Metal roofing over aging shingles - Correct way to install?

[skipmarcy]

I spent 15 years in construction management and never would I approve putting a roof over a roof...rather loose the job then do it wrong. Shingles curl from drying out, which means they dry out if they are covered or not... on your roof or in the back of your truck, they will dry out and curl over time from heat cycles. Metal over an existing roof will also breed moisture/mold/rot...your pitch is not that bad, get yourself a gator shovel and park your bucket in the right spot, with a buddy to run buckets and a few family members to rake the strays you should be able to strip it in an afternoon. Just keep several tarps handy in case the weather acts up.

Bubble wrap and an existing roof is not R-30

Spring for stainless fasteners.

I don't know what R value the shingles would be but I know the double foil bubble wrap is only about R1 value - I did my whole house in it when I built 5 years ago and researched it some before buying. What major difference the bubble wrap does is it reflects radiant heat - supposedly about 95% of it. It sure works at my place. My porches on the house have the bubble wrap under the metal and my carport on the shed doesn't - huge temp difference when the sun is shining. I agree with those that don't recommend putting the metal over the old shingles, I would spend the extra effort to remove them.

 

[nwltruck]

I would strip the old, put down felt,foam or bubble insulation then tin. The one house I have been in that used perlins the owner said when it rains it sounds like a tin shed. With a gap under the tin you will also get more hail damage ......
I've had a tin roof for 8 years. Sheeting felt tin is how I did it and I love it. I've had to replace two sheets of tin due to remodeling and the tin looked new when removed.

 

[JB4310]

Beginning several years ago, the "metal over shingles" craze took on locally. In recent years, I have seen many of these houses having to be re-re-roofed because the metal roofing is rusting from underneath.

In most of the installations that I saw, the installers simply placed the metal over the shingles (no strips, no felt, no nothing) and screwed it in.

You would at least need the felt, the metal will move (expand and contract) considerably, being right against the abrasive surface of the granular shingles would be like taking a grinder to it and would cause it to fail prematurely.

I spent 15 years in construction management and never would I approve putting a roof over a roof...rather loose the job then do it wrong. Shingles curl from drying out, which means they dry out if they

Installing a roof over an existing roof is an industry standard, and acceptably practiced everyday, everywhere. Unless forbidden by code like with architectural shingles, due to the extra weight and uneven surface.

Almost every commercial re-roof job today is going over existing. Less and less residential today due to the architectural shingle craze. But the old 3 tab shingles were "laid over" for the last 100 years with no problem.
Most towns allow 2 layers, the next town over from me allowed 3, that gets a little crazy with the 2-1/2" roof nails.

Not sure what to recommend to OP with that FLA sun issue, but I might consider leaving the shingles on, would help silence the roof and would have to help a little with the R-value. But I would research locally what happens under the metal down there. Unlike laying shingle over shingle where the weight holds the old shingles down, with the metal, especially when using the furring strips there is space under it to allow the shingles to move/curl, not sure how that would affect the metal.

JB

 

[johnrex62]

Have any of you with metal roof's checked with your local fire dept and asked there opinions of metal roofing materials? I know I am thinking about the worst but if there were a fire the roofing material can make several differences. Just a thought.

A metal roof will help since cinders or ash falling on it will have nothing to ignite. Otherwise it is not much in the way of fireproofing.

I was educated recently when I saw the writeup of one of the homes in the Bastrop Complex fire that was destroyed. It was metal roof and metal frame specifically to increase it's fire resistance. It was a melted mess afterward and completely gutted by the fire. It was pointed out that fire entered the home through the soffit vents and ignited the attic and consumed the interior of the home.

I don't know that any metal roofing product will have an advantage over another in the fire resistance department. If the exterior heat get high enough to ignite decking under the metal the gauge will not make more than a few seconds difference and all will prevent direct ignition from the spark or cinders.

 

[rkeith]

I'm a firefighter by trade...
if you install a roof over ANY existing roof you are begging for trouble should your house, barn, shed, etc. catch fire. The "layering" of the roofs will trap the heat in your house, barn, shed, etc.

From a construction perspective...
You are adding more weight to your roof and if you add leaves, snow, etc then your roof could collapse due to the weight. I would always remove the old roofing to avoid future problems; if nothing else to inspect the roof decking for problems.

 

[MMagis]

I'm a firefighter by trade...
if you install a roof over ANY existing roof you are begging for trouble should your house, barn, shed, etc. catch fire. The "layering" of the roofs will trap the heat in your house, barn, shed, etc.

Am I missing something? I'd say the fire is a bigger issue than trapping heat.

 

[Robert_in_NY]

Am I missing something? I'd say the fire is a bigger issue than trapping heat.

One problem I am aware of is if the asphalt shingles start burning under the metal roof you can't get water on it. The fire will keep burning/spreading under the metal protected from the water spray. The local firemen do not like metal roofs as they can't get on them easily and safely so they have to change how they approach a fire when it has a metal roof.

 

[clemsonfor]

Am I missing something? I'd say the fire is a bigger issue than trapping heat.

Im thinking the same think! Or either that or the smoke damage or the 3000 gallons of water that they use to put it out!

 

[rkeith]

One problem I am aware of is if the asphalt shingles start burning under the metal roof you can't get water on it. The fire will keep burning/spreading under the metal protected from the water spray. The local firemen do not like metal roofs as they can't get on them easily and safely so they have to change how they approach a fire when it has a metal roof.

Thank you! That was my point along with cutting a hole in the roof to relieve the hot, trapped gasses. It's very hard to cut through a metal and shingle roof without causing major damage to the roof. When we cut a roof we cut between the rafters so it's only minor damage. With a double roof we can't find the rafters and if one is cut... opps! We had a house fire a few years ago and the insurance company wouldn't cover the roof damage because we cut a rafter due to a double roof. It was just an attic fire. Insurance company said it didn't meet code and our damage was a result of the double roof.

Just hire some illegals to remove the old roof... everybody else does it!

 

[clemsonfor]

Thank you! That was my point along with cutting a hole in the roof to relieve the hot, trapped gasses. It's very hard to cut through a metal and shingle roof without causing major damage to the roof. When we cut a roof we cut between the rafters so it's only minor damage. With a double roof we can't find the rafters and if one is cut... opps! We had a house fire a few years ago and the insurance company wouldn't cover the roof damage because we cut a rafter due to a double roof. It was just an attic fire. Insurance company said it didn't meet code and our damage was a result of the double roof.

Just hire some illegals to remove the old roof... everybody else does it!

BUT...like was said earlier, codee allows in most places up to 2 layers. Not sure how they treat fiberglass/asphault shingles and tin? But if its legal that seems like a court case if they wont pay.

Just curious, when your on my roof about to cut a hole in it to get to the fire, how the heck do you know where my joists are till you cut them? If you plunge the saw into my roof on a joist at first you wont know till you move the saw to the side in the cut and see it?

I have a 1x6 and maybe some 1x8s in there as well. Its solid on the whole thing, unlike these new OSB sheated roofs.