Balcony Membrane and Door Threshold
A 14 ft by 22 ft second-floor balcony sits over a conditioned living room. The finished deck slopes toward the house at about 1/8 in./ft. A single-ply membrane was installed over plywood, turned up 4 in. against the siding, and terminated with a nailed wood trim. The balcony door threshold is 1/2 in. above the deck surface. After snowmelt, water stains appear on the living-room ceiling near the door and along the outer edge. The owner wants to replace only the deck boards and re-caulk the threshold.
1. Diagnose the likely water paths and explain why caulking will not solve the problem.
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- The deck slopes toward the house, so water is directed to the door and wall instead of away. At 1/8 in./ft, the slope is too shallow for reliable drainage on this type of assembly.
- The 4 in. membrane upturn terminated with nailed wood trim is a suspect termination. Nails penetrate the membrane, and the trim can trap water against the siding.
- The door threshold only 1/2 in. above the deck leaves little protection against snowmelt and wind-driven rain. Water can enter at the threshold, behind the siding, or at the membrane termination.
- Stains near the outer edge suggest the membrane may also be compromised at the perimeter. Caulking the threshold is a surface fix and will not correct slope, termination height, or concealed flashing.
2. Specify the correct repair sequence and critical flashing details.
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- Remove the deck boards and investigate the membrane and substrate. Remove wet or deteriorated plywood.
- Rebuild the deck slope to drain away from the door at a minimum of about 1/4 in./ft, with a positive outlet or scupper. Do not leave a flat or back-sloped area.
- Install a new membrane with manufacturer-approved upturns, termination bar, and counterflashing. Integrate the membrane with the wall weather barrier and a door pan flashing that extends under the threshold and up the jambs.
- Raise or rebuild the threshold as needed so it sits sufficiently above the drainage surface. Replace decayed siding and seal only as a secondary protection.
- Do not cover the membrane until the flashing sequence and drainage are inspected.
3. What testing and quality-control steps should be required before the balcony is closed up?
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- Plug the outlet and flood-test the membrane with at least 1-2 in. of water for 24 hours, or use the manufacturer-approved test method. Inspect the living-room ceiling and perimeter from below.
- Verify slope with a level and check that water drains freely without ponding.
- Inspect the door pan, membrane upturn, termination bar, and counterflashing before deck boards are reinstalled.
- Photograph the concealed details and require a final inspection of fasteners, sealant joints, and perimeter drainage. The owner should understand that replacing boards and caulk alone would not have corrected the defect.