I started with a ballpark measurement of my roof side rails. At the front they were around 44” apart, and at the back about 50” apart. I went down to the lumber yard, picked out an 2x6x8’ appearance grade pressure treated board, and headed back. The pressure treated material is important for it to stand up to the exterior exposure well, and the appearance grade stuff is so it will look pretty. I cut the board into two pieces, so that there was equal overhang in the front and the back (if your rails taper).

With your 2×6 cut, it’s time to take a closer look and measure what size U-bolts your rails with require. I knew I wanted the U-bolts to be as close to vertical as possible, not leaning either way. So I eyeballed the overall width of the rail, added ⅛” for wiggle room, and took this as the minimum inside dimension of my U-bolt. For example, if your rail measures 1 ¼” wide, then 1 ⅜” is the minimum inner dimension (or throat size) of your bolt. The length of the bolts is determined by the ‘thickness’ of your rails, plus 1 ½” for the 2×6, plus at least ¼” for the nut to thread on. Ere on the side of the longer U-bolts here, as they can always be cut off to length at the end. My rails were around 1 ½” thick, so my minimum U-bolt leg length was 3 ¼”, and I ended up getting 3 ½” long bolts.

Now that you have an idea about what hardware will work for you, trot down to the hardware store and spend a few minutes savoring the good old days. Once sufficiently savored, check out the section that has hold downs, clevises, D-rings, etc. That is where I found a satisfying selection of U-bolts, most of which were Stainless Steel. While substantially more expensive, $3 or $4 each instead of about $1 each, the stainless was worth it to me, since the whole idea of a roof rack is it sits wherever your car sits – presumably outside a lot of the time. Four correctly sized U-bolts in hand (mine came with regular nuts and a flat plate, although locking nuts would have been better — I ended up using ThreadLocker to get the nuts to stay put and not vibrate off), I also picked up a 2’ section of clear, bendable plastic tubing. UV-resistance is an important quality for this. This tubing should be able to easily slide over the bolts, and make the 180deg bend in the U-bolt. I’d say this tubing is optional, but I thought it a good idea to prevent the direct metal-to-metal contact of U-bolt to roof rail, and the damage to the rail’s powder coat finish that would result. I threw into my basket 4 stainless steel tie-downs which I screwed into the top faces of the cross-boards, for strap and bungee attachment.

My purchases in hand, I walked back up the block to my parked car, grabbed by cordless drill with the correct drill bit, and set to work. I made sure the first 2×6 section was centered between the rails, held the U-bolt temporarily in place, making sure it looks vertical, and marked the location of both legs of the bolt. Really, the most critical measurement is the spacing between the two holes for the bolt legs – everything else can pretty much adapt. Make sure the holes are centered on the width of the board. Repeat for the other 3 locations, and drill the holes, keeping the drill bit vertical as you do so. Slide the clear tubing onto the U-bolts, and cut the tubing so it extends to the top of the rail but not past when the bolt is seated fully against the rail. This will keep it from interfering with the 2×6. Slide the U-bolts onto the rails and through the 2×6’s, slide the plates onto the bolts, and spin the nuts onto the washers. Tighten them down nice and snug – you don’t want a loose roof rack – and you’re good to go! While my 2×6’s do budge forward and backward along the rails if forced, there is no concern in the connection of the U-bolts to the rails, so the load is still safe and sound.

I can carry ladders, lumber, or a kayak on the rails. I have a piece of ¾” plywood cut to the proper size, which I can throw on across the 2×6’s, fasten with 4 screws, and have a flat platform for my soft car-top carrier. I haven’t implemented anything for carrying bikes, as I have a strap-on rack for the trunk. I have a couple thoughts for what one could look like for this system, but I’d love to hear your thoughts and see what you come up with!