With a little modification any roof rack will work, and make roof top loading easy for you.
Step 1: A Solid Foundation
My minivan has a factory installed rack, I plan on adding a 2X4" to the top of each factory bow so the rack extends out enough to allow the ladder to be attached.
In my pictures I show two different ideas on ladder attachment one shows a pin through the roof rail and the other an extension that ia attached to the end of the rail, the first would be stronger but either would work.
Step 2: Give It Some Teeth
First you need to figure what you will be putting on the roof most often, in my case my canoe, the rails of my canoe are about 2 inches wide on top, so whatever teeth, notches or dowels I add to the ladders need to be at least 4 inches apart, 6 inches would work and save some work.
the three pictures give different ideas for what "teeth" you could use, the one that looks like a saw tooth would work best for a boat or canoe rail, and be the easiest to cut with a circular saw, from a standard 1X6" board or 1X4" board, a 1x6" would leave more meat in the board for strength, but would be heavier to move around the rest of the time. The dowel or square tooth would work for wider things like lumber.
To make the rails, find the height to the roof rails, now use a long board or straight edge and a level to find a 45 degree angle, the length from the end of the roof rail to the ground at a 45 is how long your ladder legs should be.
To use this with a kayak you may need to use a larger "tooth" possibly boards screwed and glued to the ladder rung and sticking up 6" to engage the sides of the kayak.
Step 3: Holding Things Together
Step 4: Load Up and Take Down
2) lay the boat or canoe on its side next to the car
3)lay the upper rail of the boat on the ladder
4) lift one end of the boat up about a foot and engage the teeth on the ladder with the rail
5) now lift the other end of the boat up until it is higher than the first end, and engage the ladder teeth with that end
6) continue this until the boat has "climbed" onto the roof rack,
6) strap the boat down, remove the ladder rails, strap them to the roof rack as well.