Weight bearing Shed with water tanks on top.....

My place has several pre-existing, very stoutly built outbuildings, stud frame and plywood sheath construction. I want to create a gravity-flow irrigation system for the site by placing several 300 gallon palletized water "totes" on top of the sheds. (The sheds have very gently sloping plywood metal-sheathed roofs so sliding off isn't an issue). No matter how I search I simply can't come up with some basic guidelines as to how much weight I can expect the sheds to hold up. I could easily place four totes on one of the sheds....this would total almost 10,000 pounds of water!! My plan is to place the tanks with one edge over the corners or walls of the shed, so some of the load is directly over the wall or corner. Where the other corners of the tanks rest on the roof span, I am wedging a 4x4 near that spot in the interior of the shed, between a roof rafter and the spot on the plywood floor that has the 4x4 stringer underneath. I have access to additional 4x4's, 2x4's etc. so I can add additional supports...but how many?

http://www2.wwpa.org/Portals/9/docs/PDF/TG7.pdf

http://www.johndee.com/vbulletin/showthread.php?14252-How-much-weight-can-a-roof-hold

http://askville.amazon.com/seriousness-snow-load-roof-hold/AnswerViewer.do?requestId=1015681

Try reading from the links above and see if that helps any. Also, try asking your search question differently. Instead of asking "how much water" will the roof hold, try a search for "snow weight on roof" or "how much snow weight will roof hold" and see if a Google search shows anything.

Most people would not be putting water up on a roof and so the question will not be asked much online. But, people do have snow on a roof and so the snow weight question will be easier to search.

That sounds to me like too much water up top. Without doing the math, and since I have not seen the roof, then of course, I do not know. But, just off the top, it sounds like too much. Another option would be to place some water up on top, but have other containers on the ground and connected? Would the upper tanks give you the pressure needed but the lower tanks expand the stored amounts?

Good luck.

By filling one tank at a time, I've answered my own question...the shed is holding up all four tanks full just fine, with a 4x4 near each interior corner..... I've discovered an easy and cheap way to add thousands of gallons of elevated storage to my site !