How much weight/ How many people can my rooftop deck hold?

  • The joists are 2"x12" over 20ft, 16" on center
  • The framing under the deck is typical 2"x4" framing (living space), on a concrete foundation

The deck was built in 1980 up to code. I went through the permits, but was not able to find a weight rating in there. Everything was approved & inspected.

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    Hello, and welcome to Home Improvement. We aren't likely to be able to give you a precise answer; you'll have to consult an engineer. And, you should probably take our tour so you'll know how better to participate here. – Daniel Griscom Jun 17 '19 at 4:57

The allowable load on the deck depends on the species and grade of your 2x12’s.

You said the deck was “approved and inspected” and it met “Code”. Therefore, I’m assuming the joists at 16” o.c. are Hem-Fir No. 1 grade or better. In addition, I’m assuming you have at least 1 1/2” bearing at each end and there is solid blocking (or adequate joist hangers to hold the 2x12’s in line) in order to meet Code. Because you called it a deck, I’m assuming it’s outside and is pressure treated (incised). (Incising reduces the strength.)

Joists spanning 20’ at 16” o.c. can support a “Live Load” (people and furniture, etc.) of 30 lbs. per square foot plus a “Dead Load” (framing, decking, etc.) of 10 lbs. per square foot.

30 lbs. per square foot is slightly below Code. (Code is 40 psf). If you use Grade: Select Structural, it will meet the 40 psf.

You didn’t give the size of the deck, but you can multiply the area by 30 psf to give you the maximum load your deck will support.

The Code does not stipulate the average weight of individuals, but you might use 180 lbs. as an average to determine number of people the deck could support.

The code allows a few increases, such as “short duration loading” and “impact loading”. I suppose both increases could be taken, but I don’t know enough about your deck to use them...so let’s just use that as a “safety factor” in case you start loading your deck up with kegs too.

If you’re thinking of loading your deck to the maximum, you’ll also want to calculate the required size of the footings, required size of joist’s supports (beams), posts fixes, required diagonal bracing for stability when they start singing “for he’s a jolly good fellow” at your birthday party...but that wasn’t your question. Party on...

  • Thank you! I believe the framing to be regular 2x4s (living quarters under it): there is no pressure treated wood there. Ok, so assuming the deck is 500sqft, 30lbs/sqft would allow for 500*30/180=83 people. Wow, that's a lot! I don't think I can pack more than 40 people there. with chairs and tables, we would be on top of each other. – topoto Jun 18 '19 at 5:16

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