Water leaves a wall in one direction, and the laps have to agree
A wall sheds water the way a roof does: in layers, each lapping over the one below, so anything getting past the face lands on a surface that carries it down and out. The weather-resistive barrier, the flashings at every opening and the cladding are not three products. They are one drainage system whose only rule is that upper laps over lower, with no substitutions of sealant for geometry. That is what makes sequencing structural rather than administrative: you get one opportunity to build those laps, bottom to top, in an order that cannot be reshuffled to suit a schedule.
The order on a combined job
- Strip the cladding back to sheathing across the whole elevation, so the barrier that goes back is continuous.
- Inspect and repair: sheathing, framing, the sub-sill under every opening, the rim. This is the reason the job is worth doing as one job, and it is invisible in every photograph of the finished house.
- Barrier on, lapped shingle-style, laps taped or detailed per the manufacturer.
- At each opening, cut the barrier, fold the side and bottom flaps in, hold the head flap up on the diagonal.
- Sill pan with a back dam and turned-up ends, lapping out over the barrier face.
- Set the window plumb, level, square and shimmed; fasten the flange to the schedule in the instructions.
- Jamb flashing over the flange and over the barrier.
- Head flashing over the jamb flashing, then drop the barrier head flap over it and tape the diagonals.
- Trim and cladding, with the drainage gap and fastener schedule the cladding manufacturer specifies.
- Sealant last, only at the joints the details call for.
Steps four to eight are the whole argument. The head flashing has to end up underneath the barrier and the barrier underneath the cladding, and there is no way to achieve that once both are already finished around the opening.
Why windows go first, never the other way round
Put new cladding and trim on a wall and then decide to replace a window, and there are two roads. Cut the finished work back to reach the flange and the barrier, which leaves a repair seam that stays visible on a run of new siding. Or fit an insert unit inside the existing frame and leave the exterior undisturbed, which is a real product with a real limitation: it inherits whatever flashing detail is already behind the old frame, sound or not.
The reverse order costs nothing. Windows in first, flashed into the barrier, cladding over the top: the sequence the details were drawn for.
Trim detail changes with the cladding
Vinyl
The opening is finished with J-channel receiving the panel ends, with undersill trim where a course is furred out. One thing gets confused here: J-channel is a trim receiver, not a flashing. It hides the panel cut; it does not carry water. The flange and the barrier behind it do the waterproofing, and panels stay hung loose so they can move through a Northern Nevada temperature swing.
Fiber cement and engineered wood
The opening gets trim boards over the flange, with the joint detail and drainage gap the manufacturer publishes, cut edges sealed and butt joints treated rather than pushed together. Above the head trim there has to be head flashing with a drip edge. A bead of sealant along the top of a trim board is not head flashing, and when it fails (sealant is a maintenance item, so it does), the water goes straight in behind the trim.
The joint we leave open
Under the window, at the bottom of the unit and beneath the sill trim, the joint stays unsealed. It is the drain. Anything entering at the head or the jambs arrives at the sill pan and leaves there, and sealing that line traps it. Well-meaning painters close it every year, and it is the first thing we look for on a leak call.
What one job buys that two jobs do not
- One continuous barrier on the elevation instead of a patchwork of tie-ins
- Flange-to-barrier integration at every opening, built in the intended order
- Sheathing, framing and sub-sills inspected while they are actually visible
- One colour decision, so cladding, trim and frame are chosen against each other
- One mobilisation, one set of staging, one disruption
The honest costs on the other side: a bigger single invoice, a longer stretch under staging, a scope that wants a dry weather window, and a project that takes an elevation at a time rather than dotting around the house. No elevation is opened wider than the crew can dry in before they leave.
If the budget only stretches to one this year
Do the windows. They are the openings in the drainage plane, they are what you feel in January, and doing them first leaves the cladding job free to be built correctly later. Where a window is only fogged rather than failed there is a cheaper middle path worth checking. See window repair or replacement. And if the cladding decision is still open, the orientation of the boards changes the labour more than the material: vertical versus horizontal siding.
When the cladding does come round, our page on siding replacement and installation walks through the options.