September arrives and the garden starts to change. The light is different, the mornings are cooler, and if you look closely at the driveway, the patio, the decking, and the render, you start to notice things you somehow stopped seeing over summer. A green tinge on the path. A dark film building on the decking boards. Render that looked fine in July now noticeably going green on the north-facing wall.

Autumn is the time to act on all of it, not spring. The surfaces that look tired now are about to go into the wettest, darkest, most biologically active months of the year, and what you do (or don't do) in September and October determines how they come out the other side.

This checklist covers every major exterior surface in order of priority, with the right GK product for each job and the specific reason why autumn is the moment to treat it. Work through it section by section and your property goes into winter with protection. Leave it until spring and you're dealing with the damage winter has already done.

Before You Start: The Autumn Treatment Window

Most biocidal cleaners, or rather the type that actually kills biological growth at root level rather than just washing it off, have a lower effective temperature threshold of around 8°C. Below this point, biological processes slow too much for reliable efficacy.

October is the reliable treatment window across most of the UK. November becomes marginal in northern England and Scotland as overnight frosts become more frequent. December through February is largely outside the reliable treatment window for most products.

The window is now. Don't wait.

Sealers have an additional requirement: they need dry surface conditions and temperatures above 5°C. For sealing, the window closes slightly earlier than for biocidal treatment. If you're cleaning and sealing, aim to complete sealing by late October.

1. Driveway

Why autumn: Biological growth on your driveway has been building since spring and is at its annual peak biomass in September and October. The organisms are still metabolically active, which means biocidal treatment is at its most effective. Left untreated through winter, algae retains moisture against the surface and accelerates freeze-thaw damage in cold spells. An untreated green driveway is also a slip hazard throughout the dark, wet autumn and winter months.

What to check:

• Green or grey-green film across the surface (algae and mould)
• Black circular spots - black lichen, not algae, requires separate treatment
• Oil staining from vehicles
• Orange rust marks from gate fittings or metalwork
• White powder on block surfaces - efflorescence

Products:

 

Method: Apply GK Driveway Cleaner at 2:1 dilution (water:product) for heavy soiling, 4:1 for lighter contamination. No rinsing required; leave to work with weather cycles. Treat black spots separately with GK Black Spot Remover applied neat and left to dwell overnight. Once clean and dry (allow 48 hours), seal with GK Invisible Stone Sealer before temperatures drop.

Full guides: How to Clean a Block Paving Driveway Without a Pressure Washer | Why Autumn Is Actually the Best Time to Treat Your Driveway

2. Patio and Paths

Why autumn: The same biological growth cycle applies to patios and paths as to driveways; peak biomass, still-active biology, and the approaching slip hazard season. Tannin staining from autumn leaf fall is a specific additional problem: wet leaves sitting on porous stone release brown and orange tannin compounds that soak into the surface and become increasingly stubborn the longer they're left. Treating and sealing before the main leaf fall prevents this staining from setting in.

What to check:

• General green or grey film across the stone (algae)
• Black spots on the stone surface (black lichen)
• Brown or orange discolouration from leaf staining (tannin)
• White powdery deposits (efflorescence)
• Surface feels slippery when wet - biofilm is present even if the surface doesn't look green

Products:

 

Method: Apply GK Path & Patio Cleaner at the appropriate dilution. No rinsing required for the biological treatment. Address black spots separately. Once clean and fully dry, seal with GK Invisible Stone Sealer, particularly important on porous Indian sandstone, limestone, and natural stone where tannin from subsequent leaf fall will otherwise penetrate quickly.

Full guides: Pressure Washer vs Chemical Cleaner: Which Is Better for Patios? | Black Spots on My Patio: What Are They and How Do I Remove Them?

3. Decking and Timber

Why autumn: Decking is the most dangerous surface in the garden when it comes to autumn slip risk. Algal biofilm on timber becomes extraordinarily slippery when wet, significantly more so than most hard landscaping materials, and a wet autumn day on untreated decking is genuinely hazardous. Additionally, untreated timber absorbs moisture through autumn and winter, accelerating the cycle of biological growth and potential timber degradation that reduces the lifespan of the boards.

