Water Leaking In Around a Window Frame: Finding the Real Source

Water travels before it appears

The single most useful thing to understand about a window leak is that the wet patch is where the water stopped, not where it started. Water gets in somewhere, runs along a cavity, a lintel, a frame chamber or the back of a sill, and emerges wherever gravity and the construction let it. That is why a stain at the bottom corner of a reveal is so often caused by a defect at the top of the opening or even further away.

It also explains why the leak seems to come and go. Many only appear with rain driven from a particular direction at a particular strength, which means still weather tells you nothing and a calm survey can miss it entirely.

Rule out the things that mimic a leak

Before hunting for an entry point, make sure it is a leak at all. Condensation running down the glass and pooling on the sill produces a very similar looking wet patch, but it appears on cold mornings rather than during rain and the water is clean. Penetrating damp from a wall defect will spread across plaster rather than concentrating at the frame. Rising damp shows a tide mark low down and is not related to the window.

Timing is the giveaway. Note whether the water appears during or shortly after heavy rain, on cold still mornings, or continuously regardless of weather. Each answer points somewhere completely different.

• Only during or after driving rain: genuine water ingress
• Cold mornings, clean water on the sill: condensation
• Constant regardless of weather: look beyond the window
• Worse from one wind direction: usually external sealant or pointing on that face

Blocked drainage is the leading cause

Windows are not designed to keep every drop out of the frame. They are designed to let a small amount in and drain it back out through slots along the bottom of the outer frame, usually hidden behind small covers. When those slots block with dirt, moss, paint, insects or old sealant, the water has nowhere to go, the chamber fills and it eventually finds its way over the internal edge of the frame.

This is the fault most often mistaken for a failed window, and it is frequently the easiest to fix. Clearing the slots with a thin brush, and checking they actually run clear by pouring a little water into the frame, resolves a good proportion of the leaks we get called to.

• Find the drainage slots along the bottom of the outer frame
• Clear them with a soft thin brush, not a screwdriver
• Check the covers over them are present and not blocked flat
• Pour a small amount of water in and confirm it runs out
• Repeat annually, and more often under trees

Sealant, sills and the surrounding structure

If drainage is clear, the next place to look is the joint between the frame and the building. External sealant has a finite life, and once it cracks or pulls away from either surface it becomes a channel rather than a barrier. It is worth looking closely at the whole perimeter, particularly the top corners, rather than only where the damp shows.

Then there is the sill. A sill needs a fall away from the building and an effective drip so water leaves rather than tracking back underneath. Sills with the fall lost, missing end caps, or sealant applied along the front edge in a way that traps water are common culprits. Above the window, defective pointing, a cracked lintel, a failed cavity tray or a blocked or overflowing gutter above the opening will all deliver water into the head of the frame and it will appear as a window leak.

• Cracked or shrunken external sealant, especially at the top corners
• Missing sill end caps or a lost fall on the sill
• Defective pointing or render cracks above the opening
• Blocked gutter or downpipe discharging over the window
• Failed cavity tray or lintel detail above the head

Faults in the window itself

Windows do leak in their own right, so this is not about deflecting blame. Perished or shrunken gaskets stop the sash sealing when closed. A dropped sash no longer compresses evenly, leaving a gap on one side. Internal glazing beads with failed gaskets can let water past the glass edge into the frame. Failed corner welds on older uPVC frames can let water through the frame itself, and old timber frames with rot at the bottom rail leak readily.

Where the sash is at fault the pattern usually follows the opening light, with water appearing along one edge of the sash rather than at the perimeter of the outer frame. That distinction is worth noting before anyone visits.

What we would rather you did not do

The most common self inflicted problem we see is sealant applied generously to the outside of the frame, including over the drainage slots. That seals the escape route, so water that gets in stays in, and a modest leak becomes a persistent one. If you seal anything, never seal the drainage.

Similarly, sealing along the bottom of a sill where it meets the wall can trap water behind it, and expanding foam pushed into a gap externally holds moisture against the frame. Leave those to a proper diagnosis.

Getting it traced properly

Because these leaks are so often about the surrounding structure rather than the glazing, a proper look matters more here than with most faults. We survey free of charge across the Wirral, Chester and roughly thirty miles around, and if what we find is pointing, a gutter or a sill detail rather than the window, we will tell you that plainly rather than quote you for a new one.

Common questions

Why is water coming in around my window only in heavy rain?

Most window leaks depend on wind driven rain hitting a particular face, which forces water past a defect that stays dry in ordinary weather, so the leak looks intermittent.

Where are the drainage holes on a uPVC window?

They are slots along the bottom of the outer frame, usually behind small covers on the outside face, and they are designed to let water that enters the frame drain back out.

Should I seal around the outside of my window myself?

Be very careful. Sealant applied over the drainage slots traps water inside the frame and commonly makes a leak worse rather than better.

Can a leak be coming from above the window rather than the window?

Frequently. Defective pointing, a cracked lintel detail, a failed cavity tray or an overflowing gutter above the opening all deliver water into the head of the frame and it appears at the window.

How do I tell a leak from condensation?

Leaks appear during or after driving rain, while condensation appears on cold mornings and clears during the day, so the timing usually settles it.

Does a leak mean I need a new window?

Often not. Blocked drainage, perished sealant and sill or structural defects are more common causes than a faulty window, and each is dealt with differently.