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Question DetailsAsked on 2/25/2014

Is it advisable to install a standing seam room over laths rather than plywood sheathing?

We wish to replace the corrugated roofing on our dairy barn that has been converted into a school. Our roofing contractor advises that it's ok to install the standing seam on the existing wood laths. the cost to add plywood to the 10,000 sq ft roof is considerable.

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6 Answers

0
Votes

As I read your question - tell me if wrong, using the Answer This Question button - I envision that your current roof has planking across the roof under the corrugated - with small gaps between planks, like this -

http://www.renovation-headquarters.co...

http://www.newcenturyroof.com/roofing...

or wide-gapped like this

http://inspectapedia.com/roof/Roof_Co...

If actually lath (generally 1/4" thick by 2 or 3 inch wide material), then would look like this (though this picture is of a wall, not a roof) and definitely NOT be suitable to mount the standing seam on -

http://thecraftsmanblog.com/wp-conten...

However, some people talk about 1 by 4 inch slatted roofing like this as lath, which might be OK if close enough spacing and good condition, which would have to be checked by an architect or structural engineer for adequacy-

http://1.bp.blogspot.com/_hEIUcve6q08...

Assuming this is 3/4" or thicker planking, this might be OK to put the standing seam over from a fastening standpoint, though I would certainly have it renailed (without removing old ones) to be sure the planks are still firmly fastened down. Depends on spacing of the lath as to whether it is suitable to carry the loads in your area.

The big problem I see is your roof will be open to full ventilation - with no insulation or sheathing at all ? This will make for a terrible amount of airflow and heat loss, immediate leaks into the school it water gets through the standing seam, and possible "howling" of the roof and rattling in heavy winds. It also makes it far more susceptible to roofing loss in tornadoes, hurricanes, and other strong winds because it will have full differential air pressure from below, rather than the restricted amount you have with sheathing coverage.

IF you can better describe the "lath" - like dimensions, if over plywood sheathing, what type of underlayment the contractor is proposing to put down under the standing seam, etc then maybe we can give a better answer.

Alternatively, if you have an architect on board for this conversion, I would talk to him. Unfortunately, you should probably do so anyway to be sure the roof can handle the snow and wind loads, because the design standards for barns are far lower than for occupied buildings. Of course, if the conversion has already been done without an architect, then good luck - probably just a matter of time before the state fire marshall or local fire department does an inspection and finds things that would be OK for a barn and not for a school. Hopefully, you have had an architect on board from day one for structural, heating, and life safety issues, and he can certainly address this issue for you.

My bottom line gut feeling - assuming your roof is the ceiling for the school (as opposed to an attic), I can't see any way around sheathing it, as well in putting insulation and underlayment in the roofing system during construction. If it roofs an airtight attic floor, then you might get away with it, but get an architect's review on it before you possibly back yourself into an HVAC and life safety code corner with a megabuck roof tht might have to be redone.

Answered 5 years ago by LCD

0
Votes

Regardless of whether you choose to install plywood over the rafters or not I strongly suggest 1x firring strips to secure the metal to. Plywood will not hold screws over a long period of time. Well, maybe if it is thick enough but then you get into some serious added weight issues. I've seen plenty of metal roofs with screws that have pulled out because they were screwed directly to 1/2" plywood or osb. As the plywood expands and contracts it pushes the screws out.


Now as for the plywood itself, it will add a lateral structural element to the roof which will help alleviate some noise and movement. However, you have to make sure the framing can handle the added weight. My opinion is the plywood is not really necessary as long as proper bracing is in place. Given that the barn has stood to date without issues there is a good chance (but not guaranteed) that it is structurally sound in terms of movement. As far as weight is concerned, that is something you should consult with a structural engineer about.


Did you frame in a ceiling or is it open to the rafters? To address others concerns about heat loss/gain I would recommend installing a ceiling to create an attic space and insulate that. Otherwise, you could use thin sheathing on the underside of the rafters (again consult a structural engineer regarding weight) and then install spray foam on that so there is still an air gap below the metal roofing. No matter which route you take, do not install insulation against plywood roof decking or the metal. It needs some air space for condensation to dissapate.

Answered 5 years ago by Todd's Home Services

1
Vote

If you have skip sheathing currently, you can go over top pretty simply. Most of the skip sheathing is better quality with better holding power than plywood in many cases (given its age and density).


Putting down a system of battens will help level the roof and give you a more consistent reveal on the roof.


Plywood is not required but will make the roof quieter.-


How are you conditioning the space, if at all?



Answered 5 years ago by WoWHomeSolutions

1
Vote

Thanks for your comments. The space beneath the roof is attic that is only used for storage. The current lathes are 1 x 6 and generally in good shape. They are approx. 8-12 inches apart now.


Sounds like a careful installation of standing seam attached to 1" lathes makes sense. Adding lathes where there are gaps would be helpful.




Answered 5 years ago by Guest_9895149

1
Vote

1) Good - 1x6 is wider than the normal gap sheathing, so good support. IF your lath is not really firmly attached, I would recommend doubling up on the nailing or better yet screw gunning at each rafter, as previously advised, for a few hundred $ more. Roofer may well insist on this anyway, as that is what is going to be holding the metal roofing down in heavy winds.

2) Also, if the roof does not have angle bracing or diagonal bracing wires between the rafters, it might require that be installed for roof stability - both for structural purposes, and also because swaying of the roof structure will cause wrinkling of the sheet metal roofing and can cause leaks - especially if standing seams separate or if it is the type which has screws penetrating the faces of the sheets and they start moving around due to roof movement.

3) A structural engineer or architect should be able to address that issue for you - most barns would NOT have adequate bracing for a structure with people "habitating" it -which a barn is not considered to be even though one or a few people are working in it on occassion. A school use if a whole different situation.

4) I would also check what the manufacturer requires for maximum gap between lath and close that up as necessary.

5) I presume you are going with an water-resistant underlayment at a minimum, to handle any future leaks ?

6) Insulation is a tougher issue, because that would usually go under the lath strips, but you certainly don't want to tear them all up. Other alternative is foam insulation board over the existing strips, water resistant underlayment over that, then new firring strips over that to fasten the roofing to, or even (if manufactured with air gaps underneath to allow for this) direct placement on the foam - though that requires special fasteners, and because the lath underneath is sporadic would not work well as a lot of fasteners would fail to hit sound material, plus the super-length fasteners are expensive !

7) Certainly it is possible to insulate under the rafters as Todd said - the main drawback of that is you do not see if there are any leaks before possible rafter damage, and of course hard to see where any leak is coming from. In your situation, if attic insulation is not already taken care of, attic floor insulation (and vapor barrier as appropriate) would probably be easiest.

Good Luck

Answered 5 years ago by LCD

2
Votes

+1 on checking with the manufacturer of the panel you intend on using. Make sure, whatever method you use, that you are in compliance with their warranty provisions and requirements.

Answered 5 years ago by WoWHomeSolutions




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