Autumn is also the moment to assess the timber's condition while you can still do something about it before winter. Boards that have greyed significantly, feel soft underfoot, or show signs of splitting need attention now, not in spring.

What to check:

• Green or black biological film on decking boards
• Decking feels slippery when wet - treat as an urgent safety issue
• Boards have greyed or lost colour (weathering and UV degradation)
• Any boards that feel soft, springy, or show signs of splitting
• Fixings and frame showing rust or corrosion

Products:

 

Method: Apply GK Decking Cleaner and allow it to dwell; the product kills the biological growth and eliminates the slip-causing biofilm. If boards have greyed or lost colour, follow with GK Wood Restorer to restore the timber to close to its original tone and open the grain for subsequent sealing. Seal with GK Invisible Wood Sealer in two wet-on-wet coats before temperatures drop, giving the timber moisture protection through winter.

Full guide: How to Prep Your Decking Before BBQ Season - the same process applies for winter protection.

4. Rendered Walls

Why autumn: Render greening is a seasonal phenomenon with a specific autumn pattern. UV levels drop sharply after the equinox, surfaces stay wet for longer between rain events, and temperatures move into algae's optimal growth range. Biological growth that has been establishing at a sub-visual level since spring suddenly accelerates and becomes visible. September and October are the window to treat before the growth peaks and before winter weather makes treatment more difficult.

Treating render in autumn rather than spring means the wall goes into its wettest, most growth-conducive months already treated, rather than spending the entire winter accumulating biological loading that you'll have to deal with the following year.

What to check:

• Green or grey-green film developing on render, particularly north-facing elevations
• Black spots or dark patches distinct from the general green film (black lichen)
• Orange or red streaking - either rust from metal fittings above, or Trentepohlia algae
• White powdery deposits on render surface (efflorescence)
• Gutters above the render - clear any blockages before treating the wall below

Products:

 

Method: Apply GK Render Cleaner working bottom to top in overlapping horizontal bands. No rinsing is required; leave to work with UV and weather cycles. Do not use bleach on K Rend, monocouche, or any pigmented render as bleach reacts with mineral pigments and causes permanent discolouration. For walls being repainted after treatment, use GK Concentrated Fungicidal Wash as the pre-paint biocide, it is self-neutralising and leaves the surface paint-ready without rinsing.

Full guides: How to Clean Render Without Causing Streaks | My Render Is Going Green: Why Does It Always Happen This Time of Year?

5. Paths and Steps: Slip Hazard Assessment

Why autumn: Slippery paths and steps are the most urgent safety issue on this checklist, and they peak in autumn. Algal biofilm, the thin, often invisible layer of biological growth that makes external surfaces treacherous when wet, reaches its annual peak concentration in September and October, precisely when surfaces start staying wet for longer and daylight reduces. Falls on external hard surfaces are a leading cause of serious injury in the UK in autumn and winter, and older household members or visitors are at significantly elevated risk.

If any path, step, or external surface at your property feels slippery when wet, this is not a cosmetic issue. It is a safety issue that requires immediate treatment.

What to check:

• Any path, step, or surface that feels slippery underfoot when wet
• Green or grey-green film on path surfaces
• Shaded or north-facing paths that rarely dry out between rain events
• Steps - particularly riser faces and back edges of treads where footfall doesn't disturb growth

Products:

 

Full guide: Why Is My Path So Slippery in Autumn?

6. Roof Tiles

Why autumn: Biological growth on roof tiles including algae, mould, and lichen accumulates year-round but is most visible in autumn when the combination of dropping UV levels and increased rainfall accelerates growth across the tile surface. Untreated biological growth on roof tiles retains moisture against the tile surface, accelerating weathering and freeze-thaw damage over winter. Soft-wash treatment in autumn, before the coldest months, protects tiles through winter rather than leaving them to accumulate further biological loading until spring.

