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TOPIC: Underlayment

Horz wood furring strips (sleepers)

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I've noticed that many builders/roofers prepare their roof decks for metal roofing in ways that defy logic. First, they'll install the underlayment perpendicular to the rake of the roof, which is correct and will provide the building with a water tight membrane, but then many of them will nail continuous horizontal sleepers (purlins) directly over that waterproof membrane on 24" centers completely defeating the purpose and function of the underlayment. Any condensate or other water that gets into that space will have nowhere to go except to collect on these many dams that were created and then eventually find a penetration in which to enter the structure. Other builders will install short sleepers, again horizontally, with gaps all along each run presumably for water to escape and eventually exit at the eave. That second approach is better than continuous sleepers but far from adequate. It surprises me that very few roofers seem to do it logically and correctly. By that I mean by providing a series of vertical sleepers installed along the rake of the roof before putting down the horizontal ones in the normal manner thus minimizing any chance of water blockage and entry as well as providing a genuinely open space throughout the underside of the metal roof to allow for ventilation and condensate mitigation. Comments are welcomed
Guest User

4/26/2008
Hi Leon You're the first person I heard mention this even though it's always bothered me too for the same reasons. Maybe it's economics but the cost of replacing a moldy attic would be worse. Maybe it's because it's residential roofing as opposed to commercial roofing where, on the latter, any screw up on details can result in big bucks settlements. Installing vertical strips first before the horizontal strapping makes a ton of sense to me. I'd even run a bead of caulk down the sides of each vertical just to be sure. Roofers are usually always on top of the importance of logical water control designs but every now and then you get these practices or habits that defy logic.
Guest User

4/26/2008
There has been several times the panel of experts have recommended exactly what you are talking about. You may have missed their posts. It has been mostly people asking questions that have suggested the pulins of lathes, whichever you want to call them. I prefer a good underlayment and neither purlins or sleepers. I do not put a metal roof on a residence without a solid deck. You are encouraging condensation problems by not having one. No, decking does not have a rated "R" value above a couple of decimal points, yet there is insulation value in the mass of wood. In the mid 80's a research firm built two identical houses, I believe in Minnesota. One house was traditional framing with fiberglass batts in the walls and ceilings for the recommended R value of homes being built at that time. The other house was built of 8" wood logs with no insulated walls. Other than that they were the same floor plan. They were set to keep the exact same temperature for the testing months of December, January and February. It was supposedly one of the coldest winters on record. For the 3 months the house with the wood logs electric heating bill was $5.00 more than the traditional built house. That was for 3 months, proving that there is an insulation value just in the mass. I have not had any condensation problems with metal installed over a solid deck. The information for the above test was gotten from a RSI issue I received probably in 1987. [email protected] Also, Owens Corning used to have a TV commercial stating that 6 inches of fiberglass insulation had more R value than 19 foot thick stone. If I really needed protection from a cold bitter wind, I think the cold would have a harder time getting through the stone.
Guest User

4/26/2008
Thanks Wade. The questions they have brought up are good and your response is great. If the choice is made to put vertical battens followed by horizontal ones on top of an existing deck, and then metal panels on top of that, the resulting air chamber should be vented with intake vents at the eave and a ridge vent. Current studies show significant summertime energy savings can be achieved in hot climates though this sort of thing which is being called "above sheathing ventilation". Todd Miller
Todd Miller
Isaiah Industries, Inc.

4/27/2008
Wow. Purlins, furring, strapping, sleepers, lath...I guess a rose by any other name really is still a rose. One more thing for the unseen panel of experts: shouldn't these strips (1X4's or whatever) ideally be of "wood naturally resistant to decay"? (Building Code phrase)
Guest User

4/27/2008
Yes, but that becomes less critical if you have vertical battens first followed by horizontal. Todd Miller
Todd Miller
Isaiah Industries, Inc.

4/27/2008
Thanks Todd
Guest User

4/28/2008
Sure. You are welcome. Todd Miller
Todd Miller
Isaiah Industries, Inc.

4/29/2008
Regarding the "vertical battens followed by horizontal ones" - can you refer me to a published spec for such a thing. I don't think I have talked to a roofer who would take such a suggestion seriously.
Guest User

7/26/2008
I would say this needs to be approved by the roofing manufacturer so you would need to get the spec from them. This is done for energy efficiency though. You may want to look into research being done at Oak Ridge National Laboratory into "above sheathing ventilation" Todd Miller
Todd Miller
Isaiah Industries, Inc.

7/30/2008