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Question DetailsAsked on 11/24/2017

Should we go with a metal or cement tile roof? Hot mop or peel and stick underneath? We live in South Florida.

We live in South Florida and have to reroof from Irma. Our current roof has many open valleys, multiple slopes, and custom clay tile. We are considering cement tiles, standing seam metal, and the Tesla tiles (as an aside, assuming they're available, how quickly can they be approved for use is Miami-Dade...are they a viable consideration?) as potential replacements. Which material will give us the best protection for our home? Should the underlayment be peel and stick or hot mopped (with four layers in the valleys)? We're hearing conflicting opinions (including one that suggests we have too many valleys for a metal roof?). Thanks for your time and expertise in helping me to protect my family and home.

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Tough question - I guess the best you can do is talk to at least three roofers for estimates both ways and listen to what theiur recommendations and objections are - then choose a system and a provider who offered that option (some may not offer both options). If using an architect on the rehabilitation, talk to him/her too - and of course how long you plan on being in the home, and what the chances of it being taken out by water before wind, also factor into how "storm-proof" a design you want to go with.

Properly done, a panel (as opposed to metal shingle) metal roof (with hurricane-rated fastenings and using heavy-gauge steel like 24 or even 22 gauge) should last as well as a cement tile one, but the details matter a LOT - the tiedowns or adhesion to the substrate is criticial. And for metal sheet roofing, if it is not put over a well screwed-down plywood sheathing it is the only thing bearing the air pressure load from inside the attic so can be pushed up and peeled off more easily.

Tiles generally blow off because of adhesion or interlock problems - so the better the interlock or anchor system (as opposed to just an adhesive or grout under them, which do not provide much pulloff resistance) and of course the heavier the tile (which works against you too by increasing roof loading on the attic framing) affect that roof type resistance.

Havings lots of valleys does complicate metal roofing installation (but tile roofing too, for that matter), and makes for more leak possibilities because valleys are (along with wall flashing) probably the most common leak source - but properly done, especially if a "seamed" valley is used (where the metal roofing is continuous across the valley rather than butting up against itwith bare ends), can work well - though the number of angled cuts does of course increase materials cost and labor.

Heavy-gauge metal roofing will probably be about 50-100% more than the tile option.

Hot mop uses tar over tarpaper, which has relatively little adhesion to the roof (though that can be helped by spraying hot tar as an adhesion layer under the tarpaper, but that is messy and prone to rippling and to leak-through into the attic) - not commonly used as a water barrier under another roofing material. Synthetic underlayment (using large plastic washers to hold it down if exposed to wind) is the "norm" these days, but can tear off in the wind just like hot mop if exposed by blow-off of the primary roofing material. I definitely would favor the synthetics for the water barrier, but peel-and-stick or adhesive applied PLUS overlying fasteners with close-spaced pull-though resistant plastic washers or lath, as mentioned below, to increase blowoff resistance. I have seen properly applied synthetic water barrier (exposed prior to roofing completion) survive 90-110mph winds for a day or more with zero damage - but stronger major hurricane/tornado winds is just so dependent on contractor care and on making sure the free edges cannot get air in under them.

An in-between solution some people use, though how well it would work in your extremely hot climate I can't say, would be 100% coverage with ice and water shield - which is a peel and stick asphaltic material designed for roof eave areas to stop infiltration from glaciering and icing. WR Grace is probably the biggest manufacturer and would be where I would go for advise regarding your area. In cooler areas it REALLY sticks to the sheathing - don't know about at 100 degrees or so air temp for your case.

A more "overkill" third option, but at about the cost of two roofs in one, is using a heavy-duty fully-adhered roofing membrane over the plywood sheathing, with conventional roofing over that as the primary protection. A fairly recent variation is membrane roofing overlaid with substantial metal or wood lath or firring strips (typically 3/4" thick or more, so a lot more than "lath) with the roofing applied over that, as a double roof - providing (with the lath strips) a greater blow-off resistance for the water barrier membrane in the event of roofing blowoff, plus a ventilation layer under the primary roofing which is supposed to significantly reduce attic temperatures and A/C costs. Probably the future of roofing design in the US.

Talk to contractors, abnd there are a lot of articles out there about Florida roofing issues and code compliance. Personally, I would not consider Tesla tiles - unproven and like other solar options likely to be vastly overpriced compared to any energy savings you will likely see in the normal case, and will likely have the same issue as metal tile with sdtorm breakage and blowoffs, even if they do get approved soon. Plus you have the significant issue of frequent blown debris and hail damage from tornado storm cells and thunderstorms, not to mention hurricanes.

As for going to 4 layers in the valleys if using hot-mop - that is not a good material for valleys - proper valley flashing is the thing to use in valleys for protection over the water barrier, to prevent it rotting out due to debris and organic decomposition wash-in accumulation. Personally, I do not see hot-mop as a good water barrier under another roofing material (including under pavers in IRMA roofs), because under a roof it tends to act as a sponge rather than as a water-shedding layer as it should.

Obviously if your roof is built to handle heavy-weight tiles, ones heavy enough to be virtually blow-off proof, that would probably be your best hurricane resistance - but most roofs are not built for that heavy a tile. Metal seamed roofing is generally the longest-lived roof short of slate (which is not an option in your case) and less likely to leak in hurricanes or heavy thunderstorms, but when it does peel you tend to get a large area of damage, not the smaller spots you tend to get with cement tiles, so a tough decision.

I think if I were in your shoes, I would require full plywood sheathing if you do not have it now, then either:

1) a hurricane washer-screwed synthetic membrane under commercial-gauge (24ga minimum, or maybe even 22ga) narrow-strip (not 24 or 26" wide-field) raised seam metal roofing with positive hold-down straps (not just screws) at close spacing - and straps that hold both panels at the joint, not just the bottom one with a slip-joint or snap-joint onto the seam from the upper piece - or if you can handle the appearance, better-yet through-screwed U-clipped seam metal roofing (which leaves the screws and U-clips over the seams exposed and sticking up at the top of the seam as well as at any mid-field screws), or

2) a synthetic hurricane washer-screwed synthetic membrane under concrete tiles which use a mechanical attachment system, not just grout.

If I had to decide on the spur of the moment, I think I would go with the metal roofing, with both edge and center-of-panel waterproof washer screws. If you have heavy tree debris on the roof from palms or such, that would push me more towards the steel option for easy cleaning, but if close to the sea (say within 3 miles or so) that weighs in favor of the concrete tiles because they do not rust (though their fasteners can). Bear in mind, the washers on the sheet metal screws (which keep the screwholes from leaking) generally go out after 15-20 years in your type area, so need replacement even though the roof panels are fine - but a WHOLE lot cheaper than reroofing. With a good brand hopefully you will get 40 years or so before sandblasting and repainting is necessary.

[On the screws - there are expensive ones (though only a small percentage increase in total job cost) with a polymer coating which self-seals in the screwhole as it is put in, under the plastic or rubber compound waterproof washer, which may (given enough years of service data) prove to be the answer to long-term screw leakage.]

And if going with tile roof, use a type with positive corrosion-resistant fasteners or clips - not just nailing at the tip edge.

Answered 3 years ago by LCD

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