Is this safe on fibre cement siding?

It can be, by its maker's method. James Hardie prohibits high-pressure washing on fibre cement and prohibits harsh chemicals such as bleach or acid on it, and what it prescribes instead is a low-pressure hose with a soft non-metal brush. By that method a fibre cement elevation is slower work. If your house is fibre cement on one side and something else on the others, the two call for different treatment.

Is bleach used to wash a house?

It depends on the wall, and that is the answer rather than a hedge. Sodium hypochlorite is what kills the growth, and the EPA-registered label for household bleach carries exterior directions for surfaces like siding, tile roofs, brick, stucco and patio stone. But James Hardie rules out harsh chemicals such as bleach on fibre cement, undiluted chlorine bleach is on the vinyl association's do-not-use list, and stucco needs a cleaner that will not attack the cement, the lime or the pigment. So no single answer covers a whole house: tell us what each wall is made of. On fibre cement, what the maker prescribes instead is a hose and a soft brush.

Can washing force water in behind vinyl siding?

It can, and the trade association names the exact way it happens: aiming the stream upward. PEPA instructs holding it level with your eye and never angling it up the wall, because upward spray forces water in behind the panels, and once it is back there it has no easy way out. That is a technique failure rather than an equipment failure, which is why the direction the wand is pointing matters more than the number on the machine.

The siding is painted wood. Is it washed like the rest?

No. The USDA Forest Products Laboratory's instruction for mildew on wood siding is a dilute solution, a bristle brush or sponge used gently, and a rinse with the stream pointed down so the back of the siding is not flooded; if a power washer is used at all, the pressure is kept low. In its section on removing paint the same chapter says the pressure needed to loosen tightly bonded paint can easily erode the wood under it, and that power washing is not suitable for paint containing lead. So look at the paint before you worry about the growth. If it is lifting or flaking, tell us when you ask for the estimate: that is a refinishing question, and refinishing is a painter's work.

What about the aluminium — window frames, screen enclosure, gutters?

The EPA-registered bleach label says to rinse quickly and thoroughly if the solution comes in contact with aluminium window frames or gutters, since metal corrosion may occur. Window frames and gutters are what that direction names. A screen enclosure and aluminium trim are the same metal, so say where your house has them when you ask for the estimate. The faces of the gutters are a separate matter: the striping on them is brush work by hand, and gutter brightening is its own service.

House washing (soft wash)

There is no such thing as one house wash. The machine a vinyl manufacturer permits is the machine James Hardie prohibits on fibre cement, and for stucco the trade associations prescribe something closer to a garden hose than to a pressure washer. Before anything is mixed or switched on, the question is what your walls are made of — and the answer is not always a single material.

Three walls, three rulebooks, and they disagree

Start with vinyl. The Polymeric Exterior Products Association — the body that was the Vinyl Siding Institute until it renamed — permits power washing, with two conditions attached. The stream is held level with your eye, and it is never aimed upward, because spray angled up the wall drives water in behind the panels. The same guidance carries a do-not-use list, and undiluted chlorine bleach and organic solvents are on it. It also says plainly that manufacturers vary, that some do not recommend power washing at all, and that it publishes no pressure limit of its own. Now fibre cement. James Hardie prohibits high-pressure washing on it, and prohibits harsh chemicals such as bleach or acid as well. What it prescribes instead is a low-pressure hose and a soft non-metal brush. That is close to the reverse of the vinyl instruction. Stucco is a third answer again. The Stucco Manufacturers Association describes a garden-hose wash with an order to it — pre-wet from the bottom up, then wash from the top down — and warns that holding the nozzle too close can erode the finish. One machine, one setting and one mix across all three is wrong on at least two of them.

