Cleaning a commercial roof is a maintenance task with two purposes. The first is to remove the things that hold water against the surface, block the drainage or attack the covering chemically. The second is to make the roof legible, because an inspector cannot assess a surface they cannot see, and a layer of debris and growth hides exactly the sort of early damage that is cheap to fix.
It is also one of the few maintenance tasks that can do real harm. A pressure lance held too close opens a lap, a scrubbing machine takes the finish off a reflective coating, and a cleaning product chosen for the dirt rather than for the membrane shortens the life of the roof it was meant to extend. The method has to follow the covering, every time. Our roof maintenance page covers the wider program this task sits inside.
What is actually being removed
| On the roof | Why it matters | Where it comes from |
|---|---|---|
| Leaves, silt and debris | Dams water, blocks outlets, holds moisture | Trees, wind, neighboring sites |
| Moss, algae and lichen | Holds water against the covering, lifts units | Shade, damp, organic debris |
| Grease and fat | Softens and degrades several membranes | Kitchen and food process exhaust |
| Chemical and particulate fallout | Attacks coatings and corrodes metal | Stacks, extracts, nearby industry |
| General dirt film | Costs a reflective roof its reflectivity | Air, traffic, time |
| Construction leftovers | Punctures the membrane under foot | Earlier work by any trade |
The last row is worth its own mention. Screws, offcuts, pop rivets and cable ties left behind by whoever last serviced the rooftop units get walked on, and each one is a puncture waiting for a storm. Picking them up is cleaning in the most literal sense and it prevents more leaks than anything else on the list.
Why it is worth doing
Drainage. This is the main one. Water leaves a commercial roof through a small number of outlets, and a blocked outlet turns the roof into a pond. Ponding water on a flat roof covers what that does to the assembly beneath it.
Chemistry. Grease and solvent fallout attack membranes continuously, and the damage concentrates in the area downwind of the source. Removing the deposit does not fix the discharge, but it slows the clock.
Reflectivity. A light colored roof was specified to run cool. Soiled, it runs warmer, the assembly below moves more each day, and the energy benefit quietly erodes. Roof coatings explains what that surface is doing.
Visibility. A roof that has been cleaned can be inspected properly. That is often the largest return on the visit, because finding a problem early is the difference between a repair and a replacement.
Method, by covering
| Covering | Normally suitable | Avoid |
|---|---|---|
| Single ply membrane | Low pressure rinse, soft brush, mild detergent | High pressure near laps, abrasive pads, solvents |
| Modified bitumen or built up | Gentle rinse, hand clearing of debris | Pressure on the surfacing, anything that strips granules |
| Metal panels | Low pressure wash, approved detergent, rinse | Pressure into laps and fixings, harsh alkalis |
| Coated roof | Whatever the coating manufacturer specifies | Pressure that cuts the film, incompatible chemicals |
| Tile or slate | Hand clearing, gentle treatment for growth | High pressure, which strips surface and loosens units |
| Gravel surfaced | Vacuum or hand clearance of debris only | Washing, which drives dirt down into the plies |
The pressure limit is the number to get in writing, from the membrane or coating manufacturer rather than from the person holding the lance. Most publish a cleaning method, and a contractor working to it is a contractor you can hold to something.
What damages a roof during cleaning
- Pressure at the laps and details. Water driven under a seam separates it, and the damage is invisible until it rains.
- Abrasion. Rotary brushes and scrubbing pads remove the top of a coating along with the dirt.
- The wrong chemistry. Solvents soften some membranes, strong alkalis attack metal finishes, and bleach corrodes fixings and gutters.
- Traffic. Hoses, cable reels, ladders and tower sections dragged across a membrane do more damage than the cleaning does. Roof walkway pads are the long term answer on any roof with regular access.
- Nothing being reported. A crew that finds damage and says nothing has turned a cheap repair into next winter’s leak.
Access and safety, which decide the price
Cleaning is the maintenance task that puts people on a roof for the longest, often with hoses and equipment, frequently near the edge, and usually on a wet surface. That makes the access arrangements a larger part of the cost than the cleaning itself on most buildings.
Three things belong in the arrangement before anybody goes up. How the roof is reached and how material is got up there. What fall protection is in place at the edges and around every opening. And where the rooflights and smoke vents are, because on an older roof they weather until they look much like the surrounding surface and will not take any weight at all. A wet membrane is also considerably more slippery than a dry one, which is the hazard crews most often underestimate on their own roof.
The wash water
Whatever comes off the roof goes into the drainage unless somebody plans otherwise, and on some roofs that is a problem rather than a detail. Grease, chemical residue and cleaning products all end up in the storm system, which is governed differently from the sewer.
Settle three things before the work starts: where the runoff goes, whether anything needs containing or collecting rather than rinsing, and what is being protected below. Planting, parked vehicles and pedestrian routes under the downpipes are all worth covering, and so is any exposed metalwork within reach of the spray.
Where cleaning is not the answer
Cleaning removes a symptom. Where the cause keeps producing it, the cause is the job.
- A drain that blocks every season may be undersized, high, or missing a strainer.
- Growth that returns within months usually means standing water or permanent shade, which is a drainage or a vegetation question.
