How you do it kinda reflects how much risk you are willing to take. My comments and suggestions are limited to maybe a maximum of 3 stories. Anything beyond that is full-blown crane work.
First off, moving a 500 lb load around on a roof that likely is not designed for that kind of concentrated weigh demands some thought--laying planks across the roof at right angles to the support structure under the roof is a consideration... So even getting it to the edge of the roof might be an exercise in white knuckles.
Getting it from the edge of the roof to the ground is your next challenge. You need something to lift the thing over the roof parapet, if it has one, and hold it out from the wall as you lower it down. You need an anchor for a
winch line of some sort on the roof. That is kind of what roofers need to lift their stuff up to a roof--maybe talk to a local roofing contractor as to how they lift roofing materials up to a roof. They may have some simple tools that can be easily set up to do this work.
Another machine to consider would be renting a "zoom boom
forklift". They are a forklift with about a 5000 lb capacity and a boom not unlike an extendible hydraulic crane boom. I've seen pictures of them lifting pallets of plywood and lumber up to pretty decent heights--maybe 3 stories with the bigger machines. They usually are 4 wheel drive.
If you are very risk adverse, plucking the item from the roof, from where they are sitting now, by crane is the way to go...call a crane service and tell them you want it rigged and placed in the back of your truck. Make sure they have 3rd party liability insurance and whatever other insurance is needed for your part of the world. If you care if they drop your compressors, you can get them to cover your goods with "lift insurance", sometimes called "hook insurance". You will stand there and watch, hands in your pockets. They take all the risk.
Finally, should they really weigh only 4-500 lbs, the weight is about equivalent to moving a piano. If there is actually a set of stairs to the roof that is wide enough you might be able to move the compressors with lots of brawn and some furniture moving dollies. I would still be super careful about the strength of the roof decking. And the strength of the stairs to the roof! Spreading the load with planks-2x12's- may be your friend here, too.
Looking at what other smart guys on this forum have to say, who suspect that they are not very valuable compressors, you may end up having to charge a pretty penny to remove them.
Hope this helps,
Jon.
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