wow what a lovely idea. I would put a wooden edge all around the roof edge to hold in the soil, now probably best time to do this as long as no really heavy snow or frost. I would layer grit aswell as soil to help drainage, but not to deep or heavy this would collapse the roof. Try sun loving drought tolerant plants ie...Alpines like a miniature rock garden OR go the grass and poppy/ daisy mix for up there. Hope this helps good luck.

What condition is your roofing felt? as this is likely to leak over time, do you want to keep the chickens dry?
'PRINCIPLES:-
The essential components of a green roof are:

TIMBER FRAME to hold the green roof elements onto the roof. In some cases, extra timber supports may be needed to ensure that the building can take the load.

ROOT MEMBRANE – this could be a pond liner or 300micron damp-proof membrane.

FILTER SHEET – this stops fine material from leaving the roof. It is generally a sheet like the type placed under patios to keep weeds out. It holds the fine soils but allows water to pass through. It can be bought at DIY stores.

MOISTURE BLANKET – essentially a woolly fleece that holds water. These can be bought commercial and are referred to as geotextile membranes. Cheap alternatives are cardboard or even old blankets/towels will do.

SUBSTRATE – this is the medium in which the plants will grow and should consist of aggregate such as crushed concrete/brick, limestone chippings, gravels or clay pellets. This should be at the base to free drainage. Above it, you need another layer, consisting of sand or soil [preferably subsoil] or both.

PLANTS – the species selected will depend on the substrate and the substrate depth. They could be seeded or plug planted'

I found this on 'Living roofs.org' - there are several sites may well help you.

Wow! Thanks to Lillium20 and Drc726 for excellent advice - particularly for steering me towards Livingroofs.org website. I am now very much looking forward to going ahead. Can I just say that I have only been a member of GOY for a few days and already I am raving about it to my gardening friends. Great stuff!!!

Good luck with your roof Cindy

Thanks again.