I would like to mount a fairly heavy shelving unit (~150lbs) to a wall with a number of L-brackets. It being a shelving unit, I would be putting all kinds of books and random stuff on it, thus increasing the weight.
This seems like a bad idea to me. I have no idea how heavy it could end up being with books, DVDs, art supplies, etc.
Is there a limit to how much weight should be mounted to a wall?
150 lbs is a very heavy set of shelves, unless the bottom sits on the floor and you only need to secure it to the wall so it won't tip forward. In any case, there is no written limit on weight, but a lot of common sense has to be used in this case. You will want to know what the rating of the "L" brackets are and if strong enough to hold your load. They would have to be securely mounted to studs in the wall with some pretty large screws. There are no anchors for drywall that I would trust for that much weight. Perhaps you might consider some legs from the bottom of your shelf unit to the floor to support the weight.
The limit is effectively defined by brackets and fixings you use and the type of wall you are fixing it to.
Make sure you get brackets and screws that can take the expected weight (plus a little bit more).
If you've got a brick/breeze block wall then (generally) the longer and fatter (size) the screw the more load it can bear - as long as you've used the correct size of rawlplug and drill bit for the hole.
If you've got a drywall (wooden studs and plasterboard) then make sure you screw the brackets into the studs. There are other questions on the site about how to find the location of studs.
Also using more brackets will spread the load so each bracket takes less of the weight thus allowing the shelf to hold more.
Finally make sure the material you choose for the shelf is capable of bearing the weight and won't bow (or even break) under the load. Real wood (or plywood) can take more weight than chipboard and thicker wood can take more weight than thinner.
If you want it off the ground consider bolting a steel L bracket to the wall and than placing the cabinet/shelf unit on this. Also look at bolting the cabinet itself to the wall.
With something this heavy, try finding the studs and not using drywall plugs because the chances of them getting pulled out and this weight crushing someone it's not a good idea. Even better if you're working with a foundation cement wall.
For extra hold look at the steel screw-in type drywall anchors... go overkill, better to be safe than dead. Also consider using the twin rack shelving units: they can hold a considerable amount of weight.
I would imagine into proper studs and good long and thick screws and thick brackets not the cheap ones you can bend with your hands 150lbs would not be an issue.
You can support any load provided proper ovehang support is provided. Use L bracket with diagonal supoort.You can reduce the span of brackets depending on the load.Also ties from the roof can be provided. A rectangular bracket with two diagonal ties and secured in roof slab and the wall is another solution.
