The antenna will work better with a proper ground plane. The Larsen NMO base kit is a good solution, or you can build something yourself.
The antenna will work better the higher up it is. The frequencies your scanner and this antenna work on are usually considered "line of sight". If the antenna can "see" the transmitting antenna, it will work well. The higher up the antenna is, the better it will work.
Attic mounts can be hit or miss. It'll be closer to RF Interference from appliances inside the home. If your attic has HVAC ducting, metal pipes, wiring or foil backed/vapor barrier insulation, it can create issues. If your roof is made out of any metal, it's going to impact reception.
Even if the attic is entirely clear, any building materials will reduce signal strength slightly. Lightning protection is still required, an inch or so of roofing material is not going to protect the antenna, and even nearby strikes can induce enough energy into the antenna to cause damage.
RG-8 is not an ideal choice for coaxial cable, either. If you have not purchased it yet, you can do better.
So, get the antenna up high. For RX only use, it's going to work better with a ground plane. Use the best coaxial cable you can afford.