The same errors turn up on site after site. None of them are subtle once you know to look, and all of them are invisible within a week of being made.
Working top-down, so upper courses tuck behind lower ones and channel water into the wall instead of over it.
It happens on tall elevations where crews start at scaffold level and work outward from there. Start at the base and lap upward, always.
The most consequential window error. Head flashing must run under the wall membrane above the opening. Wall membrane over head flashing, so water running down the wall sheds onto the flashing and away.
Get it the other way around and every drop running down that wall enters at the window head.
Sill tape sealed at the outer face with nowhere for water to go. Any water that gets past the frame sits there permanently.
The sill must drain outward, and the sill flashing must lap out onto the wall face below.
Masonry, concrete, blockwork, fibre cement and dusty timber all need priming before taping. Without it the tape looks perfectly fine at handover and releases within a couple of years.
This is the most common cause of tape failure, and it is entirely avoidable.
Pressure sensitive adhesive needs firm, even pressure to wet out against the substrate. Pressing the ends with a thumb does not do it.
Roll the full length. Where a roller will not fit, use firm hand pressure along the whole run.
Tape pulled tight across an internal or external corner bridges instead of adhering. It looks sealed and there is an air gap behind it, and it lifts within months.
Corners need the tape formed in, usually with a relief cut and a separate patch over the cut.
Foam fills gaps and provides some insulation value. It is not an air barrier and not a water barrier. It shrinks, cracks and channels water.
Foam plus proper sealing tape is fine. Foam alone is not a seal.
Silicone at the external face of a window is a weather bead. It has no continuity with the wall’s air barrier and it is not doing the job people assume it is doing.
The plumber drills through the ceiling plane on Tuesday. The linings go on Thursday. Nobody sealed it.
Seal penetrations as they are made. Keep grommets and tape on site so there is no reason not to.
Sealing penetrations and services
Self-adhered membranes need a rigid, continuous substrate. Applied over open framing they have nothing to bond to between studs and will not perform.
Mechanically fastened membranes over rigid sheathing are fine, but the fixing pattern and the lap sealing both matter more than they do over framing.
Membranes have a stated maximum UV exposure period. Exceed it and the material degrades before the cladding goes on.
If cladding is likely to be delayed, check the limit before ordering, not after.
An assembly with low permeance on both faces cannot dry in either direction. Any moisture that gets in, from construction, from a leak, or from a plumbing failure, stays in.
Decide which direction the assembly dries and make sure it can.
Insulation fitted around services rather than services routed to avoid insulation. Batts cut short at the top of a wall. Gaps at edges.
Reduces effective thermal performance and creates cold spots where condensation forms.
Wall to ceiling, wall to floor, wall to wall. The junction is where two trades meet and where the air barrier most often stops.
Trace it on the section drawing. If you lift your pen, that is the leak.
Once cladding and linings are on, the envelope is invisible for the life of the building.
Photograph every elevation before cladding and every opening before it is closed. It takes twenty minutes and it is the only evidence you will have if a moisture claim arrives in five years.
Most of these are sequence and attention errors rather than knowledge gaps. The products work. The details are documented. What fails is the moment on site where the correct thing takes ten minutes longer than the incorrect thing and nobody is watching.
Which is an argument for checking the work while it is still visible.