The fourth wall is painted wood, and its rule is about direction

None of those three documents covers painted wood siding. Its instructions come from the USDA Forest Products Laboratory, in the finishing chapter of its Wood Handbook, and they amount to a fourth rulebook. Mildew is the laboratory's term for the fungi that infect painted wood. They usually leave black deposits and often grow together with green algae. They are common on walls behind trees and shrubs, where air movement is restricted and the wall stays damp, and they can be associated with dew, which forms on the parts of a building that cool quickly: eaves, soffits, the ceilings of porches and carports. The cleaning instruction is specific in three places. Strength: with household bleach, as dilute a solution as possible — one part to five of water should be adequate, and in no case should anything stronger than one to three be necessary. Contact: a bristle brush or a sponge, gently. Rinse: a garden hose with the stream pointed down, to avoid flooding the back side of the siding with water — and if a power washer is used, the pressure kept low to avoid damaging the wood, with the stream pointed down again. Set that beside the vinyl rule. A trade association writing about vinyl and a federal laboratory writing about painted wood each give an instruction about direction, and neither is a pressure figure: the vinyl association says never to aim the stream upward, and the laboratory says to keep it pointed down. It is which way the water points. The same chapter says where cleaning ends, in its section on removing paint. Power washing, it says there, wears the less dense early-season wood faster than the dense late-season wood and leaves ridges that are difficult to repaint, and it is not suitable for paint containing lead. Separately, before any paint restoration on a structure built before 1976, it says to check the paint for lead. A wash is not a restoration, but on painted wood the paint is still the first thing to look at, before the growth. The laboratory's mildew instruction itself ends with a step that is not cleaning — refinish the cleaned surface as soon as it has dried — and that step belongs to a painter. If the paint on your siding is lifting or flaking, tell us when you ask for the estimate.

Brick and block do print numbers, and the numbers come with conditions

A wall is not always one material all the way to the ground. If yours has brick in it — piers under the wall, a porch base, a set of steps — or bare concrete block at the foundation, brick and block each have a document of their own, and unlike the ones above, these two print pressure figures. Read the conditions before the figures. The Brick Industry Association's technical note on cleaning brickwork says the brick's manufacturer should be consulted before pressurised water is used on it, and that the most appropriate method is generally the gentlest one that works. Its limit is 400 psi measured at the nozzle tip, unless the brick manufacturer permits more. From 400 to 800 psi, it says, may be appropriate in some cases, with certain types of brick, and only with that permission. Of pressure above 800 psi its definitions say something blunter: not appropriate for brick, and it may damage the brickwork. The reason it gives is that excessive pressure may damage the face of the brick, erode the mortar joints and remove finishes or coatings. The note also names the tool — a stainless steel fan tip of 25 to 50 degrees, held at least twelve inches from the brick — and an order of work that echoes the stucco instruction above: saturate the area with water first — at very low pressure, when pressurised water is used for it — before any cleaning solution goes on, and keep the surfaces below it wet until after the final rinse, so they neither streak nor absorb the runoff from above. On growth it says that masonry which stays constantly damp encourages moss, algae and lichen, and that household bleach, applied according to its directions, can remove organic material in some cases. Concrete block has its own document, from the Concrete Masonry and Hardscapes Association. It says unpainted walls can usually be cleaned by scrubbing with water and a small amount of detergent. Pressure should be kept to a minimum, it says, because water driven into the wall can cause efflorescence, the usually white deposit of salts that appears on the surface of masonry. If high-pressure cleaning is used, it recommends a limit of 400 to 600 psi, a wide tip and never a pointed one, at least twelve inches from the surface, with the spray at a 45-degree angle to the wall and never straight at it. Neither document is about stucco, and neither says how to wash masonry that has been painted. If part of your house is brick or bare block, say so when you ask for the estimate.

Low pressure does not make the chemical safe

Take the claim that soft washing is safe on everything because it is low pressure. Half of that is true, and the half it leaves out is the half that does the damage. Two things arrive at your wall, not one: pressure, and chemistry. Chemistry has its own list of prohibitions and that list does not care what the gauge says. James Hardie's instruction for fibre cement is not to use harsh chemicals such as bleach or acid, and it holds with pressure or without it. PEPA puts undiluted chlorine bleach on its do-not-use list for vinyl. The Stucco Manufacturers Association requires a cleaner that will not chemically attack portland cement, lime, or the oxide pigments that give the stucco its colour — which rules out a good deal of what is cheap and effective somewhere else. And the EPA-registered label on ordinary household bleach says to rinse quickly and thoroughly if the solution comes in contact with aluminium window frames or gutters, since metal corrosion may occur. Then look at how much of your house is aluminium: window frames, the screen enclosure, the gutters, the soffit, sometimes the whole fascia wrapped in it. A solution that is correct for the wall can be wrong for the trim right beside it. That is worth deciding before any equipment comes out, not after. Chemistry can also tell you what you are looking at, within limits. The Forest Products Laboratory's chapter gives a simple test for mildew on wood or paint: a drop or two of liquid household bleach on the discoloured spot. The dark colour of mildew will usually bleach out in one to two minutes, it says, and discolouration that does not bleach is probably dirt, extractives bleed, or an iron stain. It gives the limit in the sentences that follow: mildew can grow through a coating or under a clear finish, and in those cases it may be difficult to test for or to clean, because the finish protects the mildew from the cleaning solution. That is a test for wood and for paint, which is what the laboratory wrote it for. It is not a reason to put bleach on vinyl or fibre cement, whose makers' lists are a few lines up.

