrives at an even lower effective angle than that, with force. Clear the roof dry with a long-handled soft brush instead.
Should you empty the shed before cleaning the outside?
Yes, and for a better reason than keeping splashes off the contents. An empty shed can be stood in with the door closed, which is the only reliable way to find out where the building leaks before you test it with several hundred litres of water. It also gets everything off the floor, which is the wettest timber in the building.
Is a plastic or metal shed easier to clean than a wooden one?
The surface is, and the building is not. Moulded plastic and coated steel both wash off with mild detergent and a cloth, with no grain to raise. But a plastic shed has hollow double-wall panels that fill and do not drain, a steel shed is a coating over thin metal where every scratch becomes a rust site, and both are assembled from panels bolted together with washers rather than lapped. The cladding is easier. The joints are not.
How soon after washing can you re-treat a shed?
Longer than the surface suggests. Timber this thin looks dry within hours and is not, particularly along the bottom board and anywhere the runoff collected. Give it a genuinely dry few days and check the shaded elevation rather than the sunny one. A maintenance coat is what most timber guarantees depend on, and putting it over damp wood loses you both the coat and the guarantee.
Everything You Own Is on the Other Side of 12mm
A shed is the only structure on your property where the weather barrier, the bracing and the interior wall finish are the same half-inch board, where the floor sits two inches off the ground on bearers nobody has looked at since assembly, and where the roof is lapped at the shallowest pitch its own manufacturer will sign off on. Those three facts, and not the species of the timber, are what decide the method.
So the shed gets a pump sprayer, fifteen minutes of dwell, a brush on a pole and a rinse that never stops travelling downhill. The roof gets a broom and no water. The bottom six inches gets the most patience and the least pressure, because it is simultaneously the greenest band on the building and the only part of it with no drainage detail at all. Do the daylight test first and you will know, before you pull the trigger, exactly which joints are going to argue with you — which is more than you can find out about any other wall you own.
