If you kayak as much as I do you probably go with folks from time to time, which means transporting more than one kayak. For the average sized car this can pose difficulties. With standard roof racks it is fairly easy to fasten down a single kayak, but trying to cram two up there, well there is nothing easy about it.
Most double kayak racks made for vehicles are rather expensive and since there are so many varieties of dealer made roof racks, many store bought racks just do not fit on the car racks. This video shows how to make a very inexpensive, strong, lightweight and easy to construct double kayak roof rack that can be lashed to and roof rack you want.