I think the people at the rose forum may be of more help, but I'm usually pretty comfortable accepting mailed plants by about 4/15. And a couple weeks later would of course be even safer. I don't think you have to wait all the way to mid-May. I'm guessing that since you garden on a roof that you're in the city proper, maybe even downtown, which, if you were on the ground, would probably keep you safe from those late hard frosts. But maybe being up high negates that. I don't know. I think what you want to avoid is having outdoor plants inside (where it's likely hot, dry and dim) for more than, say, a week because it's too nasty to take them outside.

Planting on a roofdeck (presumably in large containers?) is a little different than in the ground, and my experience with mailorder is confined to dormant bareroot roses.

However, with bareroot roses, I would say the earlier the better, and midApril is ideal. You plant them and mound dirt over them till the buds start to swell and growth really starts. The plants then have longer, ideal spring weather (usually cool and damp) to form roots, and will be far better off than if you did that in May, when they may suffer from too much heat too soon. They will be bigger and you will get more of everything from them by starting them early.

Plants in active growth you need to be more careful of, but my bias would be early May. Most roses can take some frost without much problem.

April 15th I'm out there. Never lost any to spring cold. I DO advise (after planting) mulching the leaves and any new growth on bareroots until May--to avoid sun scorch.

My best results came from planting roses at the end of March/beginning April before it gets warm. They seemed to settle better than in warmer weather.

I also moved a rose bush or other shrubs in March with no ill effect. Any roses moved on a later date went to rose heaven and never returned.

Soak them well brfore planting. They'll take off.

Godd luck.