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RCI Interface “Low-Sloped Roofs Are Rotting - Case Study Resolution”
An INSPEC article on low-sloped roofs deteriorating under a sound membrane: condensation versus leakage, tracing the moisture source, and correcting the assembly.
Article by Dwight Benoy, Pam Jergenson and Gary Patrick
Read the full article (PDF)Background on this topic from Inspec
What this article covers
This INSPEC article, published in RCI Interface, looks at a problem building owners rarely see coming: a low-sloped roof that is deteriorating from the inside out while the membrane above it still looks serviceable. It follows a case from the first signs of trouble through the investigation of the moisture source and the resolution of the assembly.
The point of the piece is not that a roof leaked. It is that the water causing the damage never came through the roof at all.
Why low-sloped roofs rot from within
A low-sloped roof is a stack of materials, each with its own permeability, temperature and tolerance for moisture. When warm, humid interior air reaches a cold surface inside that stack, it condenses. The condensation wets insulation, sheathing, nailers and the deck itself, and because the membrane above is doing its job, the moisture has nowhere to go.
Air leakage from the occupied space
Unsealed deck joints, penetrations, wall-to-roof transitions and gaps around mechanical curbs let conditioned air move up into the assembly. Air carries far more moisture into a roof than vapor diffusion does, which is why a roof over a humid interior — a pool, a kitchen, a laundry, a natatorium-style space — can rot while an identical roof next door stays dry.
Vapor retarders in the wrong plane
A vapor retarder placed on the wrong side of the insulation, or interrupted where the roof meets a wall or a curb, can trap moisture rather than control it. Assemblies that were reroofed over existing layers are especially prone to this, since the original vapor control layer may now sit in the middle of a thicker, colder stack.
Moisture that was already there
Insulation that was wet before the new membrane went on does not dry out under a watertight cover. Rain during construction, a saturated existing deck, or wet fill on a lightweight concrete deck all become permanent moisture sources once they are sealed in.
What owners actually notice first
- Staining or drips that appear in cold weather and stop in mild weather
- Fasteners backing out, or a membrane that feels soft or spongy underfoot
- Rust streaks at deck flutes, or deterioration at nailers and blocking
- Interior finish damage with no rain event to explain it
- Rising energy use as wet insulation loses R-value
Because these symptoms mimic a leaking membrane, the common response is another repair or another reroof over the same failing assembly. That resets the clock without changing the cause.
Separating condensation from leakage
An investigation has to answer one question before any design work begins: is water entering from outside, or forming inside? That means looking at the whole assembly rather than the membrane alone — reviewing the original and reroof construction, opening test cuts to see the condition of each layer, checking interior humidity and pressure conditions, and correlating when the symptoms appear with the weather that preceded them. Our article on sourcing leaks to the interior of buildings covers the water-entry side of that question in detail.
Resolving the assembly
Once the moisture source is identified, the resolution is a design problem, not a patching problem. It typically involves establishing a continuous air barrier at the deck, placing vapor control where the assembly's temperature profile calls for it, replacing materials that cannot be dried, and detailing the transitions at walls, curbs and penetrations so the control layers stay continuous. Where the moisture behavior of a proposed assembly is uncertain, hygrothermal modeling can be used to test it before construction rather than after.
The deck material shapes what is possible. Our Roof Decks A to Z series looks at how individual deck types behave under moisture, including oriented strand board and structural fiberboard planks.
If your roof is showing these symptoms
INSPEC is a consulting engineering and architecture firm. We investigate, test and design; we do not perform the repair work, so the recommendation you receive is not tied to a scope of work someone else wants to sell. If a roof is deteriorating without an obvious source, a forensic roof investigation establishes what is happening inside the assembly, and roof consulting turns those findings into a corrected design. Contact us to discuss a building.