low slope roof construction

huangyhg

low slope roof construction
i am investigating a low slope (2:12) roof that is leaking.  the roof consists of wood trusses at 24" and 48" oc with plywood sheathing, roofing felt, and asphalt shingles.  a 2" air space and insulation is provided below the roof.  the attic space is ventilated.  the leaking occurs after it snows and the snow begins melting off the roof.  i believe i have a handle on what is causing the leaking.  i am now considering recommendations.  my first thoughts is to extend the ice shield all the way up the roof or provide some type of membrane over the sheathing.  does anyone have any other thoughts or recommendations for type of membrane?
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few points:
1. first of all the 2:12 slope is low for water drainage and snow shedding. the steeper the faster the water will get of the roof.
2. the 48-inch on center roof trusse spacing seems odd for plywood. if the plywood is not rated and does not have the proper thickness, you will have "drapes" between the trusses that is not good for shedding the water and the weight of the snow.
3. i would consider standing seem roof in lieu of shingles and or membranes. i am a firm believer in standing seem roofs as far as stopping leaks and wind residence. they also have small weigh per square foot.
regards,
lutfi
to be a little clearer.
the trusses at 48" on center have blocking between them to support the plywood.  no apparent sags in the roof sheathing.
this is an existing building.
i am planning on presenting some options to the owner for the roof.
1.  provide membrane and re-shingle
2.  provide membrane and standing seam roof
3.  ???????
is standing seam deck alone (no membrane) at this low slope good enough?
pre-engineered metal buildings come in with metal roofing at 1/2" per foot slopes.  i would think that a metal roofing would work as long as it is designed for the slope.
...but i'm a simple se - roofing stuff scares the hell out me.  perhaps some calls to local roofing contractors or perhaps a savvy architect might help.
i have seen standing seam be installed on slopes of 1.5:12. this is typically what pemb suppliers do.
check with various suppliers for their recommendations.
regards
lutfi
lutfi
sperlingpe...2:12 is too shallow for shingles, as lutfi noted.  as snow melts, capillarity will pull it under the shingles at such a low slope.  i agree with jae that metal roof panels would be better for this application.  the panels can extend the entire length of the slope, thus no backwater condition.
if you intend to use shingles again, i would not depend on them to be the "watershedding" course.  use ice and water shield or similar under entire shingle area or hot mop a two-ply "roll roofing" or modified bitumen system under the shingles.  treat it like tile...decorative only.
the roof slopes of my house vary from 3:12 to 1.5:12.  when i replaced the roof a couple of years ago, i covered the entire roof with ice & water barrier and shingles.  i consider the barrier to keep the water out, and the shingles as a protective cover.  it worked out fine, but be advised that the ice & water barrier cost more than the shingles did.
a metal roof was my other option, but i did the work myself and i already knew how to install shingles.  i didn't feel like learning how to do a metal roof on my living quarters.  i also didn't want the snow removal characteristics of a metal roof (most all the snow at once - usually after you've shoveled).  i compromised by installing a metal ice belt at the eaves.
a rubber membrane roof is another option.  the roofing contractors i talked to almost all suggested a rubber membrane or a metal roof, probably because they had to give a warranty.  (something you lose when you do the work yourself!)
metal roofs will probably be better, but it also may come down to a cost/benefit exercise.
i work at a full service a/e firm (lower case e on purpose).  i accompanied an architect on this site visit in case there was damage to the framing due to the water.  when we got back to the office, the archtiect (a principle in the firm) asked me to follow up.  so i am not learning about roofing and roofing construction.  i will be pricing out the options that have been suggested here.
nobody would consider altering the slope of the roof or building a flat roof over the sloped roof and adding internal drains?  yeah, i know that these options would be expensive and would change the look of the building.  would these options provide more of a long term solution less ikely to leak in the future due to the current problem?
most, if not all, shingle manufacturer's recommend placing their product on roof slopes 3:12 or greater.  other roofing material, such as modified bitumen rolled roofing and metal roofing are recommended for shallow and flat roof slopes.  replacing the shingles with a proper roofing material for this application should stop the leaking.  however, although both products will do the job, the metal roof has a much longer service life and, in my opinion, greater aesthetics.
as for changing the appearance, being as the roof is only 2:12, how much of the roof surface will be visible?