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TOPIC: DIY - Do It Yourself

PVC boards under terne roof

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1937 house, painted raised seam terne porch roof in good condition. Problem is original inadequate flashing design and leak. I removed part of porch ceiling to access underside, propped up rotted joist and rafter end, removed about 10 square ft rotted sheathing 1X8s.Underside of terne is still good, red coating seen. Will fix leak. Will replace structural wood with bolted-in treated lumber. New treated 1X8s not recommended as being too corrosive touching the metal roof. Should I use plain wood 1X8s? No problem if awkward leak site is fixed well, but if it recurs, using PCV 1X8 board is expensive but would avoid future rot. Are they too hard and expand/contract rub against the terne? By not breathing would they trap water. Any other risk? Thanks for any thoughts.
Harvey Lankford

12/5/2010
Plastic/composite lumber will probably expand and contract a bit more than acceptable ranges for structural lumber. You could use PT and just put an acceptable vinyl flashing between the metal and the beam or go with the untreated and hope you fix the moisture problem. Given the vertical orientation of the beams, it is unlikely they will rot as they will want to shed their moisture.
Eric Novotny
An informed customer is the Best Customer!

12/6/2010
Plain wood will work fine. You can also use "Blue-Wood" borate treated 1x4s as they don't have a chemical reation with the zinc coating. Just make sure you don't use copper oxide treated wood.
Nate Libbey

12/6/2010
Thanks unfortunately the specs on Blu wood say that while it is no more corrosive than CCA, this means that it IS still corrosive (but not as much as newer PT), therefore it is not approved for use in contact with metal roofing such as terne (per Follansbee) The vertical vs horizontal orientation did not help before toa void rot- all types of exposures (rafter end, joist end, roofing planks) were all rotted by the original problem The use of plain modern wide-grain wimpy lumber bothers me for this repair. The former, denser, harder, resin-loaded 1X8s of 1937 are not available so easily. So to avoid the corrosion problem of PT, to avoid the rotting risk of plain wood (if/when the leak repair fails), I thought of using the PVC. All the choices will outlive me, but I still want to do it right. I imagine there is little experience with what I am proposing to do.
Guest User

12/6/2010
Maybe I read the specs wrong on blu wood. Can't find where I read it was similiar to CCA in corrosion. (Affecting hangers and bolts is one thing, thinner older worn roof metal might be another.) Thanks ,will look into it as it might be good solution.
Guest User

12/6/2010
Boric acid treated wood is as easy as it gets in terms of treated wood. I figure that the fact that the roof is from 1937 is just rotting out is probably a good indication that the new roof should live the same amount of time if water tight. Are you sure the rot is the result of a leak or is it from accumulated condensation? Plenty of rotted out decking is from years of condensation. You can still use a vinyl flashing isolator to keep the metal off of the PT wood. http://www.duraflash.net/ Most decking ledger boards that generate leaks are a result of galvanic failed flashing.
Eric Novotny
An informed customer is the Best Customer!

12/6/2010
please see attached pic 15 X 35 ft porch, only one leak area at the complex intersection of a decorative brick wall "wing" and a valley coming down from above-left. There was inadequate original flashing design or installation at a curved area (not shown and out of sight above the rotten rafter in picture) and at the brick corner. I wondered about condensation too, but no evidence anywhere.
Guest User

12/6/2010
The staining on the brick and the pointing would tell me that is a flashing leak as well. Get the flashing detail right and you should be fine without PT lumber.
Eric Novotny
An informed customer is the Best Customer!

12/6/2010