Pressure Washing and Exterior Surfaces: A Contractor's Field Guide

Pressure washing is one of the highest-risk, highest-visibility services a commercial cleaning contractor can offer. It removes years of grime in an afternoon, and it can also strip sealant, etch concrete, drive water into a building envelope, destroy landscaping and put dirty runoff into a storm drain. The difference between those outcomes is almost entirely method and control, not brute force.

This guide covers what to clean, how much pressure each surface tolerates, how to contain runoff, how to work safely, and what to document when the job is done.

Key takeaways

• More pressure is not better. Correct pressure, nozzle angle and dwell time do the work. Excess pressure damages the surface you were hired to protect.
• Water always wins. It finds cracks, joints, weep holes, door tracks and utility penetrations. Never direct a stream where water can enter a building.
• Runoff is a compliance issue. Dirty water leaving the site, and especially entering a storm drain, is a regulatory problem, not just an aesthetic one.
• Chemistry first, pressure second. A correct detergent and dwell time lets you wash at lower pressure.
• Some surfaces are outside your scope. Sealed glazing units, delicate sealants, electrical equipment and damaged substrates should be reported, not washed.

What "exterior surfaces" actually means

A commercial exterior cleaning scope is rarely one surface. A typical property has most of the following:

Read the table as a warning about one thing: pressure is a variable, not a setting you leave alone.

Pressure, flow and nozzle selection

Three settings determine what the water does when it lands.

Pressure (PSI). The force. Enough to lift soil, not enough to damage the substrate. Most exterior commercial work happens in a moderate band rather than at the machine's maximum output.

Flow (GPM). The volume. Higher flow rinses and carries soil away faster, and is often more effective than higher pressure. A machine with good flow at moderate pressure cleans better and damages less than a high-pressure, low-flow unit.

Nozzle angle. The spray pattern. A narrower angle concentrates force; a wider angle spreads it.

Two practical rules that prevent most damage:

• Increase distance before you increase angle. Moving the wand back reduces impact far more gently than changing nozzles under load.
• Keep the stream moving. A stationary jet cuts. Travel steadily and overlap passes.

The rule that matters most: water must not enter the building

This is the single most expensive mistake in exterior cleaning, and it is entirely preventable.

Water will enter through:

• Door tracks, thresholds and weather seals. Even closed doors leak under direct spray.
• Cracks in masonry and mortar joints. Water tracks inward through the wall assembly.
• Utility penetrations — conduit, pipe sleeves, meter boxes, hose bibs, vents.
• Weep holes in brick and masonry veneer. These are drainage paths and must never be blocked or used as a water entry point.
• Window and curtain wall seals, especially aged or failed sealant.
• Roof penetrations, drains and overflow scuppers.
• Parking garage decks above occupied or storage space.
• Expansion joints and control joints in concrete flatwork.

Controls: pre-wet adjacent surfaces where appropriate, use low pressure near openings, angle the stream down and away from the building, mask or protect vulnerable openings, and have a second person watch for intrusion. If water starts entering, stop and reassess — do not press on and hope it dries.

Scope note: Building envelope repairs, failed sealant replacement, crack injection and waterproofing are outside a cleaning contractor's scope. Document the defect, report it in writing, and do not attempt a repair.

Chemistry: wash, don't blast

The most common reason contractors use too much pressure is that they have not used enough chemistry.

Rules that keep chemistry safe:

• Never mix products. Mixing an acid-based cleaner with a chlorine-based product can release dangerous gas.
• Dilute exactly as the label directs. Over-strength chemical on masonry or metal can permanently stain or etch. See chemical inventory and dilution control.
• Dwell time matters more than pressure. Let the product work.
• Test on an inconspicuous area on any surface you have not cleaned before.
• Protect plants and grass. Pre-wet and cover landscaping, and check what the product does to vegetation before you start.
• Keep the safety data sheet on site and follow the label for personal protective equipment. See WHMIS for cleaners.

Runoff control: the compliance part of the job

Dirty wash water is not the same as rainwater. It carries soil, oils, detergents, metals and organic matter, and it must not be allowed to reach a storm drain or a watercourse.

Practical controls for a commercial site:

• Contain before you clean. Block or berm the area, or use a vacuum recovery system, so wash water stays where you can collect it.
• Never discharge to a storm drain. Storm drainage is generally intended for uncontaminated rainwater only.
• Check the site's own drainage plan. Some properties have oil-water separators, sumps, or designated wash bays. Use them.
• Collect and dispose of wash water as the client's facility procedures and your local requirements direct.
• Protect catch basins with filter fabric or a drain seal before you start, and remove them afterwards.
• Plan for the worst-case volume. Estimating runoff is far easier before the water is running across the parking lot.
• Never wash a spill you cannot contain. Report fuel, oil or chemical spills to the client and follow the facility's spill procedure.

Not regulatory advice. Runoff and discharge requirements vary by province, municipality and site, and are enforced by provincial environment ministries, conservation authorities and local sewer use bylaws. Confirm the requirements that apply to your site with the local authority and follow the facility's own procedures.

Safety: where pressure washing injures people

Pressure washing combines high-pressure fluid, electricity, water, chemicals and often working at height or near traffic.

