Do homemade convertible top cleaners work?
Quick answer: They can shift light surface dirt, but we don't recommend them. Vinegar, washing-up liquid and bicarb all strip the roof's water-repellency, leave residues that pull dirt back in, or bleach the fabric -- and the damage is often invisible until the next downpour soaks through. If you have nothing else to hand, use a mild pH-neutral shampoo diluted well past the label, spot-test first, rinse hard with cold water, then re-proof straight after. A dedicated soft-top cleaner remains the right tool.
A kitchen-cupboard mix will never beat a purpose-made convertible roof cleaner. The chemistry in dedicated hood products has decades of detergent science behind it, and it is aimed squarely at the organic grime that grows on a cabriolet fabric. Specific problems like tree sap baked into the weave need targeted chemistry; most household fixes won't touch aged resin.
The appeal of the homemade route is obvious. You already own the ingredients, the internet is full of people swearing by them, and a bottle of dedicated cleaner feels like an unnecessary spend on something a squirt of Fairy might fix. The trouble is that a soft-top is not a hard surface, and almost everything that makes a household product good at its real job makes it wrong for a roof.
What people reach for first
Before buying a dedicated cleaner, most people try something already in the cupboard. Washing-up liquid is far and away the most common. Vinegar comes up constantly for algae and green growth. Bicarbonate of soda gets rubbed into stains, and carpet or upholstery shampoo gets pressed into service on the logic that it is at least designed for fabric. We have seen all of these arrive on roofs in the workshop, and each one leaves its own signature of damage by the time we get the car.
Washing-up liquid: a degreaser doing the wrong job
Washing-up liquid is a degreaser, and that is precisely the problem. The thing that makes it brilliant at lifting bacon fat off a frying pan makes it destructive on a soft-top. Modern convertible roofs carry a DWR (Durable Water Repellency) treatment that makes rain bead up and roll off the weave. A strong surfactant cuts straight through that coating in a single wash. The fabric can look cleaner afterwards, yet it has quietly lost its water-repellency -- and the owner only discovers that the next time it rains and the headlining starts to damp through.
There is a second, slower problem: residue. Washing-up liquid is formulated to be rinsed off hard, non-porous crockery. The open weave of a soft-top traps surfactant deep in the fibres no matter how diligently you hose it. That trapped residue is faintly tacky and acts as a dirt magnet, which is why a roof cleaned with Fairy often looks grubbier than it did before within a few weeks. The owner concludes the roof is "always dirty now" and washes it again with the same stuff, accelerating the spiral.
Vinegar will kill the algae and the colour with it
Vinegar does kill algae and lichen. That part is genuinely true, and it is why it spreads so readily as online advice. The catch is that acetic acid works aggressively and without discrimination. On coloured canvas and on vinyl it can permanently bleach or blotch the surface; on a black roof the harm is less obvious at first, but repeated acid exposure opens up the weave and makes the fabric progressively more porous -- harder to clean and quicker to soak through every season that passes.
For green growth there are cleaner ways to shift algae using biocidal treatments balanced for soft-top fabric, which clear the growth without the side effects. We have also written separately on why vinegar is a poor choice for lichen, where the roots sit deep in the weave and the acid does more harm to the fabric than to the lichen.
Bicarb, carpet shampoo and the rest
The others fail for related reasons. Bicarbonate of soda is mildly abrasive and alkaline; rubbed into a stain it can lift a mark, but it also scuffs the fibres and shifts the fabric's pH away from neutral, which is where soft-top coatings are happiest. Carpet and upholstery shampoos are built to foam, lift and stay in the pile of an interior textile that never sees rain -- they are not designed to rinse cleanly from an outdoor weave, so they leave the same residue problem as washing-up liquid with none of the cutting power you might at least have wanted. Anything containing bleach or optical brighteners is straightforwardly off the table; the discolouration is permanent and unmissable in daylight.
The real cost is the waterproofing
Step back from any single product and the common thread is what these cleaners do to the roof's water repellency. A roof that sheds water also resists staining, mould and a good deal of UV damage; the DWR layer is doing more than keeping you dry. Strip it with harsh chemistry and you lose all of that protection at once, usually for good unless you re-proof immediately afterwards.
This is the trap most DIY cleans fall into. Someone cleans the roof with a household product, is pleased with how it looks, parks the car and walks away -- and the roof is now in a worse state than before they touched it: cosmetically cleaner, structurally exposed. The first heavy rain finds the unprotected weave, water wicks through to the lining, and a job that started as a tidy-up becomes a damp-interior problem. Re-proofing is not an optional finishing flourish after a clean; on a stripped roof it is the only thing standing between the fabric and the weather.
If you genuinely have nothing else
Sometimes you need to shift surface grime now and there is no dedicated cleaner within reach. In that case the least-bad option is a pH-neutral car shampoo, diluted further than the bottle suggests, worked in gently with a soft brush and rinsed off thoroughly with plenty of cold water. Spot-test on a hidden corner first, near a seam or under the rear edge, and check it has dried without marking before you go near the visible expanse. Treat this as the minimum viable option, not a recommendation -- it is simply better than reaching for the washing-up liquid.
What it will not do is leave the roof protected. Whatever you clean with, a household product included, the roof needs re-proofing afterwards to put back the water-repellency the wash has worn down. The wider question of why dedicated products and a proper process beat the cupboard every time is covered in the full guide to cleaning a soft-top roof, where the order of operations matters as much as the chemistry in the bottle.