"I’ve been working on roofs around Sittingbourne and North Kent for over thirty years. Every spring and summer, I get called out to inspect leaks on houses that have just had their roofs 'cleaned' by doorstep pressure-washing crews. Nine times out of ten, the jet wash has blown out the ridge mortar, split the old underlay, and left the loft insulation sopping wet."
— Chris Baker, Founder & Owner-Operator at C.L. Baker
1. Why High-Pressure Washing Damages Roof Tiles
It is easy to see why homeowners are tempted by pressure washing. A patio jet washer takes green algae off paving slabs in seconds, so when someone knocks on the door offering to do the same to your roof for a quick cash price, it sounds straightforward. The problem is that a roof is not a solid patio slab—it is a layered weather-shield made of overlapping tiles, mortar joints, and breathable or bituminous underlay beneath.
Here is what actually happens when someone turns a high-pressure lance (often running at 2,000 to 3,000 PSI) onto your roof:
It Strips the Granular Surface Coat
Post-war and modern concrete interlocking tiles—like the Marley Modern or Redland Renown tiles found across thousands of 1950s to 1980s homes in Sittingbourne, Rainham, and Ashford—have a thin surface layer of acrylic coating or fine sand granules. A jet wash blasts that protective skin straight off into your gutters. Once the rough inner concrete core is exposed, it acts like a sponge: it absorbs rainwater, holds damp, and ironically encourages moss to grow back much faster than before.
It Shatters Brittle Clay & Kent Peg Tiles
On older Victorian properties and period cottages around Faversham, Rochester, and Canterbury, roofs are often laid with handmade clay plain tiles or traditional Kent peg tiles. Over decades of frost cycles, clay naturally Laminates and softens in places. Blasting old clay with high-pressure water cracks the tile edges, snaps the nibs holding them onto the timber battens, and leaves you needing expensive clay tile repairs.
It Floods the Felt, Battens & Loft
Pitched roofs work by gravity. Tiles overlap so rain runs downhill from course to course. Even when an operative stands on a roof or works from a cherry picker trying to point a lance downwards, the sheer force of the spray drives water sideways through the interlocking channels and upwards under the tile laps. If your roof has older bituminous 1F felt with small tears—or no underlay at all on a Victorian slate roof—gallons of dirty water pour straight onto your plasterboard ceilings and loft insulation.
It Blows Out Ridge & Verge Mortar
Unless your roof has been upgraded to a modern mechanically fixed dry ridge system, your ridge tiles, hip irons, and gable verges rely on sand-and-cement or lime mortar bedding. High-pressure water cuts straight into weathered mortar joints, loosening ridge tiles and washing away the cement haunching around chimney flashings.
Why Aged Underlay Cannot Handle a Jet Wash
This photo from one of my strip and retile jobs shows what typical 30-to-40-year-old roofing felt looks like underneath concrete tiles. Over time, old felt sags and perishes near the eaves and battens.
In normal rain, the tiles above keep the roof completely watertight. Blast it with a pressure washer, however, and water forces past the tiles and pours straight through those perished sections of felt into your home.
2. The Safe Trade Method: Manual Scraping & Biocide Treatment
You do not need high-pressure water to get a clean, moss-free roof. Both tile manufacturers and the National Federation of Roofing Contractors (NFRC) advise against jet washing pitched roofs. Instead, when I carry out roof cleaning in Sittingbourne and across Kent, I use a dry manual removal method followed by a specialist trade biocide.
Full Roof Inspection First (By a Roofer, Not a Cleaner)
Before touching a single clump of moss, I check the condition of your roof. If you have slipped Welsh slates, cracked concrete tiles, open chimney leadwork, or perished ridge pointing, they need putting right first. A general exterior cleaning company often won't spot—or know how to fix—a split lead valley or a broken tile nib.
Downpipe Protection & Safe Access Setup
Heavy moss tumbling into open rainwater pipes will quickly block your underground soakaways or drains. I disconnect or plug the downpipe outlets before starting and set up proper staging, roof ladders, or tower access so no excessive weight is placed directly onto brittle tile courses.
Careful Manual Scraping with Profiled Blades
Using hand trowels, stiff brushes, and profiled scraping blades shaped to match the exact contour of your tiles (whether double Roman, flat plain tile, or deep interlocking profiles), I gently eased off all the heavy moss cushions from the surface, interlocking side channels, and bottom tile edges.
Full Gutter Clearance & Tidy-Up
Once the bulk moss is off the roof, I completely empty out your uPVC guttering and lead valley troughs by hand, bag up all the debris for removal, and leave your pathways, conservatory roofs, and patios clean.
