There is a jeweller on Berwick Street whose window I cleaned every fortnight for six years. The glass was never the problem. The problem was a projecting sign box with a 110mm top return, twelve feet up, warm from the LEDs behind the acrylic, sheltered by the parapet above it, and occupied by the same four birds from about October onwards.
Every visit I cleaned a vertical stain that started at the bottom of the fascia and ran the full drop of the pane.
Give it a winter and that stain stops being a stain. It becomes surface damage to the frame, a soft patch in the perimeter sealant, and a set of fine scratches in the bottom third of the glass that I put there myself with a squeegee and a bit of grit I could not see.
Why Does a Fascia Ledge Above a Shopfront Collect Birds at All?
Feral pigeons want three things from a ledge and a shopfront sign hands them all three: a flat surface with a lip, warmth, and a clear flight line out. Illuminated sign boxes run at a few degrees above ambient all night. The lip on a fabricated aluminium tray stops eggs rolling. And a high street gives them a sightline down the road with nothing to drop out of the sky onto them.
Food does the rest. Chicken shop frontages on Roman Road hold birds that no amount of proofing shifts, because the pavement below is a permanent feeding station and the ledge above is the nearest dry perch to it.
The return depth that decides it
Anything with a flat top return over about 40mm is a perch. Over about 90mm it is a roost, and once it is a roost it will hold a nest. I have measured hundreds of these across shopfronts from Kingsland Road down to Whitechapel and the pattern holds well enough that I can tell a shop manager over the phone whether their sign is the cause, from the depth of the tray and the height of the parapet above it.
Signage fabricators do not think about this and there is no reason they should. The sign was drawn to a shopfitting spec in a unit in Essex by somebody who has never had to clean underneath one.
What Does Guano Do to Glazing and the Frames Around It?
Fresh droppings sit somewhere around pH 3 to 4.5. Glass itself shrugs that off – float glass is stable against everything short of hydrofluoric acid, and the panes are rarely the first casualty.
The frames go first. Anodised aluminium loses its finish in patches and greys out. Powder coat lifts at the arris where the film is thinnest. Galvanised steel angle behind a stallriser starts weeping rust down the tiles. On a listed frontage with painted timber the paint film blisters and then the wet gets into the joint below the glazing bead, which is the point at which a window cleaning problem turns into a joinery bill.
Then the sealant. Uric acid crystals plus standing moisture will soften a low-modulus silicone perimeter bead over two or three seasons, and once that perimeter is compromised, water tracks to the edge seal of the sealed unit. Misting inside a double-glazed shopfront pane is a five hundred pound problem that started as a bird sitting on a sign.
The same acid works on the fascia’s own drainage. Most sign trays have a couple of weep holes drilled in the underside and those weep holes block within a season of occupation. Water then sits in the tray, goes anaerobic, and comes over the front lip in a single concentrated run rather than spreading across the fascia. That is why the staining on a roosted frontage arrives as one hard vertical lane instead of a general grey, and it is why you can stand on the opposite pavement on Kingsland Road and count which sign boxes have birds in them without going up a ladder.
What the run-off does on the way down
The stain itself carries grit, feather, seed husk and the sandy residue that comes off the fascia coating as it degrades. That mix dries onto the glass in the sun and then a cleaner comes along and drags a rubber across it. Every scratch I have ever put into retail glazing has come from this and from nothing else. I now scrape and flood the bottom 600mm of any pane under a roost before a squeegee touches it, and I still find myself apologising to the odd shop manager for marks I made in 2019.
Is Cleaning Under a Roost a Window Job or a Biohazard Job?
Both, and the second one governs. Dry guano turns to dust and the dust goes airborne the moment anyone touches it. The organisms of concern are the ones you have read about in trade press – psittacosis, cryptococcus, the various gut bacteria – and the exposure route is inhalation of dust rather than contact with the mess.
