Scope: This article supports product selection and technical discussion. It does not replace the project designer, local building code, approved construction details or product-specific declarations.
Build the sill as a drainage tray
Prepare the sill before the window is installed. Flexible flashing should protect the sill and turn up at the ends without creating folds that hold water. The outer edge must discharge toward the drained cavity or exterior face specified by the wall system.
Connect jambs and head in shingle order
Side connections should overlap the sill treatment. The head flashing then directs water over the jamb system, while the wall underlay above laps over the head-flashing upstand. Reversing one lap can route water behind the weather-resistive layer.
Do not rely on perimeter sealant alone
Sealant is a joint component, not the entire drainage strategy. Leave the designed drainage route open and use compatible tapes or flashings on clean, dry substrates. Photograph the sill corners, head lap and repaired fastener holes before cladding conceals them.
Project checks
- Complete the sill before setting the window.
- Keep every upper layer draining over the layer below.
- Verify tape compatibility and preserve the drainage exit.