Standing-Seam Metal Roof Options
New build, zone 2, 50 miles south of Houston, 10 miles from the coast (so in Texas hurricane zone “Inland 1″). Several gable end roofs on the house. All rafters 16″ OC, decking is 7/16” zip sheathing that will be taped, etc. Attic will be unvented. OC spray foam directly on the underside of the roof decking. HVAC system entirely inside the unvented attic. No insulation above the roof deck (doesn’t happen around here).
Roofer has given me three options for a standing seam metal roof. All three use 24 gauge galvalume, and will have Ice & Water Shield at all roof/wall junctions, and “hi-temp felt” over all field areas of the roof.
First option (about $43K) is snap-lock steel over Shield/felt screwed to the zip deck.
Second option (about $45K) is same as first option, except using mechanically seamed steel with a 1.5″ seam instead of snap-lock steel.
Third option (about $51K) is same as second option, except putting down 1×4 diagonal purlins over the shield/felt and steel is screwed directly to purlins, thus creating air channels. The 1×4 lumber costs about $3K just for the lumber.
These are really the only three metal roof options I have. I’ve learned a lesson on this home about the downside of trying to use techniques/materials that aren’t common around here, and don’t want to re-learn it on the roof.
The builder recommended against using snap-lock panels on 1×4 purlins, but I don’t remember why. That’s why no option/price for that was given.
Comments on whether or not option 2 or 3 are worth the additional cost? Is the air space between the underlayment and bottom of steel worth an additional $6K over the same steel directly on the underlayment?
Thanks,
Ira
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Replies
Ira,
I would go with either option One or Two based on finding out which seam had the most resistance to high winds.
Option Three has two disadvantages: It is significantly weaker in its attachment to the roof, and comes with complications on how you will vent and provide a drainage path for the diagonal air-spaces at the eaves, gables, and peak.
Malcolm,
Are you saying that the air space isn't much of a benefit in my climate/situation? BTW, the only reason the roofer provided the 3rd option (the purlins with the air space) is because I specifically asked him to. He very seldom does it that way, and doesn't think there is anything to be worried about with regard to not having the air space. I'm the one who was/is unsure about the need for the air space in this scenario.
Thanks,
Ira
Ira,
I'm not generally a fan of airspaces above the sheathing for metal roofs. There may be some minor advantage to keeping the sheathing cooler, but to me it's offset by the complications they bring.
- You either need two layers of strapping, or as you have specified place it diagonally. If it's diagonal you need to detail the gable-ends so they also provide ventilation intakes and outlets, as many of the cavities will end there.
- You also need to work out how the drip-edge will interact with the cavities and the gutters below at the eaves.
- All these gaps bring weakness to the vulnerable edges in what y0u describe as a high-wind location - and as i said in my first response, the roof isn't attached as well as it would be if screwed directly to the sheathing.
- Because the roofing is being screwed to the furring not the sheathing, on most types of snap-lock panels the slots to screw through won't match the spacing and you may get more oil-canning.
If an unvented roof has moisture issues it will typically manifest at the ridge. This Lstiburek article covers "ridge rot."
https://www.buildingscience.com/documents/insights/bsi-088-venting-vapor
I know you did not ask and every house in TX has the HVAC equipment in the attic but I have to ask you to reconsider.
By conditioning the attic you now have much more space to condition this space has a larger surface area to insulate and putting the insulation against the roof forces you to use the most expensive and least green insulation possible.
My guess is if you did a manual J calculation for your likely to be R25 conditioned attic VS an R60 vented attic. My guess is your equipment would be 25% or more smaller and the electricity need to operate it would be 35% less and it would cost less to build.
If your house is still a set of plans find a way to get your HVAC under a flat well insulated ceiling.
Rant over.
Walta
Sounds like Clear Lake/Clute area.
Basically sub-tropical (high humidity) with a constant breeze off the Gulf of Mexico.
I would confirm with code officials, and metal roof manufacturer (warranty issues). Purlins may be fine. It's all about the spacing of the panels anyways. Personally I like No 2
Stick with the closed cell spray foam. The last thing you want are open-air soffits which will help pull off the roof when the next hurricane arrives.
I will be putting up a post in a few weeks about a radiant-barrier roof system invented by Peter Pfeiffer--a Texas based architect. He designed it over the course of 40 years specifically for your climate. (In my mind, I can hear him arguing in favor the air pathways your option three creates.) His roof system uses unpainted galvalume, which he says thwarts about 72% of the radiation that hits it. That means only a small percentage of solar radiation penetrates through to the space below. Lifting the metal roofing off the sheathing with diagonally run furring strips—spaced 24 in. o.c. with 3-in. gaps between the board ends—creates a 3⁄4-in. gap from soffit to ridge for continuous movement of air through the ridge vent. The underside of the galvalume works like a foil-faced radiant barrier, so little absorbed heat is transmitted into the attic. The metal roofing also dissipates heat quickly at night, which means it is not radiating heat inward. To create an additional barrier to heat, the area between the rafters is sprayed with closed-cell foam. In short, it is a system to mitigate solar heat gain, thereby keeping the sealed attic space below as cool as possible.
> unpainted galvalume
Strange, the data is clear that a white upper surface is cooler, especially after aging. Plus paint over galvalume is more corrosion resistant.
Agreed, low emissivity on the bottom side + a gap is beneficial.
Dorken-Delta, here in Canada distribute a product (Delta-Trela) meant for metal roofs that provides a ventilation/drainage plane without purlins or the added depth. its about 3/8" in depth and acts as vapor-permeable underlayment.
