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Question DetailsAsked on 5/17/2017

Is it necessary or important to put wood stripping under a metal roof?

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4 Answers

0
Votes

That's a great question. As a 35+ year veteran of the metal roofing industry, I get asked this question at least weekly.


Here's an article I have written on this subject: https://www.asktoddmiller.com/article...


And here's a link to a video I have made to answer the infamous batten question: https://www.youtube.com/watch?v=pEWcw...


I hope these links are helpful. Fact is, it is a complicated question to answer. There are many variables. The video and the article above go through those.

Source: http://www.asktoddmiller.com http://www.classicroof.com

Answered 1 year ago by AskToddMiller

0
Votes

Whatch yu trying to do - start a roofers brawl, asking a question like that ? Some say almost never use firring strips or purlins except on metal frame roofs, some like me say basically always, some say only when placing it over existing shingles, some hem and haw and say sometimes depending on conditions.


Here are a couple of previous similar questions with answers FYI -


http://answers.angieslist.com/Are-fur...


http://answers.angieslist.com/How-spa...


Basically speaking, ignoring sub-insulated roofs which are a special case (with rigid foam board insulation directly under the metal roofing), the roofing needs proper support to be able to handle snow/ice loads, wind loads, and walking on during installation, cleaning, and maintenance. There are two ways to do this - put in firring/nailing (though nails are not used any more for this) strips or purlins (in steel frame construction) on the roof to support it, or place the metal roofing directly on a flat surface like foam insulation board or the roof sheathing (with roof water barrier in the correct position in the layer).


Some manufacturers want to see firring strips (commonly 1x3 or 1x4) running up and down the roof, supporting the center of the flat part of the panels (not uslaly called for under the ribs because you are not supposed to step there), I have seen 1x2 or 1x3's called out for at the third-points of the flat part (the web) on wider panel roofing.


Others prefer to have the entire panel supported (my preference) with crosswise laid firring strips at specified intervals (commonly about 16-24 inches on centers up the roof).


If you use horizontal firring strips, they should have drainage slots or better (stronger) shallow curved cutouts in the bottoms so any water running down the roof on top of the water barrier does not dam up at the firring strip - and also to provide ventilation to help dry out any moisture that gets in there. Some installers try to meet that drainage need by breaking the firring into short strips (not so blowoff resistant) or by angling them on the roof with a drainage gap in between (which makes for angled fastener rows also - not real workmanlike. Some roofers, to avoid this (more so in the older days) used vertical battens on top of the water barrier and aligned along the rafters, then firring crosswise to screw the decking onto. Works, but raises the roof up quite a bit - which can be fine if looking for a 2-layer roof concept combined with ridge venting for the roofing, but not real common these days because of the extra work, and because most manufacturers are going to crosswise firring strip recommendation.


Some designs are using the up and down roof firring (sometimes in 2x3 material) with crosswise firring strips to provide a deep airflow channel so a lot of air flows from the bottom of the roof to the top ridege vent or ridge cupola - thereby significantly reducing the heat transfer between the roofing and the sheathing in the warm/hot months.


Of course, with metal shingles (as opposed to panel roofing) the mounting or engagement strip (if used) or shingles (if no mounting strip) should lie flat on the water barrier, or bridge across up-and-down slope firring strips at the manufacturer specified spacing.


And if putting them over existing shingles (a VERY bad idea in my opinion), they should go crossways on the roof otherwise you will get waves up and down the roof where they bend over the leading raised edge of the shingles and depress at the upper exposed portion of each shingle row.


It also has to be fastened down properly. This mandates proper securing screws at certain intervals (both up and down and across the roof) - the manufacturer will provide instructions on size and type and spacing of the screws - to prevent uplift and blowoff in wind. Some also provide guidance on the length, others defer to code on that which stipulates (via tables) minimum lengths for roofing fasteners or different sizes and for some types certain penetration beyond the sheathing if it is not long enough for full fastearner pullout development.


So - gave you some of the thinking - but as far as whether you go with it or not, while I recommend it (assuming your roofing is rigid enough to walk on with firring strips under it - some real cheap metal roofing these days is not), talk to your bidders, maybe the manufacturer or your selected roofing material, and your architect if you are using one on your job, for their recommendations - though with 4 people you are likely to get 3-4 different opinions.


Oh - BTW - should preferably be treated wood, but generally not available in those dimensions so adds to cost in having a contractor treat stock before putting it on the roof.

Answered 1 year ago by LCD

1
Vote

By design, some metal roofs MUST be installed over battens, some MUST NEVER be installed over battens, and some can be installed over battens OR solid decking. So, first and foremost, it’s critical to choose the right product and install it per manufacturer specifications.



And, keep in mind, for residential applications, I never suggest installations over battens unless there is a solid deck beneath the battens. Installations over battens alone are an invitation to condensation issues.


Next, I highly advise the use of a layer of underlayment over the old shingles or decking whether or not battens are used. I interpret the International Building Code as requiring it and it’s just a good practice. I normally suggest a layer of one of the higher quality synthetic underlayments, and ice and watershield if required by code.


If your home’s existing shingles are pretty curled, you could install a textured metal roof such as a metal shingle, shake, or tile directly over the curled shingles along with underlayment. You also could install a corrugated through-fastened metal roofing product and probably not have issues. However, a true standing seam, especially one without any sort of striations or stiffening beads in the center of the panels, will probably show oil canning as the result of going over curled shingles. In that case, the old shingles should be removed or battens should be installed.


