We employ all our window cleaning and carpet cleaning staff on piece rate, they get paid for what they do, regardless of the weather, if they do nothing, they get paid nothing. The rate we pay is quite good so on average they come out with a good wage throughout the year.
All our staff sign this contract prior to being taken on on a permanent basis, it suits them that they have the ability to earn well, it suits us that we don't have to pay if they are not bringing in any turnover.
You will find a lot of factory staff on a similar contract, especially in the fish industry where their workload depends on what is landed.