I have a wood shelf built out of 2X4 lumber. The shelf is 4 ft deep and 8 feet long (it spans the entire 8 feet). Does anybody know what would be the allow weight you could put on this shelf?

 

Well first off. How are you planing on securing this? You WILL need to put some type of bracing. Why such a big shelf? What are you going to use it for?

 

The load capacity of a shelf is dictated by how it's attached to the wall and how the actual shelf surface is supported.

If you lay the ends of a sheet of plywood on two sawhorses, you'll probably see a sag in the middle just from the weight of the plywood. Obviously, it isn't going to carry much weight.

If you put two 2x4's on edge on the sawhorses, then put your plywood on them, it won't sag and will probably carry a few hundred pounds.

Transferring this to a wall, a 2x4 framework will carry quite a bit of weight, but now you have to deal with that 4' width that will try to "twist" the shelf down. That's way too wide for anything except vertical posts on the front (more 2x4's). You may also need to "sister" a second 2x4 onto your post (like a jack stud holding a header) since that much shelf space will get really heavy if you fill it with "stuff".