So the fascia gets wetted down with a low-pressure sprayer and a disinfectant dwell before anything is scraped. Nobody dry-brushes. Nobody puts a pressure washer on a guano-loaded sign box, and I have watched a facilities contractor do exactly that on Rye Lane at eleven in the morning with the shop trading and the door open below.
The kit and the waste
FFP3 for anyone within the plume, disposable coverall, nitrile gloves, eye protection, and a COSHH assessment on file before the van leaves the yard. Debris gets double-bagged and taken away as waste – it does not go down the nearest gully, because that gully goes to surface water and eventually to the Lea. The pavement gets washed with the shop’s permission and a wet floor sign, or it gets done before trading hours.
There is also the law about the birds. A nest that is in use is protected under the Wildlife and Countryside Act 1981, and a window cleaner or shopfitter has no business interfering with one. Give it a winter and the same tray will have an active nest in it, at which point the proofing job you should have done in September becomes a job you cannot legally do until the birds have finished.
Which Proofing Works on a Retail Frontage and Which Is Decoration?
Spikes are the default and they are close to useless on a fascia. On a narrow ledge they work. On a 110mm sign return they give the birds a scaffold, the nest material threads through the pins, and within two seasons you have a spiked ledge with a pigeon sitting comfortably on top of a mattress of twigs and bag handles. I have pulled more nests out of spikes than off bare metal.
Post-and-wire is better for a slim cornice and almost invisible from the pavement, which matters on a frontage where the landlord or the conservation officer has views. Netting works and looks like what it is – fine over a recessed area, poor across a display frontage, and it needs tensioning properly or it sags, fills with debris and becomes the nest site itself.
Gel discs, and why I keep buying them
Optical gel – the small trays of pyrethrum-free gel that read to a bird as flame – is the one product that has surprised me. On flat sign tops it works, it is quick to fit off a ladder, and it costs a fraction of netting a whole frontage. It also needs replacing every couple of years, it goes tacky in July, and it does nothing at all if the birds are already nesting there rather than perching.
Sonic deterrents I have no time for. Neither do the pigeons.
The other half of proofing is the ledge you cannot see from the street. Recessed lightboxes, the void behind a pelmet, the gap between a projecting sign and the brickwork it is bracketed off – birds get in through a 60mm gap and the frontage looks perfectly clean while the nest sits eighteen inches back. Any survey that stops at what is visible from the pavement misses about a third of the roosting on a Victorian parade.
The honest sequence is proofing first, then a schedule. Cleaning glass under an unproofed roost on a fortnightly cycle is a way of billing a shop indefinitely for a condition that a morning’s work at height would end. I stopped taking that work in 2021 and told three retail clients on Uxbridge Road why. Two of them proofed and are still with me. One of them found somebody cheaper who is presumably still up there every other Tuesday.
What Does It Cost the Shop When Nothing Gets Done?
The visible cost is the frontage. Grey streaks down glass at eye level in a retail unit reads as a business that has given up, and the pavement below picks up the traffic of everyone stepping around it.
The invisible costs land later. Re-anodising or refinishing a shopfront frame runs into thousands and cannot be done in situ on a trading unit. A failed sealed unit on a large display pane means a specialist glazier, a crane or a suction rig, and a day’s closure. A slip claim from a guano-fouled pavement lands on the occupier’s liability rather than the landlord’s, which surprises most tenants when they read their lease properly.
The number I give shop managers
A standard 6-metre high street frontage – gel to the sign top, wire to the cornice, a full clean-down and disinfection first – sits somewhere in the £600 to £1,100 range in London depending on access, and a scissor lift on a red route adds a day of permits to that. Set that against a decade of a stained frontage and the frame refurbishment at the end of it, and the sums are not close.
The Berwick Street jeweller never did it. The sign came down when the shop changed hands in 2023 and the fitters found about 40mm of compacted guano in the tray, a dead bird, and a section of the fascia’s mild steel bracket that had rusted through to the point where the whole box was hanging off two fixings above a pavement in Soho.