Standing seam metal roofing over existing asphalt shingles?

[IHDiesel73L]

Anyone ever do this? We have an older farmhouse that had a major renovation done 20 years ago in which some new roof was installed over the new addition and the old roof was torn off and replaced over the existing house. The shingles are standard 30 year three tab that are 2/3 of the way through their life, I'm just thinking ahead and if we have the money to do it in the next couple of years I'd like to cover them with metal roofing rather than wait for a leak to develop. We don't have any leaks currently, but we do battle with grey squirrels and flying squirrels which have chewed through flashing and fascia boards and found their way into the attic and soffits. This is one of my motivations for metal roofing aside from the look.

We are out in the woods with no other houses around so we're not worried about "fitting" with the neighborhood. Our barn has grey colored standing seam roofing that is old and weathered and I'd like the house to have the same look. The barn roofing was simply bought in 12' x 3' sections and installed over purlins spaced 2' OC. The runs on the roof are 12' and 16', so with 12' panels on the 16' runs I would just plan to lap the section closest to the peak over the lower section according to manufacturer specs, seal the joints with silicone, and stagger the seams (ie: 4' panel at peak, 12' panel to edge of roof, 12' panel at peak, 4' panel to edge of roof, etc...). It seems to me that this would probably the last roof I'd ever put on this house (I'm 37 years old and plan to stay here for rest of my life)-am I missing anything here?

 

[oosik]

I see no problem to your approach. Check with the company selling the metal roofing. They may recommend a single layer of tar paper, or the like, before you lay on the metal roofing.

 

[Diggin It]

I've done standard 5 rib over shingles. Laid 1x4s over the shingles, nailed through to the decking, then laid the roof panels on them. My panels are screwed down which gets into another debate, or two, or three.

 

[M5farms]

Use a synthetic felt between the metal and shingles. The only time wood lathe strips are needed is If your deck is not level. The manufacturer approves either application but all recommend the separation sheet y
To make sure the back of the panel is not scratched. If you do not know for certain that your deck does not have any weak spots it's best you tear off completely.

 

[dieselscout80]

Could foam board insulation be added to reduce heat transfer into the Attic?

Would it be coat effective?

 

[M5farms]

Money is better spent making sure you have proper ventilation. If your attic is not conditioned make sure you have excellent air flow and add more Batts or loose fill to the ceiling.

 

[ruffdog]

I have heard that one big issue is the condensation that forms on the back side of the metal. Personally, I would rather do a complete tear-off, repair any wood, waterproof, then install the metal with ventilation behind it.

 

[IHDiesel73L]

Use a synthetic felt between the metal and shingles. The only time wood lathe strips are needed is If your deck is not level. The manufacturer approves either application but all recommend the separation sheet. To make sure the back of the panel is not scratched. If you do not know for certain that your deck does not have any weak spots it's best you tear off completely.

Makes sense. The roof over the new addition is in very good condition (no sag in the ridge, etc...) so I could go with just the felt there. The original part of the house has some sag to the ridge and waviness in the roof itself. It's all solid, just settled a bit over the last 130 years or so, so I would do lath to make the steel appear nice and straight.

Money is better spent making sure you have proper ventilation. If your attic is not conditioned make sure you have excellent air flow and add more Batts or loose fill to the ceiling.

Attic is unconditioned but has fully vented soffits. It is a walkup with 2x10 ceiling joists, so the joist bays were filled with 10" of fiberglass which is covered by 1/2" plywood throughout. There is also a gable fan.

I have heard that one big issue is the condensation that forms on the back side of the metal. Personally, I would rather do a complete tear-off, repair any wood, waterproof, then install the metal with ventilation behind it.

It would seem to me that the waterproofing is already there in the form of the existing asphalt shingle roof, so why rip it off? I guess in the case of as M5farms pointed out, if you don't know/can't assess the condition of the underlying sheathing you might want to, but luckily in my case since it's a walkup, I can see every inch of sheathing from the underside. Since the asphalt roof had no trouble shedding the record amounts of rain it saw last year I would guess that some condensation from the undersides of the metal roofing panels would pose little challenge.

 

[TnAndy]

The barn roofing was simply bought in 12' x 3' sections...

FYI: If 3' wide, it's probably NOT standing seam.

All the standing seam I've seen is 16" or so wide per pc. If it went much wider, the hidden screws on the one edge covered by the 'standing seam' wouldn't be enough to hold the metal down.

3' wide metal goes by the name "Century drain" (among other names), does have raised ribs about 8" OC, and has rows of exposed metal screws across the panels.


Standing seam makes a much better roof, IMHO, due to the screws not being exposed, (each point a potential leak) and is often 26ga versus the normal 29ga of 3' wide metal, but it's more expensive than the 3' wide metal.

 

[EddieWalker]

Maybe it's just terminology, but to me, standing seam is the type of metal roof that you don't see the screws and each piece of metal is a foot wide. Because of it's size, you have to install it onto decking. Plywood or OSB. R panels come in 3 foot widths and you can usually span 4 feet with it, so your purlins are installed every four feet. This is more cost effective and fairly common when going over an older roof. Any moisture that develops under the R panels is handled by the existing asphalt shingles. The drawbacks to leaving the shingles in place and putting R panels over it is that moisture the may develop under the metal. It can build up on the Purlins and create mold, and then rot. I've also seen moisture come out from under the metal, onto the porch and people not understand that their roof isn't leaking, it's just a lot of condensation.

If you go with standing seam, you really need to remove all of the shingles and have perfect decking in in place, then covered with felt paper or some other equally approved material. There are a lot of them out there to choose from.