For abseiling from a flat roof, you use an A-Frame trolley:
This is carrried to the roof in pieces and assembled at the top. Notice the heavy counterweights.
You simply push it sideways before each drop to access the next “strip” of building.
Or you can use a deadweight trolley:
Not as easy to use, but faster to assemble and easier to transport.
Both available here:
https://sasanchorage.com/anchorage-products/a-frame-trolley#Not cheap though, and you need training on how to use it.
I looked into this a while ago, as I was considering IRATA training. I discovered 3 things...
1st there are too many abseilers for the available work. Lots of climbers want to find some way of being paid to use their skills, and window cleaning of high-rise buildings is one of the very few ways they can.
2nd abseilers are generally paid rubbish (probably as a result of point 1). I’ve spoken to a few lads who do it, and an average rate is about £56 per day. The reasons are many, oversupply of abseilers, shortage of decent employers, building managers prefer cradles and so there isn’t as much work as you might think, insurance and training costs are very high etc etc.
3rd, OTT anchorage equipment is the best by far. I’ve heard this from several veteran abseilers. OTT (now ionic systems) haven’t made anchorage equip for at lest 10 years, but if you’re lucky, you can still pick it up second hand. They all rave about how it was the greatest stuff ever and what a shame it’s not made anymore.