Well – first of all installing aluminum roof as 3rd layer is against the building code in most states. I actually don’t know of any states that allow 3 layers of roofing.

Second – you would be installing a lifetime roof over a substrate in unknown condition – there could be much rot which you cannot see even if your attic is not finished – along the eaves, at ridge line, etc.

Third – the fastening would be not as good, because even the longer 2″ nails will have to go through 2 layers of shingles, which is about an inch, and then the substrate itself, and also nails need to penetrate substrate by at least 3/4″ – this will not be achieved with 2 layers of shingles.

Basically you should definitely remove the shingles, and fix any wood rot, before installing the roof.

Leo.

The still roof is not affected by cathedral’s ceiling lack of ventilation at all. What is affected is your roof deck (sheeting) – no ventilation can cause dry rot, especially if you use ice barrier over entire roof. I recommend using breathable underlayment so the roof substrate could be vented, albeit very mildly.

Optionally you can install a “cold roof” which is essentially a second roof substrate with spacers between it and original substrate. You would install the 2×2 “rafters” running vertically, and then a layer of plywood on top of them. This will create vented space, and you cold then use soffit and ridge vents.

As for your furnace – it needs to be vented somehow – using a through roof or through wall vent. I mean – you do not want furnace fumes in the attic.

Hope this helps,

Leo.

Thanks
Erich

I’m a little confused by your question. Do you mean installing metal roof over asphalt shingles or vise-verse? If metal roof over shingles, assuming roof deck is in decent condition and it’s only one layer of shingles, than you basically need to install underlayment + trim and then the roof. Just like there are no shingles. You would have to cut off the drip edge of the shingle roof, so metal roof trim lays nice and flat to the fascia board.

If you mean something else, please elaborate in more details.

PS – you cannot / should not install shingles over metal roof.

Leo

Considering that the top row of shingles is the same as the rest of the roof, shouldn’t caulk be applied at every row of shingles?

I know this system is far from waterproof, but I’m downright afraid of relying on caulk and underlay.

I’m really torn between choosing a system like this and standing seam.

This is something I cannot give you a definitive answer to. The reasons are many, but for me personally it’s:

A) a roofer in me saying that for best roof performance you are better off removing the shingles and fixing the roof deck if there is any rot damage.

B) a green-minded person saying that if your roof deck is in good shape, you can safely leave the shingles on the roof (only one layer though), and install metal over the top. This will prevent unnecessary non-biodegradable trash going into landfills and your roofing cost will be lower.

If you decide to leave shingles on, few things should be done:

1) all overhangs (drip edge with shingles) must be cut back to the rake / fascia board (your roofer should know what to do).

2) Make sure wooden substrate along the eaves / gables is not rotten. You could remove just a couple of rows of shingles along roof edge to fix any rotten wood.

3) install synthetic underlayment over the shingles. Felt will dry out and not serve it’s purpose in a mere five years. No ice and water should be used if you installing over shingles.

You still need to pry siding away from the walls to insert sidewall flashing. Also you will need special flashing around the chimneys / skylights, and probably will want to replace lead counter-flashing with color matching metal counter-flashing.

Good luck.