I have a 17 foot Coleman canoe, weighing in at nearly 90 lbs, not the easiest to flip, shoulder and land on the roof of my minivan with out assistance. while I have made another device to make loading the canoe easy, https://www.instructables.com/id/easy-way-to-cartop-a-canoe-or-boat/ it interferes with using the cargo carrier in the receiver hitch for my kids boats. so I dug deep into my memory and dredged this up from an old pop sci from the 60's or 70's.
With a little modification any roof rack will work, and make roof top loading easy for you.
Step 1: A Solid Foundation
you need a roof rack to tie onto first, I have seen many possible types, from the fancy expensive Yakima or Thule ones to a simple galvanized pipe one a guy I worked with had. Here is another idea https://www.instructables.com/id/Build-a-Bearer-of-Burdens/
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
while I show pegs as the "rungs" on the ladders, an easier alternative would be teeth or notches cut into both ladder planks,
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
you need a way to securely hold the ladder rails onto the roof rack, but also a quick way to remove them for storage. I figure a receiver hitch pin would cover this easily, most hardware stores sell a good variety of removable pins in the hardware isle for you to be able to find whatever you could need.
Step 4: Load Up and Take Down
1) pin the ladder rails to the roof top rails, make sure the teeth are facing upward
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.