FGIA InstallationMasters common mistakes
FGIA InstallationMasters · Understand the traps and how to avoid them.
Treating the rough opening as if it were sized for the new unit without measuring all four sides and the diagonals, then discovering the unit will not shim and square properly.
Candidates carry over habit from new construction, where the framed opening is assumed true and plumb; in replacement work the existing opening is often out of square, out of level, or built to older sizing conventions, and the candidate skips the full survey under time pressure.
What to do instead
Before selecting or ordering, measure width at top, middle, and bottom and height at both jambs, check diagonals for square, and check sill level and jamb plumb. Record the smallest width and height as the governing opening dimension and confirm it against the unit's required rough opening and shim space.
Removing the old window without first confirming the condition of the surrounding framing, sheathing, and sill pan, then installing the new unit over rot, soft wood, or a failed pan.
The exam tests sequencing and substrate assessment, but candidates focus on the installation steps they have practiced and treat demolition as a formality; concealed damage is invisible until the old unit is out, so it is easy to overlook.
What to do instead
Build a probe-and-repair step into the sequence: after removal, probe sill and jamb framing for softness, inspect the rough opening for water staining, and repair or replace damaged members and the sill pan before any new unit is set. Document the repair.
Flashing the replacement unit incorrectly by lapshing the head flashing over the flanges instead of shingle-fashion, or omitting flashing entirely because the old unit 'didn't have any.'
Replacement installs often reuse an existing opening where the original weather barrier is damaged or absent, and candidates copy what they see rather than applying the water-shedding principle; the exam rewards principle over mimicry.
What to do instead
Apply the lap principle: weather barrier behind the flanges at jambs and sill, head flashing lapped over the top flange and tucked under the barrier above, with end dams or sealed corners at the head. Where the existing barrier is compromised, integrate a new self-adhered membrane into the wall assembly.
Setting the unit directly on the rough opening sill with no shim or setting block, then fastening, so the sill bows and the operating sash binds.
Candidates conflate 'solid bearing' with 'full contact' and skip shims at the sill because the gap looks small; they also forget that the sill needs support at specific points rather than continuous blocking that can trap water.
What to do instead
Place shims or setting blocks at the sill under the jamb lines and at the mullion or lock points per the manufacturer's instructions, leaving the weep path clear. Verify the sill is level and the unit is not twisted before driving fasteners.
Fastening through the flange or frame at spacing and locations that ignore the manufacturer's schedule, or driving fasteners through the weep system and drainage slots.
Candidates generalize from one product line to all products and rely on memory of typical spacing rather than the printed instructions; exam items deliberately test whether the candidate knows that spacing and location are product-specific.
What to do instead
Fasten per the specific unit's installation instructions: correct spacing at jambs and head, fasteners set at the reinforced zones, and never through weep holes, drainage channels, or the sill pan. State in your answer that the manufacturer's published instructions govern when they conflict with general practice.
Ignoring the interior air seal and insulation details, treating the job as finished once the exterior is flashed and trimmed.
The visible exterior work dominates a candidate's mental model of 'installation,' and interior trim is seen as finish carpentry rather than part of the building envelope; the exam tests the whole assembly.
What to do instead
Include backer rod and sealant or low-expansion foam at the interior perimeter, kept off the sill drainage path, and confirm the interior air barrier connects to the frame. Treat the window as a complete assembly with exterior, frame, and interior control layers.
Confusing the water-resistive barrier sequence in new construction, installing the window flange over the housewrap at the head instead of lapping the barrier over the flange.
Candidates memorize 'flange goes over the barrier' without distinguishing sill and jamb from head, or they reverse the rule entirely; the shingle-fashion concept is simple but easy to state backwards under exam pressure.
What to do instead
Use the standard sequence: sill pan and barrier at the sill, barrier behind the jamb flanges, and the head flashing or barrier lapped over the head flange. Rehearse the mnemonic 'water goes downhill and laps outward' and apply it layer by layer.
Framing the rough opening to nominal unit size rather than to the manufacturer's specified rough opening, leaving no room for shims, flashing, and square.
Candidates use the window's call size as the opening size because the numbers look similar, and new construction plans may not restate the rough opening dimensions; the difference between call size and rough opening is a classic trap.
What to do instead
Frame to the rough opening dimensions published for the specific unit, which include allowance for shimming and flashing. Verify the framed opening for square, level, and plumb before the unit arrives, and confirm header sizing and jack studs per the plans.
Installing the unit out of square by shimming only one jamb or over-shimming a corner, then forcing the sash to operate.
Candidates chase the reveal visually instead of measuring, and a twisted frame can still look acceptable at a glance; the exam tests the measurement method, not the appearance.
What to do instead
Shim in pairs at sill, head, and jamb fastening points, checking diagonals and the frame's level and plumb as you go. Operate the sash and check the reveal gap is even before final fastening; correct the shims rather than adjusting the sash hardware.
Skipping the pre-installation check of the rough opening for the flashing and drainage path, including sill pan slope and weep clearance, in new construction.
New construction candidates assume the framer and the pan are correct because the work is new, and they treat the sill pan as someone else's scope; exam items test whether the installer verifies rather than assumes.
What to do instead
Inspect and verify the sill pan, its end dams, and slope to the exterior before setting the unit; confirm the head and jamb flashing will lap correctly and that weep holes and drainage paths remain unobstructed after installation. Reject or correct the opening before proceeding.