New metal roofing over asphalt shingles. Good or bad ?

[vidio1]

How many layers of shingles adds a wrinkle too.....

Metal over shingles is the norm here too with just a single roof. If it's been roofed before then it needs a tear-off. Btw there's a strip of material that goes between the metal and shingles kinda like a thin purlin that allows an air space to block heat/mildew problems.

 

[MESSMAKER]

Got two layers of shingles under my metal roof. It has been 10 years and it looks just fine.

 

[EddieWalker]

Purlins are a must. Attaching them to the rafters is a must. I wouldn't use nails. Depending on how thick the shingles are, I would probably use 4 inch screws with 2 x 4's on the flat every four feet.

 

[Gary Fowler]

So it looks like you have about 70% for leaving the shingles on vs 30% for tearing them off.
I like the idea of putting furring strips across the roof to attach the metal too so that you have an air gap under for good ventilation, however I don't claim to have any knowledge of roof installation techniques. All my homes have had asphalt shingles on them and never over 1 layer. My shop has metal roof but applied on top of fiberboard insulation and OSB. It is only about 8 years old so no info on how long it will last, likely my lifetime for sure.

 

[jajiu]

I just had a metal roof put on my barn. The company said with only one layer of shingle there was no need to remove them. My concern was not the weight, but the moss on the roof. They put a material down first like tar-paper, but more like black Tyvek then roofed over it. They said the moss would die without water and air and turn to dust. Roof looks great and at my age, I'll never need to re-roof again.

 

[BoylermanCT]

Purlins are a must. Attaching them to the rafters is a must. I wouldn't use nails. Depending on how thick the shingles are, I would probably use 4 inch screws with 2 x 4's on the flat every four feet.

Ditto this. My FIL and I put a metal roof on his garage and equipment storage shed over shingles. We ran horizontal 2x4's every 2 feet and screwed them into the roof joists and then screwed the metal roofing to the 2x4's. Roof still looks new after a few years.

 

[the old grind]

Last horizontal purlins I saw were used in a new build, under decking for a membrane roof. The slope was minimal and insulation filled a lot of airspace. I had advised adding forced ventilation controlled by thermo- and humidi-stats but the (ahem) architect who designed and built assure me that none such was needed.

Not sure if we got out all of the black mold, but the following Spring we cut out the decking and added vertical purlins to allow venting from eaves to parapets, then re-decked and had the pros reinstall the membrane. (btw, the house has radiant/floor heat.)

We're hearing from different climates, and not all uses are over heated spaces, so IMO there is no easy answer to cover all circumstances. (I still say that o'all size and slope can matter) As a 'preventative', I'd wait till the old shingles leaked before I covered over them. As was said, ... tearing off is to inspect the deck, if for nothing else, so I always did that. :2cents:

 

[Sawyer Rob]

Around here, folks go right over a SINGLE layer of shingles...

We have a few Amish builders who do nothing but steel, they even have the machine to make their own panels from rolls of steel. THEY say, it's just fine to go over a single layer, just as long as you put down horizontal purlins and that's the ONLY way they do it. They prefer 1" thick x 4" wide fresh cut aspen/poplar, because as it dries, it holds the screws even tighter. Pine is their second choice and NO they don't have a sawmill trying to sell their own milled purlins.

They did my brothers house a few years ago, and it's working out just fine. My brother put down the purlins and hired them to do all the steel, they did a GREAT job...

I plan to do one of my buildings the same way...

SR

 

[Budweiser John]

I personally think conventional 26ga metal roofing installed with washer head screws or nails is grosley over rated. Virtually 100% of all the fasteners are exposed to the weather and over time the sun will deteriorate the gaskets and leaks will happen.
If I were to have a metal roof installed it would be a standing Seam with concealed fasteners.
Reality is however, standing seam roofs very expensive but, last generations.
As far as installing any new roof over an old, I would compare that to the groom wearing dirty under ware to to the wedding.
B. John

 

[Deere Dude]

I personally think conventional 26ga metal roofing installed with washer head screws or nails is grosley over rated. Virtually 100% of all the fasteners are exposed to the weather and over time the sun will deteriorate the gaskets and leaks will happen.
If I were to have a metal roof installed it would be a standing Seam with concealed fasteners.
Reality is however, standing seam roofs very expensive but, last generations.
As far as installing any new roof over an old, I would compare that to the groom wearing dirty under ware to to the wedding.
B. John

The fresh underwear depends on which day of the week you get married :thumbsup:

There are screws with the rubber gasket hidden from the elements. They use cupped type washers. I would use them in a heartbeat if not to pricey.