*
I am building a large planter box for a penthous deck. The deck is elevated above the building roof. The joists are fire-treated exterior 2 x 12’s, 16″ O.C., running a 16′ span. The planter will be 37″d x 28″h x 15’l running parallel with the joists. One section of the planter (appx. 3’w) will be filled full depth with soil for an ornamental tree, the rest will have a raised bottom allowing for 16″ of soil. The owner is concerned about the additional weight of the planters on the deck and wanted me to put in additional joists. After a site visit for measurements, I realized that the only way I was going to get a 16′ 2 x 12 to the 7th floor was with a crane. So, I need to know if the deck will support the load. I have been able to find span tables at some of the industry trade sites, but not load tables. Any ideas?
Discussion Forum
Discussion Forum
Up Next
Video Shorts
Featured Story
![](https://images.finehomebuilding.com/app/uploads/2024/04/29113159/BUILDING-SCIENCE-SYMPOSIUM-1-Large-thumb-16x9.jpeg)
Check out the lineup for the 2024 Building Science Symposium featuring experts like Steve Baczek, Dr. Joe Lstiburek, and more.
Featured Video
Video: Build a Fireplace, Brick by BrickHighlights
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Replies
*
Louis,
I think you will need to calculate the load. My guess is the soil (moist) can wheigh anywhere from 75 to 100 lbs. per cubic foot. From your description, the volume of soil (36" x 36" x 28" or almost a yard) is about 21 cubic feet...a considerable weight. And that doesn't include the tree itself. In addition, its a significant point load, and the need for additional structural support will depend greatly on where it occurs along the span.
Obviously the expense of a crane is probably not feasible.
I don't have the solution. Is there any way to add vertical structural elements, thus segmenting the span and allowing you to beef up the structure with shorter members? The only other idea that comes to mind is to some how beef up the existing joists (how are they fireproofed?) and block the hell out of it.
I wish I could be of more help.
*You couldn't pull the 2x12 up with a rope? Maybe rig up a gin pole and get a police detail if necessary? I agree, that will be a lot of weight, soil is like a sponge so imagine that volume full of water. Gotta add something, most likely. Mike
*Do you really mean fire treated 2X12s, or did you mean treated like wolmanixed ???
*If I did my math right (??), 2x12x12'@16" will support 180#/ft.Your dirt will weight more than that.I suggest you find a structural engineer to do the math.First, you should figure out how to get the materials in place.
*You need to consider the species of wood, grade of wood, deflection, loads psf and other factors to determine this. As recommended, an engineer is best to consult. They then assumes liability on too when stamped.Example:2x12, 16" O.C., Southern Pine, grade #1, deflection = L/360, loads 80 live/10 dead, uniform distribution of weight: Maximum span = 15ft. 11in.Same as above except grade #2: Maximum span: 14' 0".Same as last except using Hem-fir: Maximum span: 13' 1".Just some examples to show you big differences in things like grade and species. All loads were also considered uniform which is an easier fdigure. There are too many factors for us to consider sight unseen and only an engineer can affirm it for sure.Billy
*
I am building a large planter box for a penthous deck. The deck is elevated above the building roof. The joists are fire-treated exterior 2 x 12's, 16" O.C., running a 16' span. The planter will be 37"d x 28"h x 15'l running parallel with the joists. One section of the planter (appx. 3'w) will be filled full depth with soil for an ornamental tree, the rest will have a raised bottom allowing for 16" of soil. The owner is concerned about the additional weight of the planters on the deck and wanted me to put in additional joists. After a site visit for measurements, I realized that the only way I was going to get a 16' 2 x 12 to the 7th floor was with a crane. So, I need to know if the deck will support the load. I have been able to find span tables at some of the industry trade sites, but not load tables. Any ideas?