Balcony door thresholds must provide continuity for air and water control layers while accommodating foot traffic, door swing, and structural movement. This article outlines best practices for installing and sealing balcony door sills in multifamily and commercial applications.
Balcony Threshold Challenges
These thresholds sit at the intersection of exterior decking, sloped concrete, or cantilevered slabs and interior floor finishes. Improper detailing leads to interior water leakage, condensation, or energy loss. Movement from deflection and thermal cycling increases the need for flexible, durable seals.
Waterproofing and Air Barrier Integration
- Install liquid-applied waterproofing beneath the threshold extending onto deck or balcony slab
- Use pan flashing with a continuous back dam under the door sill to direct water outward
- Seal both interior and exterior edges of the door sill with flexible, UV-stable sealant
- Lap exterior deck membrane over the vertical leg of sill flashing or metal transition plate
Drainage and Sloping Strategy
Ensure exterior deck has minimum 1/4″ per foot slope away from threshold. Provide scuppers, deck drains, or perimeter gutters to relieve ponding. Use flashing with built-in slope when threshold is nearly flush with the interior floor.
Field QA and Inspection
- Check sealant adhesion and verify bond at both interior and exterior sill edges
- Confirm continuous back dam is present and not compressed by fasteners
- Inspect drainage slope under sill using laser level or manual pitch tools
- Test for air leakage and water resistance per ASTM E783/E1105 as needed
Photograph waterproofing steps during installation for warranty and code compliance.
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