- Grease concentrated in one area is a discharge problem fixed at the stack, not on the membrane. Our page on industrial roof repair covers that.
- Debris from a neighboring roof or tree is cheaper to address at the source than to remove annually for twenty years.
What a cleaning scope should state
- The covering, and the manufacturer’s published method being followed
- Maximum pressure, and the equipment being used
- Any product being applied, at what dilution, and how it is rinsed
- Which areas are hand cleared rather than washed
- What is protected below, and by whom
- Where the runoff goes, and what is contained
- That drains, outlets, gutters and overflows are cleared as part of the work
- That any damage found is photographed and reported before anyone leaves site
Where to go next
Clean and then inspect, in that order, and treat the two as one visit rather than two. A roof maintenance plan is what turns this from a job somebody remembers into a schedule with a record behind it, and roof maintenance programs covers how that is organized across several buildings.
Common questions
How often should a commercial roof be cleaned?
There is no universal interval, because what dirties a roof varies enormously between buildings. The useful rule is to clean when something on the roof is holding water, blocking drainage, growing, or hiding the surface from an inspector, and to let the building tell you how often that is.
- Drains and outlets need clearing more than anything else
- Trees nearby raise the frequency sharply
- Kitchen and process exhaust create their own schedule
- Biological growth returns on a predictable cycle
- Reflective roofs lose performance as they soil
The practical approach is to tie it to the inspection rather than to the calendar. Somebody walks the roof, clears the drains and the gutters while they are up there, and reports what else has accumulated. Most roofs then settle into a rhythm of their own, and the record of what was removed each visit is what justifies the interval to whoever holds the budget. A roof under trees and a roof in the middle of an industrial estate are not on the same schedule and should not be put on one.
Is bleach safe to use on a roof?
Sometimes, at the right dilution, on the right surface, rinsed properly and kept out of the drains. Sodium hypochlorite is the usual active ingredient in roof cleaning products because it kills biological growth rather than just removing it, and it also attacks metal, plants and anything it runs onto.
- Kills growth at the root rather than scrubbing it off
- Corrodes exposed metal, fixings and gutters
- Damages landscaping below the downpipes
- Must be at the dilution the membrane allows
- Needs a thorough rinse, and a controlled discharge
The question to ask is not whether bleach works but what the membrane manufacturer permits. Many publish a cleaning method with named products and dilutions, and departing from it can affect both the surface and the warranty. Where there is no published method, a test area is the sensible approach. The part contractors most often get wrong is the surroundings rather than the roof: metal, glass, planting and vehicles all need protecting, and that belongs in the scope rather than in an apology afterwards.
Where does the wash water go when a roof is cleaned?
Into the building's drainage, unless somebody has planned otherwise. That water carries dirt, biological growth, whatever cleaning product was used, and on some roofs grease or chemical residue, so where it ends up is a question worth settling before the work starts.
- Roof drains usually feed the storm system
- Storm discharge rules differ from sewer discharge rules
- Grease and chemical residue may need containment
- Drains can be plugged and the water collected
- Planting and parking below need protecting
On an ordinary roof with plain water and a mild detergent, this is a formality. On a kitchen roof, a plant roof or any roof where the residue is not just dirt, it is the part of the job that can create a problem far larger than the one being solved. Ask the contractor what they intend to do with the runoff, and whether the site has any discharge consent that applies. A contractor who has not thought about it is telling you something useful about the rest of their method.
What is the black streaking on my roof?
On a pitched or shingled roof it is usually algae, which grows as dark streaks running down the slope from the ridge. On a flat reflective membrane, dark patches are more often simple dirt pickup, trapped where water stands or where the surface has become slightly tacky with age.
- Algae streaks run downslope and darken over years
- It holds moisture against the surface it grows on
- Dirt pickup on a flat roof follows the water pattern
- Neither is proof that the roof is failing
- Both reduce the reflectivity of a light surface
Whether to do anything about it depends on what it is doing. Algae and moss hold moisture against the covering and are worth removing on that basis alone. Dirt on a reflective roof costs you some of the reflectivity you paid for, which matters on a cooled building and matters less on an unheated warehouse. What neither justifies is an aggressive clean on a surface that cannot take one, so establish which you have before choosing a method. Our page on [silicone roof coating](/articles/silicone-roof-coating/) covers why some surfaces soil more than others.
Who should clean a commercial roof, a roofer or a cleaning contractor?
Somebody who knows what the roof is made of, works to the manufacturer's method, and can recognize damage while they are standing on it. That is usually a roofing contractor or a specialist working under one, rather than a general cleaning firm with a pressure washer.
- The method has to match the membrane, not the dirt
- Walking a roof safely needs roof specific training
- Damage found during cleaning should be reported, not hidden
- Lances, ladders and kit all damage roofs if misused
- Insurance should cover work on a roof specifically
The risk is not theoretical. A pressure lance held close to a lap opens it, a scrubbing machine abrades a reflective finish, and a scaffold tower or a hose reel dragged across a membrane punctures it. None of that is visible to somebody who does not know what they are looking at, and all of it leaks later. Ask who will be on the roof, what training they have for the covering, and what happens if they find damage. The last answer matters most, because somebody cleaning a roof is the person best placed to spot a problem early.