Now … as to the potential benefit of battens. They do create a thermal break to help minimize heat transfer from the metal to the roof deck. That is a good thing. However, if you choose a metal roof that is either light in color or has reflective pigment in the paint, that will be very helpful as well. Your attic ventilation and insulation will also help with summer energy efficiency.


One effective way to use battens is to cross batten. This involves putting down vertical battens first, attaching them through the roof deck to the rafters. And then you put down horizontal battens for the roof panel attachment. The resulting vertically oriented chambers can then be ventilated by bringing fresh air in at the bottom and exhausting it out at the top. This has very positive results as far as keeping heat out of the attic. It can also help avoid roof top ice dams in the winter. However, cross battening significantly raises the roof level. This means gutters will have to be re-hung. It also could cause the roof to interfere with skylights, clerestory windows, dormers, and even chimneys and plumbing stacks.


And, again, if you have good attic ventilation and insulation, those things alone are very helpful and can be more than adequate in most cases.


One thing to remember is that many of the products that are installed over battens have exposed fasteners. That can be something folks want to avoid. As a great option to the expense and bother of battens, there are concealed fastener metal shingles, shakes, and tiles available. These products usually have an integral airspace or an insulated cavity between the metal and the roof deck. This creates a thermal break which is very similar to a vertically seamed metal roof installed on battens. The dead airspace in the hollow beneath these panels acts like the dead airspace between two panels of glass in a thermal pane window. This blocks conductive heat transfer and helps keep attics cooler.


If horizontal battens alone are used, they should be installed with gaps one to the next to allow drainage. Additionally, per metal roofing manufacturer instructions, battens should never be treated lumber as salts and chemicals in the treated lumber can cause corrosion of the metal roofing panels.



Finally, if a structure has very limited insulation and ventilation (especially ventilation) in the attic, battens can be helpful, especially if a vertically seamed metal roof is being installed. Again, this is from a thermal break / thermal bridging standpoint. A home with limited attic ventilation is at HIGH risk that moisture originating inside the living space can migrate to the attic and cause condensation there. If that moisture hits a cool surface, it will condense. Condensation will lead to mold, etc. This is not good. While your home may have been fine with asphalt shingles and no ventilation, the addition of a vertical seam metal roof will reduce the roof deck temperature even just a slight bit. That can be enough to be the straw that breaks the camel’s back, and cause condensation in the attic that did not exist before.


Additionally, we want to point out … if you install things in your home like tighter windows, house wrap and siding … those things will reduce the ability for moisture inside the home to exit through the walls. This can cause higher moisture levels in the attic. So, if you plan those improvements now, do not do something to the roof that could make it more prone to condensation when those other things are done.



In conclusion, battens can sometimes be helpful but we find those occasions to be few and far between, limited primarily to homes with insufficient attic insulation and ventilation. And, a great alternative to battens are the metal shingles, shake, slate, and tile products which have an integral dead air space between the metal and the roof deck, serving a function similar to that of battens.

Source: www.asktoddmiller.com

Answered 1 year ago by AskToddMiller

0
Votes

Todd - good to see you back on this forum - seems like it has been awhile.


I was surprised to see you first post disappear - guess Angies List thought your links to uyour series of metal roofing Youtuve videos was too commercial ?


A question regarding your answer (#1), and a clarification on two of our comments for readers:


1) You said some metal roofing should never have firring strips - I have certainly seen that, with some of the very thin steel or weak aluminum decking, which could never stand even a foot of free span over firring strips without crumpling/buckling under someone walking on it, so they would have to lie flat on the water barrier over the sheathing. But can you amplify in what case a metal decking product would ALWAYS have to have firring strips (other than supporting strips where there is no solid sheathing under it) ? Aside from the ventilation and energy efficiency factors, I cannot think of a metal panel (as opposed to shingle) product where a firring strip would be necessary in every instance.


2) about treated battens / firring strips - with metal decking which does not have a corrosion protective back coating I can see that being a potential concern, though I have to admit I have never seen a manufacturer instruction sheet or warranty prohibiting treated firring strips. Hopefully, one would not have enough moisture on the underside of the metal decking to cause signficant corrosion. The few metal roofs I have seen taken up with treated wood under them had only slight corrosion blistering and corroded strips across the backs, but no penetration depth to it.


I do remember decades ago when US Steel and Kaiser Aluminum were big metal panel roofing suppliers and ACA/CCA (which are salts) treated wood was first commonly available, they did recommend (but not insist for warranty coverage) that narrow strips of roofing felt be put across the top of the firring strips to avoid direct contact and possible corrosion. This was when all metal roofing was bare steel with no protection on the underside.


3) We both mentioned water barrier (normally large-roll synthetic under metal roofs today). To make clear to readers, if firring strips/battens are used, this should be located UNDER the firring strips so the ventilation airspace is right under the metal where condensation is most likely to occur, and to keep any roof/boot/flashing leakage running off over the top of the water barrier from contacting and corroding the underside of the roofing metal like it would if the water barrier were directly under the roofing (though that is the case with roofing direct on sheathing without firring strips). If an open-grid framed roof or widely spaced old- solid roofing planks as the "sheathing", it is important to tightly stretch this so it does not sag down and hold water between the framing / roof sheathing boards and the metal roofing - obviously, FAR better to put down a solid sheet sheathing first when doing a reroof in that sort of situation.


Answered 1 year ago by LCD




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