Low-Pressure Biocide Treatment
Scraping removes the visible moss, but microscopic spores remain in the pores of the tile. I apply a gentle, low-pressure wash of professional-grade biocide across the entire roof pitch. This soaks into the tile without damaging the granular finish, killing off remaining moss roots, green algae, and black lichen over the following weeks and slowing down regrowth for years.
3. How Different Roof Materials React to Cleaning
Not all roofs across Kent are built with the same materials. Here is how common domestic roof coverings hold up against high-pressure jetting versus manual trade cleaning:
| Tile / Slate Type | What Pressure Washing Does | Correct Trade Approach |
|---|---|---|
| Concrete Interlocking Tiles (1950s–Present Estates) |
Strips the colour coat and protective granules, leaving rough grey cement exposed to frost and rapid moss return. | Profiled scraper matching the tile trough, clear interlocking side channels, apply biocide. |
| Machine-Made Clay Plain Tiles (1930s Semis & Suburban Homes) |
Drives water under the short single laps and breaks weak tile nibs, causing tiles to slip down the roof. | Light hand brushing and scraping from roof ladders/staging, replace any frost-shattered tiles, biocide spray. |
| Handmade Kent Peg Tiles (Faversham & Period Cottages) |
Destroys soft heritage clay surfaces, dislodges wooden or metal pegs, and saturates historic roof timbers. | Extremely careful manual clearance only where moss blocks drainage valleys; never jet wash heritage clay. |
| Natural Welsh Slate (Victorian Terraces in Medway & Swale) |
Delaminates older slates, snaps corroded nails, and floods the loft (many Victorian slate roofs have no felt underneath). | Gentle hand removal of moss around overlaps and lead valleys followed by biocide application. |
4. A Word of Warning About "Coloured Roof Coatings"
Quite often, companies that pressure wash roofs do it as part of a hard-sell pitch for "thermal roof coatings" or coloured paint sealants. They blast the roof with a jet washer (which strips your original tile finish), and then offer to spray on a thick coloured resin paint for several thousand pounds, claiming it will insulate your home and save you from needing a new roof.
As a career roofer, I strongly advise homeowners to steer clear of painted roof coatings:
- Tiles need to breathe: Clay and concrete tiles naturally absorb and release small amounts of moisture. Sealing them in thick plastic paint traps damp inside the tile, which freezes in winter and causes the tile face to spall and flake off.
- It glues the overlaps together: Spray-on coatings bridge the small gaps between tiles, stopping condensation from escaping your roof space and blocking drainage channels.
- It does not fix what is underneath: Painting the top of an old tile does nothing to repair rotten timber battens, corroded nails, or torn underlay beneath.
5. When Should You Clean a Roof vs Repair or Replace It?
Moss removal is great preventative maintenance when the tiles underneath are still structurally sound. Heavy moss holds rainwater like a wet blanket against your roof and rolls down into your guttering every time there is a storm, causing damp fascia boards and overflowing downpipes.
However, if your roof is already leaking inside the loft, if dozens of tiles are crumbling, or if the wooden battens and felt beneath have rotted out after 50+ years of service, cleaning the moss off will not solve the problem. When I come out to look at your roof, I will tell you plainly whether a manual clean and small roof repair is all you need, or whether your money is better spent putting towards long-term remediation.
Common Questions About Roof Cleaning & Moss Removal
What if someone Has already pressure washed my roof and now it leaks?
Give me a ring on 01795 904566. I can inspect both inside your loft space and up on the roof pitch to see where the water is getting in. Often, jet washing dislodges lead step flashings around chimneys, cracks brittle valley mortar, or shifts single tiles out of alignment. I will explain the exact point of failure and give you a straightforward quote to make it watertight again.
How long does a manual scrape and biocide treatment take?
For a standard semi-detached or terraced house around Sittingbourne, Medway, or Faversham, manual moss removal, full gutter clearing, any minor tile replacements, and biocide application usually takes between one and two days depending on the pitch, access, and how thick the moss build-up is.
Will the biocide wash harm my garden plants or pets?
No, when applied properly. Because we scrape away the heavy moss dry first and disconnect or bag the downpipes, we only need a controlled, low-pressure application of biocide that absorbs directly into the porous tile surface rather than running off into your flowerbeds or pond. We always ask that pets are kept indoors while we are actively spraying.
Do you use subcontractors or agency labour on site?
Never. I personally survey, quote, and carry out the work on your roof myself. When you hire C.L. Baker, you get thirty years of local Kent roofing experience on your roof—not a franchise crew with a rented pressure washer.