It can be done but there will be some loss. Generally the higher the frequency, the bigger impact of barriers like roofs and walls. So 70 cm downlink will suffer noticeably. I’ve done it and make the following recommendations.

Get the most gain you can. While omnis are possible, you may lose patience with limited passes as I did when I started with turnstiles and moxon turnstiles. Get or build a yagi or other directional antenna and turn with a cheap tv rotor (I prefer old alliance u100s as they are inexpensive used and quite rugged). The biggest one you can afford or squeeze into that attic space available. Because you don’t need to weatherize, consider just rigging up an Arrow or Elk (I did it with the Elk) at fixed elevation.

Get a preamp. Its night and day, believe me, when you have one with less than ideal antennas. I use the ARR 25 watt switched units. Youll really benefit from the preamp on 70 cm downlink. If you do VO52 or A07 mode B, a preamp on 2M is helpful, but I’ve found these sats quite usable without one from the attic. While this might just be my QTH, I found a lot more QRM on 2M than 70 cm and the preamp just tends to make the QRM louder.

Keep the antenna away, best you can, from any possible sources of coupling (metal), including house wiring, ductwork, flashing, and even foil backed insulation batts. You cant avoid everything up there, but just try your best. The effect, exhibited often in the form of directional SWR issues and RFI is a combination of how big the metal is and how far away. In my case, I had to move things around due to nearby heating ducts.

Good luck!

Mike WB1AAT