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TOPIC: Roofing over existing roof

purlings question

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Hello -- we just had a metal patio roof put in and it is nearly finished --about a day to go. The contract said that purlings would be used. However, they did not do this. Upon asking I was told that they had decided not to do the purlings but attach it directly to the existing roof, which is single-shingle, with some pieces missing from a storm. They told us that it would make no difference in cost to put purlings on or not. Please let us know before we pay the other half of the bill if this sounds fishy, or could it be that the purlings are not always used, depending on the particulars of a certain roof. Seems to me like we should be getting a discount. Thanks - EB
elsa backus

6/18/2004
The answer to the question of "To purlin or not to purlin" is first determined by the profile of the roofing and how its manufacturer says it should be installed. Some products should be installed over purlins, some should not, and some can go either way. One purpose of purlins can be to help provide extra ventilation and even energy efficiency. This is most easily done if vertical purlins are installed prior to the horizontal ones so that eave to ridge venting can occur. If the roofing was installed over the old shingles, I hope that an underlayment was put between the two. Otheriwse, the scratching against the mineral surface of the old shingles could cause some problems over time, particularly if your metal roof panel is designed to expand and contract.
Guest User

6/19/2004
Attn: Todd-- Thanks for the advice. It seems to us that the contractor would not have specified purlins originally (he spelled it "purling") unless there was a reason. No, we did not see any underlayment going on between shingles and metal. The metal was put on the old roof with a screw gun. The reason for not doing the purlins was given that it would have raised the height too much so that the patio roof ran into the flashing above and then they could no longer "warranty the job" if that was done (whatever the heck this might mean !). As for what the manufacturer is of the metal product, we'll need to check into this to see what's recommended. Thanks EB and LB
Guest User

6/19/2004
Their expressed concern about raising the metal panels too high in relationship to the roof they tie in to is certainly valid. Transitions like that are always tricky, and doubly so if the metal panels are so high that they create a negative pitch flowing back to the upper roof.
Guest User

6/19/2004
Todd, What is a "negaive pitch flowing back into the upper roof" Would you exlain that to Elsa in plain terms please? She is having problem understanding why the other contractor would not give a waaranty if they use purlings.
Guest User

7/31/2007
This has been going on for three years? When you raise the level of the lower roof by putting it on purlins, then the flashing off of the higher roof also needs to be raised. If you keep that flashing the same then you will create a bad situation with (probably) a negative (uphill) slope to the flashing. The problem is that, depending upon the roof configuration, it may be a problem to mount that flashing higher on the upper roof, Todd Miller
Todd Miller
Isaiah Industries, Inc.

8/1/2007