Common Roof Issues – Capillary Water Action in Roofing
What is Capillary Action?
Capillary action describes how water moves through narrow spaces, cracks or overlaps in roofing materials, often seeming to defy gravity. The movement occurs due to the forces of adhesion from water sticking to surfaces, cohesion from water molecules sticking together and surface tension. In roofing, it typically occurs:
- Through tiny pores in materials like concrete, masonry or wood.
- Along overlaps in steel roof sheets or membrane roofing sheets, especially if seals degrade or gaps remain.
- Beneath flashing where water is “drawn” upward or sideways into the roof assembly during rain or in warmer more humid environments.
As you may expect, even small amounts of water being drawn into the roof space below the surface is never a good thing. That’s why capillary water action can be problematic in a big way.
How Can Capillary Action Damage a Roof?
When capillary action pulls water below the roof surface into tight spaces serious issues can start without you even realizing it. Damage to the roof system caused by capillary action includes:
- Hidden Roof Leaks From Capillary Draw: Capillary draw is a roof leak that is caused by capillary action. As capillary draw occurs water sneaks under asphalt shingles, the flashing and overlaps, reaching into roof cavities, insulation and decking.
- Mold, Rot and Decay: Persistent accumulating moisture from capillary intrusion breeds mold, causes wood rot and leads to roof deck deterioration.
- Rust and Corrosion: In metal roofing systems, trapped moisture accelerates rust at the laps and seams.
- Efflorescence and Plaster Damage: In masonry or concrete roofs, capillary water brings salts to the surface, causing visible damage and crumbling.
How to Repair Capillary Action Damage Within the Roof Structure
The following table outlines common capillary action problems, their signs, and appropriate repair approaches:
| Problem Type | Signs | Repair Approach |
|---|---|---|
| Flashing Leaks | Drips at roof penetrations | Reseal or replace flashing properly with drip edges and weatherproof sealants |
| Mold, Rot or Decayed Roof Decking | Musty smells, spongy wood | Remove affected materials, treat with fungicide and replace decking |
| Rust/Corrosion at Laps | Red stains, weakening metal | Clean rust, prime, reseal joints and use bituminous or elastomeric sealants |
| Efflorescence/Crumbling Plaster | White deposits, soft plaster | Chip out the damaged plaster, treat substrate, apply damp-proof barrier and replaster |
