Hello,
Currently I am working on an outdoor pavilion. I am looking for a product that can be installed between standing seam metal roofing and plywood. I want to prevent the accumulation of moisture between the metal roof and the plywood so the metal roof won’t rust. We are trying to avoid installing furring for simplicity sake.
Any ideas? Thanks!
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I used one of the non-asphalt barriers such as Tri-Flex or... the other main brand escapes me at this moment, but if you tell your lumber yard you're looking for the better quality product that can be used instead of tar paper they'll know.
The stuff I've used comes in 5' wide rolls and lasted through two months of high winds before we finally got the metal on.
You will run into several opinions about what to put under a metal roof. The very traditional method is wood nailers running up the roof so moisture can drain away from the metal and the sheathing. Some folks still put down an underlayment under the nailers for added protection. Probem with this method is the edge trim treatment so you don't get bees, bugs, bats, etc. in the space under the metal. There are several trim solutions from homemade to commercial products.
I've seen a lot of folks put metal roofing directly on felt or synthetic material (designed as metal roof underlayment). My personal opinion is you will not get the 100 years plus out of the roof with this type of system. Coated steel products do corrode quickly when placed in a moist environment - both galvanize and galvalume.
Improperly stored coated steel roofing - stacked on a horizontal lift uncovered outside - will quickly form "white" or "black" rust which is the start of the coating degradation. Metal roofing in continuous contact with anything that retains moisture will have a shortened life span.
I find most folks don't think in long time horizons anymore when it comes to their dwellings or outbuildings - do the job at the best price and let someone else worry about it 25 years from now. My house was built in the early 1700's in the moist, humid mid-Atlantic. I have two roofs, the original shake roof and a standing seam that I guesstimate to be well over 100 years old - attached to nailers on top of the shakes. A coat of asphalt impregnated coating every 15 or 20 years and it's like a new roof.
You need to decide how quick/cheap you want to do this job and if it is worth it to you to invest in a lifetime roof.
Tell me about your asphalt impregnated coating on your standing seam roof. I have a 1938 installed standing seam roof on an 1820 house and cannot get coatings to stay on it. The last attempt was with Acrymax (high dollar) and it is failing big time from underneath (inspite of meticulous prep) and above (oxidizes big time in sun light, fine in the shade). I figured I would try some Cool Seal as a last resort.
Bill
When we accepted responsibility for a base metal roofing claim in the steel industry, our research folks insisted that we let a mild rust film develop on the surface. This drives the owners nuts but the rust is really etching the surface for the coating to adhere to. Most folks rush to paint a roof as soon as a small spot appears and many times while the surface is still shiny.
There are a number of products out there for roof coating - from the Henry brand at Home Depot to Sherwin Williams to house brands at farm supply stores. The product is sold in 5 gallon pails and is very thick - I use an extra heavy duty 1/2 inch drill with a commercial paint stirrer to get it thoroughly mixed - usually about 15 minutes of work. Use a wire brush and wipe down the lightly rusted roof to remove any loose particles and either roll or brush on the coating.
Beware of folks who stop by and say they are in the area coating barn roofs - most use a cheap, thin silver paint - thin enough to spray on. And a year after application, it's ready again - and you've bought in to a never ending maintenance project! I can get 15 to 25 years out of a roof coating - depending upon the extent of rust, how clean the surface, quality of application and the weather conditions over the years.
http://www.roofcoatings.org/pdf/tn05.pdfView Image
Where would this moisture come from? are you planing for leaks?
Since this is an outdoor pavilion, the temperatuire should be similar on both sides, and no moisture will be trapped in an 'attic' enclosure to condense.
So there should not be any measureable amt to worry about,
The main reason for underlayment in this situation is as a slip sheet, IMO, and tarpaper is fine
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The main reason for underlayment in this situation is as a slip sheet, IMO, and tarpaper is fine
Depends on the metal. If it's terne, then it needs to be isolated from the felt per manuf. instructions. AFAIK, that spec exists in several of the prefinished metal instructions, but not all brands.
These questions are usually answered in the manufacturer's specs but they do vary from brand to brand.View Image
Thanks. I've never done Terne.What about CU? I remember doing some govt work almost 35 years ago that had a book of specs a mile thick where construction paper was required for a slip sheet between the tarpaper and the CU. I'm thinking it was because of thermal expansion...
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construction paper was required for a slip sheet between the tarpaper and the CU.
That's a totally unneccessary step left over in the SMACNA manual from the days when terne was king.View Image
No one wants leaks but they do happen even with the best of installations. The folks who make coated products (galvanize, galvalume, terne, etc.) have seen it all, so they are always conservative. It might be a little more work, but nailers will give you the extra insurance you might want from a coated steel product. If I was presnted with a corrosion claim, I would deny it if the metal does not have a breathing space.