Internal Damp Near Upper Windows — Possible External Causes

Internal Damp Near Upper Windows — Possible External Causes

When damp or mould appears on internal walls near upper floor windows, it is easy to assume the window itself is leaking or that condensation is the cause. In many cases, however, the actual source is an external building defect above or beside the window opening. Understanding these causes helps landlords direct repairs correctly.

Failed Pointing Above the Window

The mortar joints in the masonry directly above a window opening are a common entry point for water. Wind-driven rain is absorbed through eroded joints and tracks downward, appearing on the internal wall at or below the window head. Because the water has entered above the window rather than through it, replacing or servicing the window itself achieves nothing.

Lintel Defects

The lintel spans the top of the window opening and supports the masonry above. In older buildings, stone or brick lintels can crack over time. Steel lintels can corrode and expand, opening gaps between the lintel and the masonry. Both conditions allow water to enter above the window and appear internally at the window head or reveals.

Deteriorated Window Surrounds

The junction between the window frame and the surrounding masonry is sealed with mortar, mastic, or a combination of both. Over time, these sealants crack, shrink, or lose adhesion, creating gaps that admit water. On upper floors exposed to driving rain, even small gaps can allow significant moisture ingress during storms.

Stone Sills and Sub-Sill Drainage

Stone or concrete window sills should project beyond the wall face and incorporate a drip groove to throw water clear. If a sill is cracked, sloping back toward the wall, or has lost its drip detail through erosion, water pools against the wall and enters below the window. This appears internally as damp at or below the window sill.

Why Condensation Is Often Wrongly Blamed

Condensation does occur on cold window surfaces, particularly single-glazed windows in winter. However, if damp persists regardless of season, appears on the wall rather than the glass, or is localised to one window rather than all windows, an external source is more likely. A professional investigation can distinguish between condensation and penetrating damp.

Investigation Approach

An external defect investigation examines the masonry, lintels, sills, and sealants around the affected window from outside. For upper floor windows, this requires safe access to height. Rope access allows close inspection without scaffolding and enables targeted repairs during the same visit if defects are identified.

Common Repairs

• Repointing mortar joints above and beside the window
• Renewing mastic sealants between frame and masonry
• Repairing or replacing cracked sills
• Addressing lintel defects where accessible
• Clearing blocked weep holes in cavity trays above windows

Key Message for Landlords

Damp near upper windows is frequently an external masonry problem rather than a window or condensation issue. Before investing in new windows or telling tenants to improve ventilation, have the external fabric inspected. Correct diagnosis saves money by ensuring repairs target the actual source.