Why seams are the weak point
A waterproof fabric is only a waterproof garment once its seams are sealed. Every stitch line is a row of needle holes through the barrier, and water under pressure — rain driven by wind, a pack strap pressing wet fabric — finds them. This is why the presence or absence of seam tape is the single most reliable shop check for whether a jacket is genuinely waterproof.
The standard solution is seam tape: a narrow strip bonded over the stitch line with heat and pressure. Bemis, the largest tape maker, describes its tapes as multi-layer constructions engineered to bond to specific fabrics — which is why tape is matched to the laminate, and why Gore requires approved tape applied by certified factories as part of its garment certification.
Fully taped versus 'critically taped'
Fully taped means every seam in the garment is sealed. 'Critically taped', 'strategically taped' or 'partially taped' means only some are — typically the seams most exposed to rain: shoulders, hood, chest, sometimes the upper back. The term 'critically taped' has no standardised definition; which seams a brand considers critical is its own decision, and two jackets with the same label can differ.
Partial taping is a legitimate cost and weight decision for jackets intended for lighter use — but it belongs in the water-resistant-to-light-rain category of expectations, whatever the marketing says. For sustained rain, fully taped seams are the baseline this site applies everywhere.
Where seams fail, and what to do about it
Seam tape ages. The adhesive bond flexes at the shoulders, cuffs and zip lines thousands of times, and years of flexing, washing and heat cycles eventually lift tape edges. A lifted tape edge shows as a visible bubble or peeling strip inside the jacket, and it leaks at that spot in rain — localised and repeatable, unlike condensation.
The repair route is professional. No manufacturer publishes a DIY tape-rebonding procedure; Gore, Arc'teryx and Patagonia all route lifting tape to authorised repair. What you can do is prevention: Gore's care documentation notes that accumulated body fats, sunscreen and residues can cause seam tape to peel, which is one more reason regular washing extends a jacket's life.