Cracks around every door and window frame? The gap nobody filled correctly

Look along the edge of almost any door or window frame in an older building and you'll find it: a fine crack running down the line where the frame meets the wall. Sometimes it's just a hairline in the paint. Sometimes it's a real gap you can slide a card into, with the plaster crumbling away and, on external windows, a damp stain bleeding in below the sill. It's so common that people treat it as normal — fill it with putty at repaint, watch it come back, repeat.

It keeps coming back because the crack isn't really a plaster problem. It's a joint problem, and it was built wrong at the point where two very different materials meet.

Why the frame–wall joint always wants to crack

A door or window frame and the wall around it are made of different stuff — timber, aluminium or uPVC frame against brick, block, concrete or plaster — and different materials move differently.

• Timber swells and shrinks with humidity, a lot, across the seasons.
• Aluminium and uPVC expand and contract with temperature far more than masonry does — a sunlit window frame moves noticeably between afternoon and night.
• The masonry wall barely moves by comparison.

So at the joint between them, one side is always moving relative to the other. If that joint is filled with something rigid — sand-cement mortar, hard wall putty, ordinary filler — the very first season of movement shears it. The rigid filler can't stretch, so it cracks clean along the line, exactly where you filled it. Repaint, refill with the same rigid stuff, and you've simply scheduled the next crack.

On top of that, the gap is often just badly built: packed loosely with mortar dabs and offcuts, with voids behind, no proper bond to the frame, and — around external windows — no seal at all against driving rain. So it cracks and it leaks.

The two things that were missed

1. The joint needed to be flexible, not rigid. A gap between two things that move relative to each other must be filled with a material that can stretch and compress — an elastic sealant or a flexible, polymer-modified filler — so it moves with the joint instead of tearing. This is the core fix and the one that's almost always skipped.

2. The frame needed a proper bond and, outside, a weather seal. The filler has to actually grip both the frame and the masonry (many rigid mortars won't bond to smooth aluminium or uPVC at all), and around external openings it has to seal out water. A rigid mortar that won't stick to the frame leaves a hairline path straight through — capillary heaven for rainwater.

How to fix it so it stays fixed

1. Rake it out properly. Don't skim over the old crack. Cut and rake out the loose, cracked filler back to sound material, and clean the gap of dust and crumbs. A flexible filler over a loose, dusty joint fails as fast as a rigid one.
2. Prime / bond the surfaces so the filler grips — especially important on smooth aluminium/uPVC frames and on old, dense plaster. A bonding agent here is the difference between a seal that holds and one that peels off the frame in a year.
3. Fill with a flexible material, not sand-cement. For a moving joint you want an elastic sealant or a polymer-modified flexible filler that can take the seasonal movement. For wider structural gaps, pack the back and finish with the flexible seal.
4. Outside, seal against weather. External window and door perimeters need a continuous, waterproof, UV-stable seal — no gaps at the sill, where most leaks start. Slope any sill fill so water runs out, not in.
5. Then make good and paint. Only once the joint itself is flexible and bonded does the cosmetic repair actually last.

What we'd use

• SUMO Flexit 2K, a two-part polyurethane adhesive (R2T / IS T5-TS2, the highest deformability class), is built for exactly this kind of multi-substrate, moving joint — it bonds strongly to difficult surfaces like metal and uPVC as well as masonry, and its high flexibility lets it ride the seasonal movement between frame and wall instead of cracking. For bonding and sealing thresholds, frames and mixed-material joints that move, this is the tool.
• Where you're making good the plaster and mortar around the opening and want it to resist cracking and grip an old or smooth surface, gauge the repair mortar with SUMO SBR Latex — it adds the flexibility and bond that plain sand-cement lacks, so the patch flexes a little and holds onto the frame edge instead of shrinking away.
• On the tiled side of a wet-area frame or threshold, grout the tile joints with SUMO Sumoxy 2-Part epoxy grout — waterproof and stain-free — and keep the actual moving frame-to-wall joint as a flexible Flexit 2K sealant bead, not rigid grout.

That endless crack around the frame isn't the wall's fault and it isn't bad luck. It's a moving joint that was filled with something that can't move. Rake it out, bond it, and fill it with something flexible — and the line you paint over this time stays closed.

SUMO is a brand of Adheseal India Company. FLEXIT and SUMODRY product specifications should be confirmed from the current technical data sheet before quoting.