Most window failures don't announce themselves on installation day. They turn up a year or two later — a draught along the head jamb in February, a soft patch of plaster under the sill after a heavy rain, a casement that won't latch cleanly once summer humidity swells the frame.

By then the cause is buried behind trim, siding and plasterboard. Which is why the most useful thing anyone can do with a window is get the installation right at the moment it happens. The errors below are the ones that come up again and again, and nearly all of them are cheap to prevent while the opening is still open.

Poor Flashing Is Behind Most Window Leaks

A window is a hole cut in the building envelope, and water is patient. Flashing is not a strip of tape you add at the end; it is a layered system that hands water from the window back out to the exterior, one piece at a time, always downhill.

The typical mistakes are easy to spot on a finished job and hard to fix: no sill pan at all, with the installer relying on a bead of sealant under the frame; head flashing lapped the wrong way, so water runs behind the flange instead of over it; side flashing butted rather than overlapped, leaving a joint that holds water like a gutter.

Work from the bottom up, the way a roofer would. Sill pan first, with a turned-up dam at each end so water cannot run sideways into the wall cavity. Then the sill flashing, then the jambs over it, then the head last, sitting over everything below. Tape the flange to the water-resistive barrier with a compatible flashing tape and follow the window manufacturer's written sequence, because flanged and flush-fin windows are detailed differently.

• Sill pan or a properly sloped sill, with end dams, on every installation.
• Shingle-style laps: sill at the bottom, sides over it, head over the sides.
• Tape applied over the flange and onto the barrier, pressed down, not just stuck.
• Weep holes at the sill left clear — never caulked, never painted shut.
• Sealant used as a supplement to flashing, never as a substitute for it.

Uneven Shimming and Forcing the Frame Into Place

A window frame is not rigid. Twist it slightly and the sash binds, the weatherstripping stops meeting, the lock no longer lines up. Yet plenty of units are still installed by pushing a marginally oversized window into a marginally out-of-square opening and driving screws until it stops moving.

Shimming errors are subtle. Shims placed only at the two ends of the sill, letting the centre sag. A single stack in the middle of a wide unit, which acts like a fulcrum. Shims that aren't used in pairs, so the frame can rock. Nothing at all under the hinge side of a casement, which is exactly where the weight hangs. And checking plumb with a 600 mm level on a 1.5 m frame, which will flatter almost any installation.

Set the unit on a bed of shims under the jambs and under the meeting rail, use shims in pairs with the wide end against the frame, and check level, plumb and the diagonals of the opening before a single fastener goes in. Measure corner to corner both ways; a discrepancy of more than a few millimetres means the opening needs attention, not more force.

Test the operation before you fasten

Open and close every operable sash at each stage of fixing. Start with the jamb nearest the lock, then the opposite side, checking the sash still moves freely after each screw. If it binds, stop and re-shim. Adjusting a window is a five-minute job at that point and a demolition job afterwards.

The Wrong Sealant in the Wrong Place

Sealant is the most misunderstood product on the job, and the errors run in both directions. Too little gives you a thin bead that cracks at the first cold snap. Too much gives you a window sealed on every face, including the drainage path, so water that gets in has no way out.

The principle worth remembering is that the interior side is the air barrier and the exterior side is the drainage plane. Seal the inside properly and let the outside drain. That means no fat bead across the weep holes, no caulking the gap at the bottom of the exterior flange, and no sealing the joints between sash and frame on a sliding or hung unit.

Foam causes its own problems. Standard expanding foam in a narrow cavity will bow a jamb and jam a sash. Use a low-expansion window-and-door foam, apply it in one pass, and never fill a deep cavity in a single go — two thinner passes, with time to cure, distorts far less. For deep or wide joints, backer rod first, then sealant, so the bead has something to sit against and can move.

Choose a sealant rated for exterior movement rather than a general-purpose painter's caulk, tool the bead properly, and check that it is compatible with the frame material, the flashing tape and any coating already on the wall.

Measuring the Rough Opening, Not the Window

It sounds obvious and it still happens weekly: someone measures the old window, orders the new one, and finds it will not fit the hole. Windows are commonly specified by rough opening size, and the frame is smaller than that by design, leaving room for shims, foam and sealant. Read the manufacturer's rough opening figure rather than assuming the two numbers match.

Measure the width at the top, middle and bottom, and the height at the left, centre and right, then use the smallest figure. Check the diagonals. Check the depth of the reveal against the frame and the intended trim, because a deeper frame may need jamb extensions. On the exterior, confirm there is a slope or a sill detail that sheds water away from the wall, and allow enough headroom for a head flashing above the unit.

Gaps Behind the Trim

The gap between the frame and the rough opening is the shortest path for cold air, and it is easily covered up. Foam the perimeter consistently rather than in a few spots, and pay attention to the head, which is often skipped because it is awkward to reach.

Then look further in. Where the interior air barrier meets the window frame, is the joint continuous? Is the space behind a jamb extension sealed? Trim carpenters regularly close up a window only to hide an unsealed gap that will whistle for the next decade. Seal those junctions before the architrave goes on, and trim away excess foam so it doesn't push the trim out of line.

Handling, Storage and Sequence

Windows stored flat for a fortnight will take a set. Standing them on a corner dents the frame. Protective film left on in direct sun can bake onto the profile and becomes very difficult to remove.

Keep units upright and braced, remove the old window only once the new one is on site and inspected, and line up the exterior faces of adjacent units so the facade reads as one plane. Afterwards, keep ladders, boards and bags of mortar away from the frames — a surprising number of dents happen after the window is installed, not during.

A Final Walkthrough Before the Trim Goes On

The fifteen minutes you spend checking the installation before it is closed in will save you an unreasonable amount of trouble later. Work through this in order:

1. Operate every sash, lock and vent. They should move without force and latch without lifting.
2. Re-check the diagonals and the reveal on all four sides.
3. Run a hose gently at the sill and head for a few minutes and look for moisture inside.
4. Confirm the weep holes are clear and the sill track drains.
5. Check foam and backer rod for gaps, and trim anything proud of the frame.
6. Look at the exterior bead: continuous, tooled, and clear of the drainage path.
7. Verify the head flashing sits above the flange and laps over the sides.
8. Remove manufacturer labels where instructed, fit screens, and note any care instructions.

Catch a problem at this stage and it's a shim and a bead of sealant. Catch it a year later and it's a repair. Same mistake, very different bill.

Photo: StartupStockPhotos / Pixabay