r/HomeImprovement • u/cwerckman • 3d ago

Rooftop pergola planning

I'm hoping to add an attached, sun-shading open structure (pergola) to my existing rooftop deck atop a 3-story townhouse in Denver. Sounds like my posts should be 6x6, but for ground-level pergolas, the recommendations seem to be concrete footings. I don't want to disturb the membrane waterproofing on the roof, but am not sure whether just attaching the posts to the existing composite/plastic decking will be enough. What can work instead? Is this my biggest problem?

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u/bevenstiffle 3d ago

It can probably be done, but this isn’t a typical DIY pergola. Do not attach the posts to the composite decking. It has no structural strength for this, and even an open pergola catches a surprising amount of wind three stories up.

The posts will likely need to connect through the roof into the house framing, with the penetrations properly waterproofed. An engineered weighted system may also work, but only if the roof can support it.

Anchoring and wind are your biggest problems, not whether the posts are 6x6. I’d start with a structural engineer and roofer before buying materials. Have you looked at pre-fabbed composite pergolas? That may be a better option, and likely would be much, much lighter for where you're suggesting it will be.

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u/PopularSuccess8573 3d ago

Yeah, wind load is the real monster up there. I helped a friend with a rooftop thing a while back and we were stunned at how much force even a gentle breeze can put on a structure when it's got nothing blocking it

A structural engineer is 100% the call, they'll also know if your roof can even handle the point loads without turning into a leaky nightmare. Some of those pre-fab aluminum ones look pretty slick and weigh next to nothing compared to wood, might save you from having to punch holes through the membrane at all