Is my plan for a plywood roof deck ok?
I have a newly installed rolled tar roof, and with summer here I’d love to use the roof a bit. I was thinking of a nice simple deck: 2 or 3 sheets of plywood to distribute the weight, and a picnic table on top of it. And I was thinking of adding pressure treated 2×4’s mounted the “flat” (wide) way to the underside of the plywood to let it vent.
Planning to prime and paint the plywood, 2×4’s and picnic table with outdoor deck paint.
Any thoughts about the plan? Is that likely to be fairly low impact on the roofing material? Or is there another way you can think of adding a deck that would be lower impact? That’s the primary concern.
Any thoughts for what paint and primer I should use? And what material and thickness of plywood?
Thanks for any help.
Replies
Not a good idea. How will you anchor this material to the underlying roof structure? Any anchorage will penetrate the water barrier provided by your roofing. There are ways to construct a deck over a low-slope roof but it gets quite complicated when it comes to managing water. Search the FHB archives for an article on exactly this matter.
I guess this is more of a base for a table than a regular deck. We're not anchoring it to the roof, it just sits on top of it. That has the advantage of being able to move it around a bit, since the deck consists of two 4x8 sheets of plywood, each separate (not nailed to each other, but connected temporarily while in use, but easily separated when moved).
As far as water drainage, the roof has a clear slope towards the drain, and we're planning on mounting the 2x4 supports such that water will flow to the drains.
Do what you will I suppose. I would agree that the roof membrane needs protection from the concentrated load imposed by the table legs. The real concern is with the potential for penetration(s) resulting from foot traffic (e.g. pointy heels), a dropped utensil (e.g. a carving knife perhaps), etc. This becomes a risk as you begin to enjoy the space and lose sight of the potential hazards.
Table legs directly on the tar are definitely a problem (hence the platform to distribute the load), but this roof is plenty tough for everything else you mention (and gets all that whether I put a table here or not, people come up here all the time).
The story so far, see attached pic. A surprisingly shoddy Lowes picnic table with lumber that looks like it was cut by a gorilla, and the two plywood planks that will make the deck being painted. Giving them lots of coats.
I can't decide whether to mount the 2x4's under it vertically (the normal way for a deck) or horizontally. I'm thinking horizontally will be easier on the roof since there's more surface area and there's a bit of flex. But horizontally might lead to warpage...
I am not an expert in roofing, so I will defer to others on here on damage to the roofing itself, but wanted to jump in and say that your underlying roof structure was likely not designed to handle much of a live load. Depending on your local snow load it may be designed for construction live load only (20 psf), or maybe even less. This may be ok for a couple of residents hanging out from time to time, but I would not be hosting any gatherings up there..