J‑bar mounts typically fit round crossbars about 1.0"–1.6" (25–40 mm) in diameter and flat bars about 0.6"–1.6" (15–40 mm) in thickness.
Common materials include metal and steel for racks and baskets, aluminum for track components, PVC and polyester for soft pads, and various padded or molded materials used at contact points to protect the hull and roof.
Capacities vary by style: some J‑cradle mounts list around 80 lb; many soft‑pad and J‑bar options list roughly 150–176 lb; and rooftop cargo baskets or heavy‑duty carriers can list capacities up to about 200 lb.
Carriers attach in several ways: they can be bolted or clamped to roof beams or crossbars, strapped down (soft pads can work with or without side rails), or mounted directly to base tracks or track/tower systems.
Common protective features include padded contact points and thick soft cushions, wear‑resistant plastic brackets on mounts, and non‑slip, shock‑absorbing bottoms on soft pads — all designed to cushion contact, reduce rubbing, and improve grip between hull and roof.
Many rooftop carriers are rated for a single kayak; some are designed to carry two. A few listings specify capacity by number of kayaks rather than by weight.
Features that ease loading include quick‑fold or fold‑down designs, rollers and spring‑loaded saddles, tilting or lift‑assist mechanisms, and included heavy‑duty straps plus bow and stern tie‑downs.
