Hello all.
I am building a wooden pergola attached to the house. It will be covered with twin wall polycarbonate sheets. The roof will be only slightly inclined, and the whole patio is closed by house walls from two sides (thus, placed in a corner). I would like to keep as much light as possible for winter period (the patio is facing south) so the polycarbonate sheets will be clear, but temperatures in my area can reach up to 100F during the summer so I am concerned about the heat build-up on the patio in that season. I will place some roll-up shades between rafters to keep direct sun light away, but my concern about the heat beneath the roof remains.
I am thinking about leaving a ventilation gap on the top side of the roof: about 4″ wide space between house wall and top edge of the poly sheet. To keep the rain away, I would overlap this opening with another layer of poly sheets about 8″ long, about 4″ higher than the roof, and I’d use side flashing profile turned upside down – with the dripping lip up – on the top edge of lower layer of sheets (thus, the roof itself) and once again on the top layer of sheets, but this time with the dripping lip turned down.
Does that make any sense? Is it allowed at all?
If needed, I can attach a sketch…
Thank you,
Darko
Replies
Really, noone to talk me out of the idea, or give any comment?
Sorry Darko.
Attach that sketch and maybe …..
My shop has a polycarbonate roof and it sometimes gets too hot, but it has something your pergola probably won’t— 4 walls. I think it’s the greenhouse effect you are worried about but unless the space is inclosed it won’t happen. If you shade the roof your temperature should be the same as any other shaded outdoor space
Thanks Calvin and Mike for your replies.
Mike, as long as it should be the same (and not for sure), I'd like to think about possibilities upfront. Once the twin poly will be installed, there will be no easy way back in case if I will be on slow cooking mode beneath the roof...
Anyway, I am attaching a sketch - I hope clear enough. What are your thoughts?
Thank you in advance,
Darko
Do you plan to cover the walls?
Other question is if you get leaves on the roof. with a shallow slope, you may end up having to clean them out once in a while.
How big is this going to be?
Is the edge you plan to gap covered with a roof overhang and gutters?
No, aside of two house walls (to which the pergola is attached, it is placed in the corner), the other two sides are not going to be closed. The pergola covers about 500sqft. And yes, a bit higher than the "gapped edge" there is an overhanging roof with gutters.
I am aware of cleaning that I'll have to do, I'll live with it...
With the overhang above, why not just leave a gap? You could still put the trim at the top of the low slope roof to discourage driving rain to move away from the house. (with the two open sides, driving rain will still get things wet)
But with the two open walls, and low slope roof, it is not clear to me that you would get a significant air flow escaping along the wall. Air moves in response to force, and while hot air is less dense which allows cooler air propelled by gravity to push it out of the way, it is not clear to me that you will see a significant benefit from this gap.
You might want to consider incorporating that overhang in your thinking, and make the pergola a little higher, and the gap wider.
Uncle Mike,
Thank you for your response. If I am getting it right, you are actually not objecting the idea ;)
Unfortunately, the heights of both roofs are fixed, consoles for rafters were placed while pouring concrete in house building stage. Space between roofs is about 4 feet, which feels a bit too much to keep the gap sheltered.
The gap should be wider?
Thank you,
Darko
Just thinking out loud.
With two open walls, you really are not providing for a weather-protected space, so that everything under it needs to be chosen to endure weather exposure, including doors and windows to the house.
I find myself wondering about all the flashing this will take, and how the lexan will make it all visible.
Calling this a pergola led me to think of a more self-supporting structure, independent of the house, rather than a porch roof, integrated to the structure and protected from the weather.
Semantics should not deter you from building what you want.
I live in dry climate so I have not noticed any problems, but you can get condensation inside the polycarbonate. Installation instructions should cover this.
Thank you all for the comments.
“[Deleted]”
I want to build something similar at my house too, and not sure about the size of the ventilation gap. I’ve heard that it might be a nice idea because it helps reduce the dampness and mold on the roof. But I’m not sure that 4 inches are enough.
I going to call https://carportaustralia.com.au/patio-roof/ and ask for their advice, but it would be nice to see some opinions online. This thread is helpful, but I am still unsure about the size of the ventillation hole.