Seal before the rains: the hairline-crack map of the outside wall

Every outside wall develops cracks; the maintenance difference is between sealing the ones that keep water out cheap and painting over the ones that mean something else. This is the sorting page - what's a cosmetic line, what's a sealant job, and the junction list where the rain actually enters.

The sort, in plain measurements

• Hairlines under 1mm (the fine network in render, the craze at corners): a good exterior acrylic filler brushed across, painted on schedule - they're weathering, not movement, and the plaster coat is doing its slow job of absorbing the sun.
• The 1-5mm moving class - the single straight crack above a window's corner, the line under a sill, the split where the extension meets the main wall - is a movement joint: the wall opens and closes a fraction with the day and season. Rigid filler cracks inside a month here; the only thing that lasts is flexible sealant (a polyurethane or exterior acrylic-latex caulk), pushed into an opened V-groove, tooled, and repainted with an elastomeric coat over it if the look matters. The sequence is the whole skill: widen the hairline into a proper V with a scraper's edge so the sealant has depth to grip, brush the channel clean, dampen (cementitious products want the water), bead, tool, leave, paint.
• The line you don't seal: wider than 5mm, a horizontal crack under a window sill paired with one over it, stair-steps through blocks with a bulge, or anything where you can slide a folded card into the gap - those are the structural chapter's subjects, and the sealant that makes them quiet until the next inspection is how the next inspection goes badly.

The junction list, where water actually enters

Most wall water isn't through the flat face - it's at the places the drawing shows as two materials arguing: the parapet's top edge and its cracks at the coping, under window sills (the sill-to-wall joint is the most under-sealed line in the house - a continuous flexible bead and a working drip moulding over it), the eaves where the roof meets the wall, around every pipe collar (the AC sleeve and the waste-pipe hole are drilled first and sealed second, and the seal ages - renew them on the two-year walk), the wall-to-floor base on the weather side where splash-up lives, and the render's own lift edge - a hollow tap means re-anchoring or re-coating, not caulking. A torch, a scaffold plank or a ladder, a notebook, and a Sunday-morning hour of this list beats every product on this page's other half.

The timing, because the desk has watched it fail

Do the sort and the seal two weeks before the rains, not after the first wet patch - fresh sealant and fresh paint need their cure week dry, and the June version of this job is a rainy-day version of the leak chapter. The moss and the dark green streak in a hairline are the map legend for where water parks: they grow in the damp line before the plaster fails, and the honest summary the desk repeats: a sealed 1mm crack costs the tube; an unsealed 1mm crack costs the room.