There is no one right answer. It always depends what screws you used, what the wall is made of, how many set screws you used and how deep the shelf itself is, i.e. where the load point is situated.
That being said, we often mention a 300lb load capacity for our recommended installation. This holding strength comes primarily from the two upper panhead wood screws inserted more than 2" into a standard kennel dried fir stud. When we talk about this, we are referencing 300lbs, distributed more or less evenly across a shelf 24 inches long, installed into two studs with four screws. We'll say the shelf in this example is no more than 4" thick (this is because of the set screws and their possible length). Anything thicker and you will need to use the steel bracket. Anything heavier than this 300lbs and you will need to reinforce the wall by putting up 2x6's, doubled up, between the studs - this means more opportunities to attach to the wall with screws.
We always urge you to consider safety first - don't cut corners, don't try to skip out on using either set screws or glue, and always be safe and smart about the way you install the product. We love an 8" thick barn mantel as much as anyone, but the standard aluminum bracket install (especially with corners cut) is not going to be the right choice there, cost-efficient as it may seem - for those heftier projects, you will want to pay the extra money for the custom made steel bracket. The last thing you want is something falling off the wall and someone getting hurt. You CAN sometimes get away with using an aluminum bracket for these heavier-duty projects, but you need to respect the limitations and make the necessary adjustments/reinforcements to do it.
We have tested the aluminum bracket in real time under the following conditions:
The shelf held more than 300lbs without sagging. (Weight spread over a 24-Inch long shelf)
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