I routinely cartop a 17-foot folding sea kayak that weighs about 80 lbs. I use a standard rack system with a product from Thule called a "rollercoatser", which allows rolling the boat up onto the rack with minimal effort. I then use two ratcheting tie-downs on the rack and two more on the bow and stern.
At 150 lbs., I'd check what your roof rack is rated for. If you have an aftermarket system, you should be OK, but some factory racks are pretty cheesy.