DIY: Roof-Rack For Sheet Building Supplies

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I needed a way to transport sheet building supplies (e.g. plywood or Gib plaster board) that typically come in 1.2m x 2.4m [4' x 8'] sheets. I wanted a flat-bed roof-rack for the top of our car that would fully support the sheets whilst in transit. I also wanted the roof-rack to be articulated so that I could still open the boot [trunk] when the (unloaded) rack was on the car. Here's what I came up with...

The roof-rack is designed to slot between the roof-rack bars that are already on our car.
The plan, below, displays the roof-rack from underneath. In this plan the longitudinal members (orange) are rebated to accept the deck (yellow) so as to create a flush surface.
The deck only spans 50% of the length so that the unloaded roof-rack doesn't catch too much wind (particularly at the front).
The roof-rack and load is fully supported by the roof-rack bars - the longitudinal members do not touch the roof of the car.
The deck is made from 12mm [½"] H3.2 treated plywood. All other timber is 50mm x 50mm [2" x 2"] H3.2 treated pine.
Pairs of Ø50mm [2"] holes are drilled in the deck to facilitate tie down. These holes line up with the middle set of roof-rack bar supports.
The rear, hinged, support is cantilevered such that it can swing up but not down.
The 'hinge detail' photos, below, show the heads of the vertical bolts sticking up proud of the timber - these were later recessed so as to create a flush surface.
If you build your own one of these, please ensure you check that it's legal in your locale. And don't put more than a sensible amount of weight on it (the stack of plywood in the photos, below, is 'maximum capacity' on my one).

Roof-rack plans (underside)