An awning rail is a piece of aluminum channel attached along its whole length directly to the trailer. The awning fabric itself slides into this channel, thus forming a leakproof full length seam between the trailer and the awning. According to my way of thinking, this feature alone makes the rail preferable to the bracket arrangement, which permits rainwater to run down the side of the trailer onto the "porch". The picture below shows such a rail, and a bag awning slid into it. As you can see, the rail is used not just for deploying the awning but for transport of same.
I highly recommend you install one of these- once you have one, you can use it to string just about anything from. My first awning was a tarp wedged into the rail using a piece of spline...it traveled with its three supporting poles rolled up inside. Worked just fine.
I've since installed a regular pulldown awning just like the big boys have using the same original rail for the fabric. Couldn't be happier!
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