Cause of Black Mold on a Roof
LESLIE: Larry in Connecticut is dealing with mold. What’s going on at your house?
LARRY: I put a new roof on my house. My old roof had – looked like black mold on it.
LARRY: It’s black. I don’t know. I don’t know how bad it is, but anyway they took that roof off and put a whole new roof on; new plywood and everything. And eventually that black started coming back on …
LARRY: … and I’m wondering – I have the exhaust fan from my bathroom running up there and I told one of the guys who was redoing my bathroom to run it to the eaves and run that, you know, the exhaust air out but they didn’t do it. So my exhaust fan is going right into my attic.
TOM: If the bathroom exhaust fan is discharging into the attic, that could possibly be contributing to this. The other things to do is look at your attic ventilation system. The best ventilation system is a combination ridge and soffit system where the air is intaking at the overhang of the roof – the soffits – and then it’s exhausting at the ridge. If you don’t have that type of system, that could be part of your problem right here. How old is your house?
LARRY: It’s about – well, 1973; three, four years.
TOM: OK. A 1973 house sometimes has soffit vents but there are not always enough. And sometimes in a 1973 house you will see perforated soffit material that was installed over solid plywood soffits. So you need to be absolutely certain that your soffits are wide open; that you don’t have insulation that’s pressing down into it. 1970 houses are famous for this problem; especially when it comes to delamination of the roof sheathing. You mentioned that you replaced the roof and some plywood at the same time. I’m not surprised. And the reason that would happen is because the ventilation is not working correctly. So you need to look very carefully at that soffit.
You know, a quick way to do this – do you have like a vinyl soffit material or aluminum soffit material at the overhang?
LARRY: They’re vinylish; vinyl.
TOM: Alright. So here’s a little trick of the trade. Go around that vinyl soffit material and take your hand and push it up. I bet you you’re going to feel plywood on the other side of it because that would have been the original soffit and a lot of times when they side these homes they put perforated vinyl soffits on top of solid plywood soffits which, of course, is useless. You’ve got to have good air infiltration because if you don’t you’re going to get condensation on the underside of the plywood sheathing and if you get condensation on the sheathing you will get mold and delamination.
LARRY: Good. How about putting one of those vents in the roof, you know, to suck the air up; you know, those roof vents?
TOM: Useless. Useless.
TOM: Yes. Continuous ridge and soffit vent is the way to go. It works much better than a roof vent. It works better than an attic fan because it works 24/7/365 and if it’s installed properly, as the wind blows against the side of your house the air enters the soffit; it rides up underneath the roof sheathing where it takes out moisture in the winter and heat in the summer and it exhausts at the ridge vent, which is always depressurized because it’s at the peak of the roof. So that’s the system that’s the most effective way of flushing your attic space.
LARRY: How about the existing mold that’s in there now? Should I just wait …
TOM: If you dry it out it’ll go dormant; I wouldn’t worry too much about it. It’s not a space that you’re interacting with.
LARRY: Should I vent out my exhaust fan? Should I vent that out (inaudible)?
TOM: Absolutely. Yeah, you shouldn’t have any bathroom or kitchen exhaust fans dumping into the attic. They should go all the way to the outside.