When it comes to materials you have two options, steel or aluminum. You might see the occasional wood roof rack, but they are not a great long-term solution as they will weaken over time and are not as durable. Think of your roof rack as an extension of your vehicle; it should be as rigid and strong as possible.
Steel has always been a popular DIY roof-rack option, as it is easy to acquire and relatively inexpensive. If you have factory roof racks that run from front to back, you can build your own steel crossmembers using one-inch square tubing cut to size and secured to the factory racks using carriage bolts and a fork latch.
For a more custom design you'll need to do some metal cutting and welding. A MIG or flux-core welder is a great starting point. You'll also need factory or aftermarket roof mounts to attach your rack to. This DIYer used one-inch round metal tubing and welded up a custom roof rack that had exactly what he needed to carry his kayak, storage boxes, and grill.
If you don't have the welding chops for steel, 80/20 aluminum is a great DIY option that simply bolts together. Bob Clagett from I Like To Make Stuff built an aluminum rack for his Land Cruiser. He now has the ability to haul full-size sheets of plywood as well as anything else he can fit up there, including off-road lights, a roof-top tent or a Christmas tree.
With any custom roof rack, you'll need to lay out the exact measurements required and constantly square things up. With 80/20 aluminum, you can custom order to the exact measurements you need and then attach it all like an Erector Set. The 80/20 is great because the T-slots allow you to secure all types of attachments by simply using a bolt and T-nut. You aren't limited to fixed points on your rack to tie your gear down; just slide your hooks or eyebolts into position where you need them—very similar to how Front Runner roof racks work.
Aluminium Roofing Sheet Production Line
Aluminum racks are nearly as strong as steel but nowhere near as heavy, which means the gas mileage hit you'll take will be minimized.
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