Roof Cleaning Glossary | 15 Moss & Soft Wash Terms

15 terms explained

Manual Profile Scraping: Using shaped metal or nylon blades matched to the exact profile of your concrete tile or flat slate to lift thick moss cushions off by hand.
Low-Pressure Soft Wash: Applying cleaning and biocide solutions through a low-pressure fan nozzle (similar to a garden hose) rather than a high-pressure jet.
Quaternary Ammonium Biocide (DDAC / BAC): Biodegradable algicide and fungicide sprayed onto scraped roof tiles to kill remaining moss spores, green algae and white lichen.
Bryophyte Moss Cushions: Thick spongy clumps of rootless moss that wedge into the overlapping joints and water channels of concrete roof tiles, holding damp against the roof.
Crustose Lichen (White & Yellow Spots): Slow-growing symbiotic organism tightly bonded to the surface of older tiles and slates, which releases gradually over several months after biocide treatment.
Granular Surface Coating: The factory-applied coloured sand or acrylic layer on concrete roof tiles that protects the tile body and is stripped away if hit with a high-pressure lance.
Interlocking Water Channel: The narrow side groove down the edge of each concrete roof tile designed to carry rainwater down the roof, which overflows into the felt when blocked with moss.
Freeze-Thaw Spalling: Flaking and cracking of roof tiles or ridge mortar in winter when water trapped inside wet moss cushions freezes and expands.
Downpipe Isolation (Bung / Disconnect): Temporarily disconnecting the downpipe shoe or plugging the outlet during roof scraping so dislodged moss cannot wash down into the underground storm drain.
Gutter & Swan-Neck Flush: Emptying every handful of moss from the gutter trough and flushing the top offset bends after the roof scrape is finished.
Roof Ladder (Cat Ladder): Lightweight aluminium ladder with a padded wheeled ridge hook that spreads weight across the roof peak so tiles are not cracked underfoot.
Ridge & Hip Mortar Pointing: The sand-and-cement bedding under half-round ridge tiles, inspected during a roof clean in case moss roots have loosened the edge.
Lead Roof Valley Trough: The open V-channel between two intersecting roof slopes where falling moss collects first and blocks heavy rain runoff.
Preventative Biocide Re-Treat: A quick low-pressure spray applied from the eaves or a tower every 2 to 3 years to stop new moss spores taking hold on a clean roof.
Reduced 13.5% Residential VAT Rate: The Irish VAT rate that applies to exterior cleaning and routine maintenance on private dwellings.