How many and how heavy of container plants can a rooftop deck handle?

Question

We have recently had a rooftop deck constructed on the third floor of our MN home, and since we have virtually no yard, are considering using some of the space to "go green," while still leaving plenty of room for entertaining.

The deck is 8x24, made of cedar, by a reputable contractor. It sits on the third floor on a nearly flat section of roof, above the enclosed living space on the 2nd floor and an open porch on the 1st floor. We have several lightweight pots we use for tomatoes, other vegetables and flowers outside. When filled with potting soil and plant material they weigh 25-30 pounds each. At about 15" in diameter we could put a row of about 15 of these on the deck with still ample space for sitting/entertaining -- or up to 3 such rows, if in the future we wanted to grow more of our own vegetables.

As what point would the weight of the plants potentially become a problem for the structural integrity of the deck and/or roof?

Answer 1

While I realize that it's disappointing not to have every question easily answered by the internet for free, there is no way to correctly and reliably answer this question than to consult with (yes, pay money $$$ to) a civil/structural engineer, preferably one licensed to practice in your local jurisdiction (State, since you are in the USA.)

For 5 different houses, built differently, there will be 5 different answers. There is no "General" answer, and the stakes are at least your house, if not the lives of your family if you get it wrong and collapse the place while you are all asleep, etc. It is NOT a good place to "guess."

Joining a community garden near your house might make more sense as a way to get garden space.

Answer 2

If the deck is supported by the roof, you cannot place any pots there at all. The roof is typically structured to hold whatever may lay on it in a seasonal fashion, (live load) snow comes to mind here, and the weight of the materials that make the roof itself (dead load).

Now there is a full time load added (the deck) to the roof that takes away from the load that the roof is designed to carry.

If the deck is supported by the exterior walls, then it is up to how the deck is built, and that will be up to the builder to tell you. Decks by code when built, not on a roof are supposed to support 60 lbs/ sq. ft. With your deck, one side may be, over a bearing wall, but there is no way to tell even with pictures. It also depends if the builder used the proper sized framing to handle that....