Hello Joan,
I have two such systems in my garage. Someone involved with construction inspection told me that if there is anything above the fire sprinklers a pullye system could contravene fire regulations in an underground parking lot.
In any event, in my garage I use pulley system at the front and one at the rear of the kayak. A carabiner on the bottom pulley is clipped to a loose fitting webbing sling which goes around the kayak. The bottom pulley is double as is the one above, attached to the rafters. In addition, I have single pulleys to guide the rope to the side of the garage and along the wall.
4 mm rope is strong enough but a bit uncomfortable on bare hands when raising the kayak. 6 mm rope is probably a good choice. 9 mm rope has a nice feel but is bulky, more expensive, and requires larger pulleys. I used cheap pulleys from Canadian Tire but some stores may not carry double pulleys of a suitable size.
On the weekend, I can send you some photos.
Philip Kubik