Todd,
I've never tried one of the mesh underlayments on a roof but have several reservations.
- A 3/8" gap which is impeded by the mesh may provide some drainage, but I can't see it yielding any useful ventilation.
- Most metal roofs rely on screws and gasketted fasteners for their attachment, both of which would have to compress 3/8" to be secured to the sheathing below. The panels either have to dimple that much, or hope the loose fasteners remains watertight.
I'm not in a climate where the heat of a roof is a problem, so I've only thought about this from a moisture point of view, but it seems to me that providing ventilation and drainage under metal panels to dissipate moisture is really dealing with something you created by adding a gap in the first place. No gap, no moist air to condense
Would be interesting to compare to a rainscreen on a wall - which is clearly beneficial, but has some differences.
Jon,
Yes. While I'm not wild about mesh products used as rain-screens on walls either, they at least provide a valuable capillary break - a function they don't on a roof.
All that aside, I'll be interested to read Peter Pfeiffer's blog. It sounds like he may have figured out how to effectively use diagonal furring. Something iIhaven't seen done.
On option#3, steel purlins would be WAY BETTER & cheaper than wood purlins. Although being in a high wind area, I agree with the others, that installing the metal roof directly to the roof sheathing is best practice.
MBCI makes an elevated deck standing seam metal roof but in a hurricane zone, I probably wouldn't use it. Although it does carry some Florida approvals.
https://www.mbci.com/products/roof/standing-seam-roof-systems/lokseam-/
https://www.mbci.com/products/roof/standing-seam-roof-systems/ultra-dek-/
Although Options #1 & #2 DO NOT allow for the roof sheathing to dry UPWARD since metal attached directly to wood prevents any drying going UP. Sheathing can dry inwards as long as you don't use closed cell spray foam or similar vapor closed insulation directly underneath the roof sheathing.
No one seems surprised by the quotes on the roofs. In 2013 I put a 40 year galvalume roof (basically one large gable and a second small intersecting gable) on a 3,200 square foot house. The total cost was $8,500, which was well below the $17,000 the builder had estimated.
But $50K for a metal roof? When did that happen? I know everything is bigger in Texas, so maybe this is a monster house.
Steve,
Being a specialty product the labour component of metal roofs has always been disproportionately high. The last house I designed had a simple shed roof. I calculated the materials at 12K and the time for two guys to install at three days, but all the quotes came in north of 30K.
My experience supports Malcom's statement. It's a specialty product. We are in Sedona, AZ. Got quotes from the 5 roofing companies for standing seam roof... Quotes ranged from $40K to just over $50,000. We ended up doing it ourselves. Shocked to learn it was only $10,000 in materials (and we did a thicker gauge than the cheapest quote). We also added gutters to the order and some extra trim material in the $10K! I kept strict track of time: The wife and I made ~$800/hr (combined) for our time and had a neat learning experience. It was hot up there, VERY loud, and pretty sharp.... and I was cared enough that I bought a climbing rope and 2 harnesses. (I wasn't scared by the end, which is even scarier). I also agonized and researched for HOURS how to handle a few transitions, which I didn't count in the time. (It was easy, but... not until you knew what you were doing, and I (incorrectly) thought what we were doing was "special".) Anyway, we still "paid" ourselves more than a NYC lawyer.
I chose Option #1 offered above (even for the quotes). After doing it, I have ZERO concerns about the quality and strength and longevity of a "snap-lock" standing seam roof vs mechanically locked.... (It's a full overlap snap... all the way up and down... and double that It's a really cool system, and *extremely* solid feeling (we had to pull one apart and... wow... NOT fun.... that will definitely be the last thing to go on this roof.
Flying Beet,
That's what drove me into it too. There was just too much money on the table.
Once you learn the four or five common flashing conditions , then it's pretty simple. I agree - it's very reassuring to put on a roof you know won't leak and lasts as long as they do. After probably forty roofs, I finally got round to my own last summer. I only fell off once.
Metal roofs are pretty common on new upmarket builds in the Atlanta metro area. (Farmhouse, anyone?) So maybe there is more competition here. In any case, I suspect it's more about labor than material costs.
On my 2013 roof, it cost $800 to go from a 25-year warranty to a 40-year warranty. I wish everything on that project had turned out be half as expensive as the estimate. ;-)
Sheffield Metals (a company that supplies the rolls of metal to the roofers) recently posted a video that said their cost of steel has gone up 75% since August 2020. The roof will take about 85 squares. The living area is about 3400 sqft, but there are also two large porches (about 1100 sqft) and an attached garage (about 1000 sqft). All gable end roofs (four "sections") with an 8.5/12 pitch and four dormers. Unfortunately, a lot of roof surface compared to the living area.
I talked to Peter Pfeiffer (architect in Austin, TX mentioned in an earlier reply). The system he champions is the galvalume snap-lock panels on 1x4 diagonal "lathes". He has specific recommendations on the details (edges, flashing, etc.). I only talked to him for about 15 minutes, but the main reasons behind his design are the ones mentioned in Kiley's reply.
I will talk to the roofer to find out why he doesn't recommend snap-lock panels on top of purlins/lathes, and maybe have him discuss his concerns with Peter. It may be due to requirements for windstorm certification because we are so close to the gulf coast.
Assuming Peter's design details aren't significantly different than what the roofer planned to do for a mechanically seamed roof on wood purlins, the estimate should be about $49K.