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I want to put a pergola on my flat roof but am not sure if it's possible to construct one safely. My roof is a wide open flat surface built to be enjoyed as a rooftop deck with tufflex deck layer. I want to attach the posts directly to the roof (that seems the most secure) but I don't think that is possible due to compromising the integrity of the roofing layer (I assume it would require redoing the entire roof to create brackets that would be more properly sealed from leaking). On the other hand I'm not sure how to calculate if concrete blocks are enough to prevent it from flying away which would be catastrophic as it would fall 3 stories. Unfortunately there is no structure I can attach it to other than the ~4ft walls around the roof that are capped with a metal sheet.

Has anyone done this or do you have ideas on if it can be done safely? I was originally planning to just attach the pergola to concrete blocks but I'm worried about gusts of wind and have no insight on if this is safe or what amount of weight is needed to secure it. For the pergola frame I plan to use bamboo due to its light weight and strength: my idea is that the center of mass would be much lower using bamboo vs a traditional hardwood or even an aluminum frame. It would also put less load on the roof itself, but I sacrifice potential stability as cedar pergolas are 5x the weight I believe.

The roof looks like this: roof example

In summary, is it possible to construct a pergola on a flat roof safely given my constraints (roofing material, no obvious part of the house to attach it to, etc)?

Edit: The roofing material is actually tufflex

Edit2: Added a more representative picture due to commentary

  • Well that one is just an example of the general structure (to show there is almost nothing to connect to). There is a stairway to the roof and its built to hold furniture. These roofs are pretty common for new construction in the pacific northwest and people often grill and have parties on them. Most commonly you will see the standard table/umbrella combo though some people have put up posts for hanging lights. I have even seen a rooftop with multiple heat lamps on it which I thought for sure would topple over in the wind. – James May 21 '18 at 23:04
  • Hello, and welcome to Stack Exchange. Imagine your pergola being blown off the roof and falling three stories onto something/someone, and know that you probably can't be sure it would never happen. – Daniel Griscom May 22 '18 at 0:55
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Yes, I’ve done this, but it was designed into the roof. That is to say, we tied into roof beams with round steel pipes. The pipes were bolted into the roof beams and extended up through the roof about 12” or so. We caped the pipes so they didn’t leak and then used “plumbing jacks” (like ones made for plumbing vents,) to keep the roof from leaking. We then bolted cross beams to the pipes for the gazebo floor and kept the floor off the roof about 6” or so for drainage and cleaning. We then installed posts and a roof. (The structural system used bolts not just nails.)

Also, the roof beams were designed to hold the gazebo up, but it was also designed to hold it down (I live in a high wind area.) That means the beams need to be secured to posts and the posts need to be secured to the foundation adequately.

If you use your concrete block idea, be sure that the roof is designed to support that much additional weight. Remember, roofs are usually designed to just support snow loads not live loads with all your drunken friends celebrating your 30th birthday.

Wind factor increases the higher you go up. The effect of putting your gazebo on a three story structure means you’ll have significantly higher effects (lateral loads and uplift) of wind than one sitting on the ground.

BTW, if you’re creating a roof top gathering place, you’d better install a guardrail to keep your friends from wandering over to the edge and falling off. (Blood is difficult to get out of sidewalks.)

  • The roof is designed as a deck so the walls are certified by the city as guardrails and it is designed to hold more than just water weight (snow/pooling) as it is meant for supporting reasonable live loads. The wind factor is what I'm most worried about; how do I ballpark the weight needed to secure a pergola frame on a 3 story structure? What kind of wind gusts do you get in your area? I have a hard time teasing out the calculations from documents like this: awc.org/pdf/education/std/…. I think the top of the pergola would be at about 40 ft. – James May 22 '18 at 18:59
  • @James At that height, we’d use 35 lbs. per square foot of wind uplift here. That’s a lot. If your gazebo is 12’ x 12’, then that is about 5,000 lbs. of uplift. If concrete blocks weigh about 28 lbs. each, it would take about 180 blocks, minus weight of lumber. That would crush your roof. Plus, the gazebo needs to be designed for lateral wind load too. – Lee Sam May 22 '18 at 19:20
  • thanks for the 35 psf calculation. My project will not be fully covered. It will look like this: bambooimport.com/image/Bouwpakketen/…. The square footage of the actual roof elements would be closer to ~10-14sqft and the wood is circular so the wind vector would not apply the entire force to that surface area; which would make 14sqft conservative. For this design would it be accurate to calculate the weight based off that sqft? This would put the necessary support weight at a more manageable 500 lbs. How do I address lateral wind load? – James May 23 '18 at 18:29
  • @James If it’s made like your picture, I don’t see any resistance to wind you’d have to worry about. The weight of the structure should be adequate to resistance any lateral force due to wind. You may need to worry about seat cushions flying away more than the pergola. Are you in a high seismic zone? – Lee Sam May 23 '18 at 19:02
  • I'm not sure if it's considered a high seismic zone (earthquakes are rare but significant), but I am in the Seattle area which definitely carries the risk of a potentially large (7.0 for the Seattle fault line) earthquake; larger for the fault line out in the ocean. I'm far enough away that the maps I've read consider my risk "fair" instead of moderate to severe. – James May 23 '18 at 21:52

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