That is illegal, by the by. You are required to remove any snow from your vehicle that may impede another driver's ability to drive or maintain control if it dislodges from your vehicle. It is no different than an unfastened or unsecured load, and dangerous to boot. The bridge could potentially suffer damage in a worst case scenario.
In most cases it is difficult to go on the top of trailer and it takes time. If you fall down from there you could get seriously injured or even get killed.
That is also illegal in finland. If something like ice flyes to your windcreen and there becomes a scratch. Truck driver has to pay the windshield.