We are planning to re-roof our house with metal
I am not a builder, so please bear with me. Our house is in Northwest Florida, a red brick ranch style built in 1960. We currently have GAF Timberline shingles that were installed in 2000 and they are failing. We are planning to re-roof with white Galvalume snap-lock panels, the kind with the tabs on the panel with slots.
At this point we are considering several alternatives: (1) placing a layer of reflective foil on top of the old shingles, then screwing down 1X4 lathing to create an air space, then installing the metal, or (2) placing tar paper or synthetic underlayment on top of the old shingles, then installing the metal – no foil, no lathing, no air space, or (3) removing the old shingles and then doing (1) or (2).
Please advise as to the pros and cons and which method you think would be the most cost effective. Of course, let me know if you need more information. If you wish, you may email me at [email protected].
Thanks,
Donald Nelson
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Replies
In Florida, the radiant barrier will help, even if you have a well-insulated attic (but you may not, since it's Florida). Even better would be putting the radiant barrier over the 1x4s so that the foil is facing an airspace below it, rather than above it. This will make sure the foil's performance never gets compromised by dust accumulation. If you follow this approach, you should install new underlayment underneath the 1x4s and over the shingles.
Donald,
Do you know how many layers of asphalt shingles you have on your roof? If you have more than one layer, that would definitely tip the scales in favor of stripping the roof down to the existing sheathing.
There is only one layer of shingles on the roof. There must be something I don't understand about a foil radiant barrier - I thought the foil would have to face up toward the metal in order to reflect the heat back toward the metal and away from the decking.
Also, I meant to ask if the lathing should be laid vertically (eave to ridge) in order to allow ventilation.
Thanks very much for the quick responses.
Radiant barriers are kind of mind blowing: they can work to reflect thermal radiation back towards a source, or to prevent the source from radiating in the first place. And they work equally well to do either! The fundamental rule is that they have to face an air gap. But for a given air gap, you can actually put it on either side and get the same effect when it's new and shiny. Given that it doesn't matter for initial performance, the long term performance should be considered. And facing down means that dust won't settle on it, so it maintains that performance long term.
Congratulations on the "cool roof". It's on my bucket list for our new home.
Are there any concerns about cell signals using the foil and metal panels?
Clara,
There is no clear answer to the question. I wrote an article about a related question: Do Foil-Faced Building Products Block Cell Phone Reception?
In that article, I quoted Robert Palardy, manager of technology at Louisiana-Pacific, who said, “There are plenty of documented cases where people report diminished [cell phone] reception in houses with aluminum siding and metal roofs.”
For more information, read the article.
A word of warning if you ever plan to have solar PV on the new roof. Leaving the existing shingles in place greatly increases the chances of needing to re-enforce the roof structure later when the solar is to be installed. Whereas, if you remove the old shingles, then install a much lighter weight metal roof (metal roofing is usually around 1.2 psf, and the existing shingles are around 2.7 psf, a new solar array is around 2.5 psf), you have almost guaranteed that the roof is already strong enough for the solar array. Some jurisdictions allow a prescriptive process for solar, that if you can show the roof has only one roofing layer, and was a permitted structure, no engineering is required by a PE (depending on your roofing quotes, the PE evaluation can sometimes cost as much as the shingle removal, not to mention the costs for re-enforcement if they are needed).
Removing the existing shingles would be the smart move if you plan to have solar in the future. You can also contact a local solar installer for their 2 cents, as they will know your regions permit process better than i would.
To Nate G: Please help me understand why new underlayment would be needed over the old shingles if you use lathing. Is it due to condensation that may form on the underside of the foil?
To Mr. Sullivan: I need to read more on radiant barriers - do you know of a publication that a layman could understand? Also, is there any difference between installing the lathing vertically vs. horizontally?
Thanks
Donald,
New roofing underlayment is a standard part of any roofing job. If you really want to save money, and if your old asphalt shingles are still waterproof, I suppose you could save a little bit of money by skipping the underlayment. But the underlayment makes for a cleaner, neater installation, and it doesn't cost much.
The 1x4 purlins or 2x4 purlins are installed parallel to the eaves and ridge. If you run the purlins parallel to the rakes instead (running up and down), you won't have wood where you need it when it's time to fasten your roofing panels in place.
Here is a link to an article with more information on radiant barriers: Radiant Barriers: A Solution in Search of a Problem.
Mr. Holladay,
After reading your article, I'm wondering if we should skip the radiant barrier. I consider our house to be well insulated, but we do have ductwork in the attic. The simplest and least expensive method of installing the new roof appears to be putting down new underlayment on top of the existing shingles and then installing the metal. I have no idea if removing the old shingles and/or creating an air space with lathing would be cost effective. I would appreciate your thoughts on this.
Thanks
If you have ducts in the attic, a radiant barrier makes sense. IMHO, in a climate like Florida, it also makes sense irrespective of that, especially if you have the opportunity to get one relatively cheaply due to a re-roof situation. A lot of the anti-radiant-barrier sentiment expressed here comes from people who live in places where one can forget what the sun looks like for 6 months out of the year. They are not much use when it's cold, but in a sunny climate where your space conditioning is almost all cooling, and especially with ducts in the attic, I'd do it.
To Nate G: If we go ahead with the lathing and radiant barrier, would it be reasonable to leave the old shingles on? Can you recommend a brand or type of radiant barrier material? Seems like there are a lot of different substrates for the foil.
Thanks