4/14/2009<< Back to Facilities Management Roofing Category Home
Wet Roof Deck?
Compiled by FacilitiesNet Staff
Wet roof decks simply can't be ignored. Recovering a wet roof deck won't make the problem go away. That's because recovering a roof deck will likely only cause the roof to blister, or cause other serious problems that won't be covered by a warranty.
Here's how to tackle a wet roof deck the right way. First, identify wet areas in the roof. To do so, one of three types of moisture surveys should be completed: infrared, nuclear or capacitance. None of these actually measure moisture. Instead, they measure the effect moisture has within the roofing materials.
For best results, match the type of moisture scan with your roof since each moisture scan works differently. Infrared surveys measure the heat retained or lost in insulation that has become damp. Ballasted roofs aren’t a good candidate for infrared surveys because the rock itself retains a lot of heat, giving potentially false readings. Nuclear moisture surveys measure hydrogen atoms in the roof, meaning that any membrane with a large hydrogen chemical component will send positive readings. Water is a good conductor of electricity, and capacitance surveys measure electricity traveling through the roofing material. This won’t work on a roof with wet or ponded areas, and may require modified instruments on EPDM roofs.
The amount of moisture found in the roof, if any, may dictate the choice of whether to recover or replace. The more areas of moisture found, the more expensive it will be to replace those areas.
If you have 100,000 square feet of roof and four 8-by-10-foot areas are wet, replacing those sections makes sense. But if 30 percent of your roof is wet and it’s scattered throughout the roof, the labor to replace all of those sections probably equals the cost of just tearing off the entire roof.
But what if the roof is leaking just after a recent replacement? It's not that farfetched — due to poor design or installation, many roofs experience water leakage soon after construction.
In many instances, water leakage through a roof membrane can go unnoticed because a vapor retarder at the bottom of the roof system captures the water. The captured water absorbs into the insulation, significantly decreasing the thermal value of the insulation and causing premature deterioration of the roof system.
A properly prepared and implemented design can prevent water from migrating into the roof. Generally, the membrane in the roof’s field does not allow bulk water leakage. Most leaks through a system arise from unreliable detailing.
Roofing system manufacturers provide standard details for perimeter conditions, which typically have the flashing exposed and terminated on the wall surface. They rely on sealants to prevent water infiltration. Manufacturer details typically do not address leaks around the roof system.
For example, in most instances, roof terminations consist of surface-mounted conditions — exposed termination bars or metal flashing — or reglet-set flashing — a small cut in a wall system to insert the metal flashing. In a brick masonry wall, these types of terminations are not appropriate because water can bypass the flashing, infiltrate through the masonry, and migrate into the insulation. Instead, the design of roof flashing for a masonry wall should incorporate a through-wall flashing that extends through the masonry to capture and divert water out of the wall above the flashing.
Building Envelope: Keep Water Out of the Roof by Eric J. Seaverson, P.E.
The Roofing-Mold Connection