What to check:

• Green, grey, or black growth on tile surfaces
• Darker discolouration along ridge lines and valleys where moisture accumulates
• Dark streaking running down from roof to walls below

Important: Do not pressure wash roof tiles. High-pressure water removes the protective granular surface of concrete tiles and can crack or dislodge clay and slate tiles. Roof tile cleaning requires a soft-wash approach; low-pressure or no-pressure chemical application. If working at height, consider engaging a professional contractor.

Products:

 

Method: Apply GK Pro by low-pressure sprayer at the appropriate dilution. Do not rinse; leave it to work with UV and rainfall over the following weeks and months. GK Pro's minimum two-year residual biocidal action means a single treatment provides lasting protection without requiring annual intervention.

7. Garden Furniture and Outbuildings

Why autumn: As garden furniture comes in for winter storage, it's worth cleaning it before it goes away, not after. Furniture stored dirty over winter can develop mould and staining that is significantly harder to shift in spring, and contaminated furniture stored in a garage or shed can spread biological growth to adjacent surfaces.

Similarly, sheds, garages, and outbuildings with timber or rendered surfaces accumulate biological growth through the year that should be addressed before winter rather than left to worsen.

What to check:

• Green or black biological growth on metal, plastic, or timber garden furniture
• Mould or algae on shed and outbuilding walls
• Any surfaces that will be in contact with the house wall or stored items over winter

Products:

 

The Autumn Order of Work

If you're tackling the whole property, this is the sequence that makes the most sense:

1. Clear gutters first: Before treating any wall surface below a gutter line, clear the gutters. Treating render or walls below a blocked gutter, then having it overflow in the first autumn rain, undoes the work immediately.

2. Treat roofs and high-level surfaces: Biological material dislodged or washed from roof surfaces during and after treatment will run down onto walls and hard surfaces below. Treat from the top down.

3. Treat rendered walls and vertical surfaces: Once gutters are clear and roofs treated, treat render and walls. Allow dwell time before treating the surfaces below.

4. Treat hard surfaces - driveway, patio, paths: Apply biocidal cleaners to all horizontal hard surfaces. Address black spots and specific stains with the appropriate targeted products.

5. Treat decking and timber: Clean and treat decking before temperatures drop toward the sealer application threshold.

6. Seal: Once all treated surfaces are clean and fully dry, allow 48 hours after any rinsing and then apply sealers. Stone sealer on patios, driveways, and natural stone; wood sealer on decking and timber. Complete sealing by late October in most of the UK.

7. Garden furniture and outbuildings: Clean furniture before storage. Treat outbuilding surfaces.

Autumn Checklist at a Glance

 

All Products in This Checklist

All available from GK Products UK with free delivery on orders over £60 to mainland England, Wales and parts of Scotland:

• GK Driveway Cleaner 5Ltr - £22.79
• GK Path & Patio Cleaner 5Ltr - £22.79
• GK Render Cleaner 5Ltr - £22.79
• GK Decking Cleaner 5Ltr - £22.79
• GK Multi-Surface Cleaner & Sanitiser 5Ltr - £22.79
• GK Pro 5Ltr - £84.00
• GK Concentrated Fungicidal Wash 1Ltr - £7.20
• GK Black Spot Remover 5Ltr - £19.99
• GK Wood Restorer 5Ltr - £40.00
• GK Invisible Stone Sealer - from £60.00
• GK Invisible Wood Sealer 5Ltr - £21.60
• GK Oil Out 5Ltr - £82.80
• GK Stinky Pinky Rust Remover - from £11.99
• GK EFF OFF Efflorescence Remover 5Ltr - £34.98
• GK Anti Mould Paint - coming soon

Not sure which product is right for your surface or staining type? Call the GK team on 07557 111 297 or email sales@gkproductsuk.com, we're happy to advise on the right treatment for your specific situation.