david gunn
2/19/2010
Here is the mystery. I'll spell out some of the pieces of the puzzle although this may not all be related. At this point I’m throwing darts. I'm helping a client track down a problem on a 5 year old home. Since the house was new the owner has been fighting roof leaks (or so they think) The roof is a copper, standing seam roof on a fairly low slope, maybe 2/12? The house is effectively a rectangle. The roof has a ridge down the middle of the long dimension of the rectangle. Along the long sides of the house is an integral copper gutter behind a parapet wall. The assembly is as follows, from the top down: 16" wide copper panels, 30 lb felt paper, ice and water shield, 3/4" plywood deck. Below the roof is a more or less unvented attic (only about 24" tall at the ridge). Below that are ceiling joist with r-30 batt insulation and a drywall ceiling. There are a number of roof jacks installed for ventilation either side of the ridge (see photo) that allows some heat to escape from the attic space. This is a three story town home. On the third floor there are two mechanical rooms that contain gas fired furnaces. Those mechanical rooms have open ceilings therefore basically tying them into the small attic space above them. The 3rd floor of the house has a very low humidity level, ranging between 30 and 35% over the past few weeks. This has caused a lot of separation of wood crown moldings. (I’m interested to see what that does during summer months with the ac running). The owner thought the separating wood work was an indication of a roof leak. However, I think it is more related to the humidty levels. Recently they’ve been running the heat. There shows evidence of leaks (or so we thought) in a few locations. We thought it was linked to the integral gutters on the roof and one of the leaks may have been found with a small spilt joint in the gutter. However, three good roofers have been unable to find leaks in other locations. In looking for the leak(s) we pulled apart one of the copper panels on the roof to find moisture under the copper panel at the bottom 4 or 5 feet of the roof slope. As a check we pulled another panel on another area of the roof to also find moisture under the bottom 4 or 5 feet of the roof slope. My thoughts: Do we need more attic ventilation? Should we cut in some roof jacks for ventilation toward the lower end of the slope to vent the attic? Since there is a parapet wall with an integral gutter is there no way to install any type of typical soffit or eve vents. Is the low humidity in the house an indication of something else other result from typicaly winter heating? Are the furnaces in the 3rd floor mechanical room introducing too much additional heat into the attic when it’s cold outside? My HVAC contractor is worried about adding too much attic ventilation because he is worried the air ducts in the attic could sweat during our hot, humid summer months. I’ve seen that problem before as well but I’ve got to think these jacks can’t be adding that much venting. The interior of the 3rd floor mechanical rooms have heavy mildew on the walls and hvac equipment telling me there is substantial humidity levels in those areas, probably during the warm summer months and because they are effectively the lowest point in the attic (thinking the heavy humid air would settle there first). Those rooms are also not well sealed against the surrounding conditioned areas so the mildew there doesn’t surprise me. Obviously I need to eliminate any potential leaks in the integral gutter but those don’t seem to explain the moisture on the underside of the metal panels. Any thoughts or suggestions?
david gunn
2/19/2010
Another question was whether the standing seam joint could be leaking on such a low slope. It is probably a 1" tall seam. If so why would it happen only at the bottom of the roof slope?
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2/19/2010
I think you are on the right track with the mechanical room. Where are the flues vented? The fact that you have the mildew condition in that area is indicative of the source of the issue. The most likely answer is usually the right one. The mildew in that room is probably from moisture from the room but I would also be suspect of reverse movement of moisture in the hot and humid summers down there just as your HVAC contractor was concerned about sweating duct work. That room needs to be sealed regardless. One of the combustion by products of burning gas is water that is carried in a hot exhaust stream. If the flues are not vented through the deck and properly sealed, you could certainly have enough leakage to push too much moisture into that attic space and cause problems. I would also want to see that mechanical room sealed off from the attic space completely and sealed from the living space as well. Just double check that flu drafting is not negatively impacted and you are still exhausting the gases properly. Once that is done, I suspect you will have cured the issue, however, the fact that the sweating in on the perimeter edge of the roof could also be due moisture migration from the living space inside the walls. If the building envelope is complete and intact, moisture diffusion is usually slow enough to either side of the envelope that it will not cause problems in terms of condensation. If you have larger bulk moisture movement, you could certainly get sweating on those panels. I would check the condition of exhaust fans, where the showers and bathrooms are, dishwasher, and other sources of moisture. Post back some follow up as I would like to see what the resolution is.
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