Replies

1. 
   Piffin | Jan 11, 2003 07:57am | #1

   Yes. Definitely!

   A lot of the old pattern will telegraph through, especially in certain lighting and thermal conditions

   The grit from the shingles will wear on the back of the metal finish until it is down to bare metal and it rusts through - one purpose behind using metal is that it lasts longer but yours might not

   The fasteners will have farther to travel and there will be more flexing in the shank in that grey area between the roofing and the sheathing where the old comps seerve as a loose shim. Some fasteners will break off because of this

   Since you can't tighten the fasteners down equally with the comps crumbling under you will find that some of the neoprene washers don't seat properly and some will leak. If your sheathing is boards rather than plywood, It is highly likely that you will hit some airholes that you don't know are there.

   Some fasteners will undoubtedly run into the head of a roofing nail and be slanted off so they won't hold as well and the deformation will cause leaks and unsightly lumps/wrinkles

   You will have a harder time dealing with penetrations like stink pipes.

   The right way to do this if you don't want to tear off the old shingles, would be to strap the roof with 1x4 furring strips or even 2x4 run horizointally. That way you can shim them if the roof is uneven to get it straight to a plane, You are sure it is well fastened against wind, You can easily get trim to cover the strapping at the edges.

   You make sure to place the furring leveled straight across and then all your fasteners line up easy for a good looking roof.

   .

   Excellence is its own reward!

1. 
   teach | Jan 11, 2003 04:30pm | #2

   Thanks for your reply. I am a high school construction trades teacher and I have built several wood frame buildings with metal coverings. I've always put the metal on new wood. On my house I was hoping to save a little time and effort, but felt in the back of my mind that placing the metal flat on the shingles would probably shorten the life of the roof. I already have a large quantity of 1X4 so guess I had better go sahead and use them. Thanks again for your reply.    teach  

1. 
   Piffin | Jan 11, 2003 05:44pm | #3

   Good! I get tempted too to take shortcuts for myself that I would never take for a client and usually regret it.

   Excellence is its own reward!

2. 
   Bruce | Jan 11, 2003 05:49pm | #4

   We bought our house with a similar situation ... metal over asphalt.  Was done by the previous owner to cure the leaking problems they had.  Never had a lick of trouble since we moved in.  After a massive snowfall in '92 here, many people in town did the same.  I haven't heard anything about a rash of problems from them wither.

1. 
   Piffin | Jan 11, 2003 06:02pm | #5

   Yes, it'll work for a few years but won't last the lifetime irt is intended to. And I'm betting that there are a few leaks around screws that don't show up because of the layer of comps under it..

   Excellence is its own reward!