• Never point the wand at a person. High-pressure water can cause serious injection injuries and tissue damage. Treat a wand like a loaded tool — critical
• Wear eye and face protection, gloves, boots and hearing protection. Debris flies at speed — critical
• Protect against electrical hazards. Keep cords, plugs and connections out of water, use ground fault protection, and never spray near live electrical equipment, panels or fixtures — critical
• Manage slip hazards from wet, soapy surfaces, especially on slopes and ramps — critical
• Control traffic. Work areas on or near driveways, loading areas and parking need cones, signage and, where required, a spotter — critical
• Use fall protection. Roof work, ledges and elevated platforms require proper fall protection and, in many cases, training and a written procedure. Never work at height beyond your competence or equipment.
• Watch for overhead lines. Keep lances, poles and water streams clear of power lines.
• Manage heat and cold. Pressure washing is physical work in weather, and burners and engines add heat.
• Fuel safely. Refuel engines and burners only when equipment is off and cool, in a ventilated area away from ignition sources.

The Canadian Centre for Occupational Health and Safety publishes guidance on high-pressure cleaning hazards and on general workplace safety that is worth building your procedures around.

Scope note: Rooftop work above your training or equipment, and any work on a roof with structural concerns, falls outside a cleaning contractor's scope. Report the condition and decline the task.

Preparing and protecting the site

• Walk the property with the client and photograph pre-existing damage: cracks, rust streaks, failing sealant, chipped paint, damaged asphalt, loose pavers, existing stains.
• Identify and protect electrical outlets, exterior lighting, cameras, signage, planters, benches, vehicles, bike racks, gas meters and utility boxes.
• Move or cover vehicles. Overspray and detergent spotting on nearby vehicles will become a claim.
• Protect landscaping with covers and pre-wetting, and know what the detergent does to grass and bedding plants.
• Control overspray. Wind direction changes the job. Note it, and stop if conditions make containment impossible.
• Sign and barricade pedestrian routes. Slip hazards on wet flatwork are real, and the general controls are in slips, trips and falls prevention.
• Confirm access to water. Water supply, hose runs and backflow prevention are common site issues; sort them out before the crew arrives.

Sequence and method

1. Inspect and document the site condition with photographs.
2. Advise the client of anything that should not be cleaned, and get written direction.
3. Set up containment for runoff and protect drains.
4. Protect or move vehicles, furniture, planters and signage.
5. Sweep and remove loose debris, gum and litter. Pressure washing does not remove bulk debris.
6. Pre-treat with the correct detergent and allow dwell time.
7. Wash top-down on vertical surfaces, and work away from the building on flatwork.
8. Rinse thoroughly to remove detergent residue. Leftover detergent streaks and attracts soil.
9. Recover or contain wash water and dispose of it as directed.
10. Remove protection, restore the site and check for damage.
11. Photograph the finished result and record the work.

A blameless record of pre-existing damage is the single most valuable document in exterior cleaning. It is also where a deficiency record helps, and the workflow is described in deficiency tracking.

Inspecting exterior work

Exterior cleaning is judged in daylight and from a distance, which hides a lot. Score these items:

• Areas specified in the contract cleaned to the agreed standard — critical
• No damage to surfaces, sealant, paint or landscaping — critical
• No water intrusion into the building during or after work — critical
• Runoff contained and disposed of as directed — critical
• Warning signage and barricades in place during work — critical
• No detergent residue, streaking or visible wand marks
• Joint, paver and asphalt condition unchanged
• Drains and catch basins clear and protection removed
• Pre-existing damage and defects documented and photographed
• Site restored, equipment removed, no trip hazards left

Appearance-level standards for outdoor hard surfaces sit inside a graded approach; the wider framework is in the APPA cleanliness levels explained.

Where this fits in the quality system

This sits under our the janitorial inspection checklist, which sets out the standard these checks are measured against.

Frequently asked questions

What pressure should I use for commercial concrete cleaning?

It depends on the concrete's condition, age and finish, but general flatwork is usually cleaned with moderate pressure and a 25-degree nozzle, letting detergent and dwell time do the heavy work. Decorative, stamped, sealed or damaged concrete needs lower pressure. Test an inconspicuous area first, and always increase distance before increasing pressure.

Can you pressure wash brick or masonry?

With low pressure and the right chemistry, yes. High pressure drives water into the wall assembly, erodes mortar and can spall the brick face. Use a wide-angle nozzle, keep the stream angled down and away from openings, avoid weep holes, and pre-wet surrounding surfaces. If the masonry is already cracked or crumbling, document it and do not wash it.

Is pressure washing runoff allowed into a storm drain?

Generally not, when the water carries detergent, oil, grease, metals or other contaminants. Storm drainage is normally intended for uncontaminated rainwater. Contain the water, protect catch basins, and dispose of it according to the site's procedures and your local requirements. Confirm the rules with your local authority.

Why did pressure washing damage the surface?

Usually excess pressure, too narrow a nozzle, holding the wand still, or using an aggressive setting on a soft or already degraded substrate. Over-wetting can also cause damage indirectly, by driving water behind cladding or into joints. Correct chemistry with lower pressure is the safer method.

Can pressure washing remove mildew and algae permanently?

It removes the growth and the staining it leaves, but it does not remove the conditions that cause it: shade, moisture and organic buildup. Expect recurrence on north-facing and shaded surfaces. A correct detergent and a defined cleaning interval manage it better than repeated aggressive blasting.

Clean the surface, protect the asset

Exterior cleaning is one of the most visible services a contractor provides and one of the easiest to get wrong. Controlled pressure, correct chemistry, contained runoff and a documented before-and-after record protect the client's property and your reputation at the same time.

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