How to Vent a Flat Roof Properly
LESLIE: Now we’ve got Ken on the line who’s dealing with a very moist situation in the attic. Tell us what’s going on.
KEN: OK, yeah. Say, we live – Dorothy and I – about three blocks from the Pacific Ocean, next to the Columbia River. I purchased a house here with a flat roof. Basically, it has about a 3/12 pitch. It had a torch-down system on it and I opted to go – an IAB Teflon system.
KEN: And it was about a $10,000 system. Well, after they put it in, about a year later I noticed mold on the underside of the roof. And they put three vents – about 8-inch-diameter vents – in the ridge. And when I saw the mold, well, they said, “Well, you’re not getting adequate ventilation in the (inaudible at 0:10:32).” Well, anyway, they put six more vents in there and they had messed up and left about a half-a-dozen little holes where they had bad leaks. And so I had water in between the IAB system and the torch-down and my plywood.
KEN: So I put the fan in there and my question now is – and putting those additional six vents on the bottom.
TOM: Are you talking about – when you say “the bottom,” are you talking about the underside of the overhang at the soffit level?
KEN: Well, no. I don’t have any overhang.
TOM: You don’t have any of the – OK.
KEN: It’s a flat roof and it just comes to the walls. And so, after they put the six – three on the – or four on the top ridge about – I had mentioned to them we should put some vents down low and they said, “No, you don’t need vents up here with that little” – but probably wouldn’t have had to have but they were incompetent.
KEN: And they did because they left about a half-a-dozen holes and leaks in that Teflon.
TOM: Alright. So here’s the situation. So you had a minimal amount of vent. You spotted some mold, you added additional vents and now you’re – are you still seeing mold in the attic or not?
KEN: No, I think – they came in and they wiped it down with whatever.
TOM: So you’re not seeing the mold anymore in the attic. And the question is do – is it possible to have too much ventilation? The answer is no. In fact, a perfectly ventilated attic space is going to be at ambient temperature all the time.
Now, because it’s a flat roof, it’s much more difficult to vent. If this was a pitched roof with an overhang, you would have soffit vents down across the soffit, ridge vents across the peak. It would essentially be wide open all the time, constantly circulating air. And what that does is in the wintertime, it takes out moisture, which can condense and lead to mold. In the summer, it takes out heat which, of course, drives up your cooling costs. So I don’t think it’s possible to have too much attic ventilation.
Did you also mention that you put a fan in there?
KEN: Well, I put a fan in there to dry out the moisture first and that’s what my concern was. Maybe I shouldn’t have put the fan, because I’m spreading those mold spores around by doing that.
TOM: Well, if the mold was treated, I wouldn’t worry too much about that. But here’s the thing about fans. Now, the fans – the attic fans – are only going to work on a heat-sensitive switch unless you wired them somehow differently. And so those fans – those attic fans – typically only work in the summertime; they don’t work in the wintertime.
KEN: It was in the fall, so it was relatively cool.
TOM: Right. But there’s – but those attic fans work on a thermostat, which is generally, if it’s installed correctly, set at around 100 degrees. So it would have to be an awfully warm, fall day for that to kick on. I would say that if you’re not spotting the mold any further and the attic doesn’t seem to be leaking, you addressed all those issues, that you’re just good the way it is and I would just stop right there and enjoy it.
KEN: Right, right. Well, this is about the eighth house – we retired in the last four years. It’s kind of like watching gold rush here; you’re always going to find something in an old house. So, we enjoy it but it’s a lot of work.
TOM: Alright, Ken. Well, thanks so much for calling us at 888-MONEY-PIT.