Bike Tarp or Bike Cover? What a Tarpaulin Really Does

A builder's tarpaulin costs about a tenner and it is genuinely waterproof. If you have a bike living outside, the obvious question is why you would spend more than that. It is a fair question, and the honest answer is that a tarp does one job well and three jobs badly — and the three it does badly are the ones that decide whether your bike is still working properly next spring.

What a tarp genuinely does well

Let's be fair to the tarp first. Polyethylene sheeting is waterproof, it is cheap, it comes in sizes big enough to swallow a cargo bike, and it does not care about UV or frost. For keeping rain off something for a few days, it is hard to beat on price.

If your bike is going under cover for a fortnight while you are away, or you are throwing something over it in the back of a van, a tarp is a perfectly sensible answer. The problems start when the tarp becomes the permanent arrangement.

Problem one: the tarp traps the water it kept out

This is the one that catches people out, and it is the reason so many riders end up convinced that covering a bike is worse than leaving it bare.

Polyethylene sheeting is completely sealed. Air does not pass through it. So when a bike goes under a tarp still wet from the ride home, or when the ground underneath is damp, the moisture that is already there has nowhere to go. It evaporates in the warmth of the day, condenses on the underside of the plastic when the temperature drops overnight, and drips back onto the bike. Repeat that cycle for a few weeks and you have created a small, permanently humid greenhouse around a machine made largely of steel and aluminium.

The bike would have been drier in the open, because at least there it would have had wind and sun to dry it out between showers.

A fitted cover behaves differently for two reasons. It is shaped around the bike rather than draped over it, and it is open at the bottom rather than sealed to the ground, so air moves underneath and carries moisture away. Bicycling put it plainly when it named BikeParka in its bike cover guide: "Unlike many designs, the Bike Parka is breathable, which helps reduce corrosion and mildew."

Problem two: wind

A tarp is a rectangle. A bike is not. Whatever you do with bungees, you are left with loose sheeting, and loose sheeting in wind does two things: it flogs, and it abrades.

Flogging sheeting works against the points it touches — the saddle nose, the ends of the bars, the top of the mudguard — and over a winter it will wear the finish off them. It also works the tarp itself loose, which is why tarps end up in next door's garden after the first real gale.

A cover cut to the shape of a bike, tensioned at the wheels and drawn in with toggles, does not have loose fabric to flog in the first place. That is a fit problem, not a fabric problem, and no amount of bungee cord solves it.

Problem three: you cannot secure it

A tarp has eyelets at the corners, placed for tying to scaffolding. It has nothing where you would want to pass a lock through around a frame.

This matters more than it sounds. A cover that cannot be secured to the bike is a cover that walks off, and a bike under a tarp that has blown away is a bike with no protection at all on the night it rains hardest. Covers designed for bikes deal with this directly — the BikeParka URBAN, Longtail, Longtail WS and Cargo have lock-through panels or slots so a D-lock passes through the cover and around the frame.

And the thing nobody mentions: a tarp advertises the bike

A blue builder's tarp bungeed over a bike-shaped object in a front garden tells everyone walking past exactly what is underneath it, and that whatever is underneath is worth covering. A fitted cover in a plain colour reads as ordinary street furniture. It is not security, but it is not nothing either.

So when is a tarp the right answer?

Genuinely, in these cases:

• Short-term storage of a bike that is already dry — a few weeks in a garage, not a winter in a garden.

• Covering a bike in a van or trailer, where the job is keeping dirt off other things rather than keeping weather off the bike.

• Covering something that is not a bike at all, which is what tarpaulins are actually made for.

What a tarp is not good at is the thing most people buy it for: a bike that lives outside permanently, gets ridden in the rain, and goes back under the cover wet.

Bike tarp vs bike cover: common questions

Can you use a tarp to cover a bike?

You can, and for a few dry days it works. For permanent outdoor storage it usually makes things worse, because sealed plastic traps moisture against the bike instead of letting it dry, and loose sheeting flogs in wind and wears the finish off whatever it touches.

Does a bike tarp cause rust?

Not directly, but it creates the conditions for it. Rust needs water and time. A sealed tarp over a damp bike holds water against the frame and drivetrain instead of letting it evaporate, so the bike stays wet for longer than it would uncovered.

Is a tarpaulin or a bike cover better for outdoor storage?

A fitted cover, for anything longer than a couple of weeks. The tarp wins on price and loses on fit, airflow and security — and those three are what determine whether the bike is still shifting properly in March.

How do you stop a tarp blowing off a bike?

Bungees at the corners and weight at the bottom edge help, but you are working against the shape: a flat sheet over a bike will always leave loose fabric for the wind to get under. A cover shaped to the bike and tensioned at the wheels does not have that slack to begin with.

Does a bike cover need to be breathable?

It needs to not trap moisture against the bike, which is what most people mean by breathable. A cover that is shaped around the bike and open at the bottom lets air move underneath and moisture escape, while still keeping rain off from above. A sealed sheet does neither.

Is a bike cover worth the money over a tarp?

It depends entirely on how long the bike is outside. For a fortnight, no. For a bike that lives outside year-round, a cover costs a fraction of a drivetrain and rather less than a set of seized brake and gear cables.