The rear roof rack brackets are bolted through all the way, through the rear plywood bulkhead. Main concern is to fully seal it. Front roof brackets are bolted through the front left and right corner structural wood. Again, sealing, since rot of these members is common in older units. These four attachment points almost perfectly coincide with the locations of the hydraulic tubes, helping to ensure that rack payloads are equally borne by the hydraulics. Supposedly you are allowed to add up to 250 lbs to the stock weight that has to be borne by the hydraulics. Keep in mind this includes the weight of whatever is stowed in the overhead bins, and the weight of the rack itself.