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Hi,
I'm currently renting a house that I am thinking about purchasing. The house is a double A-frame with a cathedral ceilinged loft upstairs where the attic space should be. The year before I moved in the landlord (who was living here) installed a standing seam metal roof over some old (defective) asphalt shingles.
On Tuesday, I paid for a home inspection and the issue the inspector was most concerned about had to do with ventilation. Apparently, there is none. He noticed some unevenness to the drywall on the ceiling of the cathedral ceilings (patchiness), and outside, he noticed moisture stains underneath the eaves and soffits and at the top of the "A"'s. He said that this could be an indication that moisture was collecting underneath the roof and could possibly rot out the rafters.
I certainly do not want to buy a house and then be left, in a few years, with no functional roof. My question, then, is a) does standing seam metal over asphalt shingles reduce the ventilating capacity of a roof? b) what can be done to ventilate this roof, short of constructing a double roof such that air circulates inside? And, how much would this cost for a standard sized house? c) What is all this I hear about cathedral ceilings with a particular type of insulation NOT having to be ventilated at all?
Thanks for the opportunity to ask!
Yours,
Ryan
New Hampshire
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This is a pretty complex issue and, frankly, there are varying opinions on it. I will try to address it a bit though.
First, the installation of the metal roof has not worsened anything. Fact is, there are potential ventilation and moisture problems with A-Frames regardless of the roofing material installed on top. Current building codes, in fact, call for such cathedral ceilings to have vented airspaces. In years past, though, that was not the case.
There is some potential for condensation to form on the back side of the metal roofing panels. Generally, this is an unusual occurrence and the underlayment which should have been installed with the new roof would prevent this from being a problem.
The greater concern with a building like this is that moisture from inside the building would condense in or on the roof decking when the dewpoint is hit. Dewpoint is determined by a combination of temperature and ambient humidity. (Think in terms of the "sweat" on the outside of the cold glass of lemonade I enjoyed on my front porch yesterday on a very hot and humid day here in Ohio!)
In "normal" (i.e., buildings with a vented attic) construction, the moisture that is generated inside the home can escape through ventilation. In your home, that opportunity does not exist.
One of the best things you can do is look for ways to reduce humidity inside the home. Humidity is created by taking showers, doing laundry, having houseplants, wood-burning stoves, etc. If you minimize these things or make sure that their resulting humidity is vented to the outside, you can lessen the potential for problems. (I am not suggesting that you need to quit taking showers!) You might also run a dehumidifier, especially during the seasons of high humidity and cool nighttime temperatures (which could cause the roof deck to reach dewpoint).
Certainly, another option is a vented roof over your current roof but that would come at an expense and you'd have to make certain that the building could handle the weightload.
As for the drywall and other problems noted by the inspector ... those may or may not be caused by past condensation problems. There could be other sources of the moisture. I'd take a hard "common sense" look at them. You might also want to have a roof inspection done by someone experienced with metal roofing.
All Best.
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I'm currently on a project that is having metal decking installed over perlins, we are having a major problem with oil canning. What are some causes of oil canning and how best to solve them.
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Depending on the type of roofing assembly you currently have, that is one with proper attic ventilation and if the existing asphalt shingles are in reasonable condition, then they may act as the underlayment. If however you intend to place the metal directly over the shingles, then most building codes call for a minimum of 2 layers of type 15 felt of one layer of type 30. This will stop any abbrasion with the metal while acting as a good moisture barrier. A safe bet is to always check with the manufacturer as they know their products design and needs to meet your climatic area and the local building codes.
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I am alittle confused over your terminology. Metal decking that is typically applied over purlins is not a roof covering material but a structural part of an assembly that is designed to have a roof covering applied over it.
Possibly you are referring to is a vertical rib roofing sheet either in through fastened or standing seam profile. A through fastened panel usually has multiple ribs rollformed the length of the sheet that add strength and minimize oil canning.
That leaves a standing seam profile that is typically rolled with raised edges and flats in between typically from 12" to 24" in width.
These products are each roll formed from coils of steel either naturally finished or in pre painted. A small coil of steel at 10,000 lbs coulf have up to 4,000 lf of roof sheets. At times it is difficult to control the stresses across the width of the coil typically 3-4' wide which can cause the middle of the coil strip to be fractionally longer.
When the product is roll formed the flatter products tend to show the stresses which show as waves. To be fair with the manufacturers, this may not be evident at the time of manufacture and only show up after installation as the various installations can cause the roof material to expand and contract at different rates at differant times of the day.
The roofing contractor should inspect the roofing material as it is delivered and note any problems immediately to the manufacturer and have the rep come out. There are certain standards set down by the industry and if the product fails to meet they will replace the order.They would normally have a mill test report on the steel and can check the order back.
This issue is made worse at times if thinner base steel is used especially with portable rollformers.Again contact the manufacturer or steel supplier. Good luck.
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In the trailer park where we reside (panhandle of Florida), the trailers that are having metal roofs installed, the installers are removing all the roof vents (vent hoods near the roof edges and roof venting at the peak). They are installing 2 inches of rigid insulation between the old roof and the standing seam metal roof. Shouldn't the roof continue to be vented or can the venting be done away with?
tom
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Mobile homes and trailers are built under a differant code that permenant dwellings and I'm not sure of the function of the existing hoods nor the construction of the trailers in question. Typically the roof assembly in trailers was constructed with the cavity as thin as possible and filled with ridgid insulation. I have also seen where they did leave an air space between the insulation and the roof material and the only way to ventilate it given the low slope was at the sides.
I am assuming the reason the owners are having the upgrading is for the insulation which then requires a new roof and a SS roof is a great roof to put on low slopes. The suspect that the assmbly you talk about is an economical choice. A 1" air space over the insulation will increase the efficiency of the assembly if it is vented however it will require more costs to create that situation.
In plain terms what they are doing does not meet the building code for permenant homes however it probably does on mobile trailers. It would be better to have a 1" air space and vent it to stop heat transfer however you will be into more costs. The system they are putting on will increase the energy efficiency of the trailer at a reasonable costs. Hope this helps.
6/20/2002
Isaiah Industries, Inc.
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