The order of the work, and the time the chemistry needs

Both stucco documents put the same step first: the wall is wet from the bottom up before any cleaner touches it. The Stucco Manufacturers Association then washes from the top down. The Portland Cement Association's stucco repair manual gives the same pre-wet, and it sets a least-aggressive-method rule for that work — begin with the mildest method that works and escalate only if it does not. It specifies fan tips in the fifteen to forty degree range and keeping the stream moving, and it gives no pressure figure at all. Then the chemistry needs time. The exterior directions on the EPA-registered bleach label — written for exterior surfaces like siding, tile roofs, brick, stucco and patio stone — are to hose the surface first, keep it wet for at least ten minutes, and rinse. The way the label keeps it wet is by telling you to put more on. Nothing clings on its own; you go back over it. The label's optional wording also says to avoid applying the solution in direct sunlight, which in July here is a scheduling problem rather than a footnote. A wall in full afternoon sun will not stay wet for ten minutes, so the order that works is the order the shade allows.

What a wash cannot do

Hairline cracks in stucco are the standing example. The Stucco Manufacturers Association states that hairline cracking is typical, is usually not a defect, and does not jeopardise the wall's water resistance, because the weather-resistive barrier that actually keeps water out sits beneath the stucco rather than on its face. Sealing is not exterior cleaning and we do not do it, so we have no reason to tell you anything other than that. The second limit is the house itself. A wash takes off what is sitting on the wall and what is growing in it. It does not restore a finish that is already gone, it does not fill anything, and where paint is lifting at an edge, water and cleaner will find that edge — which is a thing to know beforehand rather than discover afterwards. Some vinyl manufacturers do not recommend power washing at all. If yours is one of them, or if the elevation is fibre cement, the method is the maker's to set. For fibre cement, James Hardie prescribes a low-pressure hose and a soft non-metal brush. Where a house is more than one material, the elevations are not all the same job, which is why the method is a question for each elevation rather than one for the whole house.

Common questions

Is this safe on fibre cement siding?: It can be, by its maker's method. James Hardie prohibits high-pressure washing on fibre cement and prohibits harsh chemicals such as bleach or acid on it, and what it prescribes instead is a low-pressure hose with a soft non-metal brush. By that method a fibre cement elevation is slower work. If your house is fibre cement on one side and something else on the others, the two call for different treatment.
Is bleach used to wash a house?: It depends on the wall, and that is the answer rather than a hedge. Sodium hypochlorite is what kills the growth, and the EPA-registered label for household bleach carries exterior directions for surfaces like siding, tile roofs, brick, stucco and patio stone. But James Hardie rules out harsh chemicals such as bleach on fibre cement, undiluted chlorine bleach is on the vinyl association's do-not-use list, and stucco needs a cleaner that will not attack the cement, the lime or the pigment. So no single answer covers a whole house: tell us what each wall is made of. On fibre cement, what the maker prescribes instead is a hose and a soft brush.
Can washing force water in behind vinyl siding?: It can, and the trade association names the exact way it happens: aiming the stream upward. PEPA instructs holding it level with your eye and never angling it up the wall, because upward spray forces water in behind the panels, and once it is back there it has no easy way out. That is a technique failure rather than an equipment failure, which is why the direction the wand is pointing matters more than the number on the machine.
The siding is painted wood. Is it washed like the rest?: No. The USDA Forest Products Laboratory's instruction for mildew on wood siding is a dilute solution, a bristle brush or sponge used gently, and a rinse with the stream pointed down so the back of the siding is not flooded; if a power washer is used at all, the pressure is kept low. In its section on removing paint the same chapter says the pressure needed to loosen tightly bonded paint can easily erode the wood under it, and that power washing is not suitable for paint containing lead. So look at the paint before you worry about the growth. If it is lifting or flaking, tell us when you ask for the estimate: that is a refinishing question, and refinishing is a painter's work.
What about the aluminium — window frames, screen enclosure, gutters?: The EPA-registered bleach label says to rinse quickly and thoroughly if the solution comes in contact with aluminium window frames or gutters, since metal corrosion may occur. Window frames and gutters are what that direction names. A screen enclosure and aluminium trim are the same metal, so say where your house has them when you ask for the estimate. The faces of the gutters are a separate matter: the striping on them is brush work by hand, and gutter brightening is its own service.

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