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randlefactor 12-01-2012 12:02 PM

Asphalt Shingles over metal roof - See Pics
2 Attachment(s)
I was inspecting the drip edge on my roof when I noticed that under the asphalt shingles is plywood on top of a ridged metal roof on top of plywood. Is it normal practice for a roofer to do this type of install?
Will this effect the install of a ridge vent? I'm trying to cool down the roof during summer and prevent ice dams in winter.

House Info: Upstate NY. Colonial-ish style. 1830 single family 2000 sq ft. 1.75 story (upstairs bedrooms have partial sloped ceilings). Sloped roof. Converted porch (cold!).

mae-ling 12-01-2012 12:06 PM

is this a normal house?

randlefactor 12-01-2012 12:11 PM

.see above details

Gymschu 12-01-2012 12:18 PM

Not normal where I'm from. Sounds like a former HO or a contractor got lazy and didn't remove the corrugated metal roof. I would suppose that the metal does provide a certain amount of airflow under the plywood and shingles, which isn't a bad thing, but, it is unusual. If you're planning on a new roof, now would be the time to have it done RIGHT. I would think putting in a ridge vent with the way the roof is now, would be a B**** cutting through shingles, plywood, metal, and more plywood.

randlefactor 12-01-2012 12:26 PM

Ya that's what I was afraid of. The roof was just replaced 2 years ago (we moved in last spring), so we won't be getting a new one any time soon. If I can get a ridge vent installed and somehow maintain the gap that the metal roof provides, maybe I'll end up with super-ventilation by having a thin layer of air drafting from the roof edges up and out of the ridge :)

randlefactor 12-01-2012 12:32 PM

...error post by mistake

joecaption 12-01-2012 02:16 PM

The first row of shingles was also installed wrong.
Not enough over hang.

mae-ling 12-01-2012 04:30 PM

Joe - I agree, although that is how some do it.

joecaption 12-01-2012 04:40 PM

Why? Sure way to defeat the whole reason for the drip edging.

randlefactor 12-01-2012 07:51 PM

The thing that bothers me about this double layer roof is that the drip edge is not tall enough to cover the gap. Also to your point about the short shingles, yep, the water does find its way to the underside of the shingle and not take full advantage of the drip edge because there is no overhang.

mae-ling 12-01-2012 11:11 PM

sure - but then it just runs down the metal roof!!!!!!! lol

carpdad 12-02-2012 09:18 AM

From the photo, it looks like you have a shed roof (flat plane from main wall to porch outer wall). Ridge vent does on top where two roofs meet in angle.
First, and I ask this time to time, do you have ice dam problems? One sign may be icicles hanging from the gutter. Research ridge venting before making changes to your roof. You will have to close that gable vent, for one thing, although it also looks like a small window.
For your metal roof porch, if it is enclosed, is it all season living space? You mention ventilation through the metal roofing, but I don't see the outlet for that air to exit. You can put a specialty vent strip along the top of the roof, but strip of metal roof has to be cut and these vents are costly.
That porch sounds like it needs some things upgraded, especially insulation. I would not worry about ventilating that shed roof for now until you decide how you are going to use the porch.
I assume the porch does not have a gutter. If you put a gutter there, I would recommend removing the metal roof, put down ice dam shield and reroofing with number of box vents between the rafters with soffit or reframed facia vent.
You have 2 layers of roofings and plywood decks. That is about maximum the rafters should take.

randlefactor 12-02-2012 10:52 AM

1 Attachment(s)

Thanks for your intuitive feedback. Here's a picture to better explain the situation. The red arrows show where ice damming occurred during the first tiny snow of the year (3" maybe). The room closest to the cars is an enclosed finished porch with a vaulted ceiling so I doubt there's much insulation. It's always colder in there than the rest of the house. I'm guessing that's due to the old windows, insulation, etc... Even though it has one heating forced air duct and is open to the rest of the house.

randlefactor, Upstate NY

carpdad 12-04-2012 10:23 PM

That little snow suggests that there is a roofing problem than ice damming. The fact that roof was cut to the drip edge where drips of rain could get sucked back under the roof, over the drip edge and into the house (capillary action).
---temporary solution: recreate overhang by slipping roofing or narrow flashing under the first course and secure with caulk and some nail.
---better solution: remove enough roofing to lay down ice shield.
---best: combination of ice shield, soffit vent and ridge vent, air barrier insulation in the attic.
Fast solution is to install heat cable into the gutter.

The porch cathedral ceiling probably does not have enough insulation and vent space. I would use the heat cable for now, extend the roof over hang, sheet metal cover for facia, but for the future, I would reframe/add to the rafters so that you have vent space and approaching R30, or more. Consider getting rid of cathedral ceiling and putting in a flat ceiling for much better insulation and venting. Fiberglass insulation loses insulation value when air flows into it. When creating vent space, separate the vent space and insulation with air block, plywood or foam panel. Cellulose is better air barrier, but I can't recommend it along the facia because water damage is greater with cellulose.

jagans 12-06-2012 04:01 PM

No Air flow
1. It is never a good idea to sandwich materials together that have vastly different rates of expansion and contraction under thermal load. The plywood will rack, and nails will back out.

2. For ventilation you need an intake and an exhaust. You have neither. The metal roof should have been removed, the deck inspected and repaired or replaced, then the new roof should have been installed. It was actually easier to tear off the metal roof than to do what they did, and now you have two plywood roof decks, a metal roof, and a shingle roof as dead load on your structure.

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