Fitting a casement window correctly is one of the most fundamental skills in joinery. Done well, the installation is weathertight, compliant with building regulations, and trouble-free for decades. Done poorly, it leads to draughts, condensation, callbacks, and compliance issues. This guide covers the full installation sequence from opening preparation to final adjustment.
Preparing the Opening
Remove the existing frame cleanly, taking care not to damage the surrounding masonry or plasterwork. Check the reveal for any defects — spalling mortar, damp patches, or cracked lintels — and address these before proceeding. The lintel must be sound and properly bearing; if in doubt, the structural condition of the opening should be assessed before any new frame is installed.
Clean up the reveal faces, removing loose material. The opening should be dry before the new frame goes in. If there is existing mastic or sealant residue, remove it fully — new sealant will not bond reliably to old product.
Positioning and Packing
Offer the frame into the opening without fixings. Check it sits level on the cill — use a spirit level across the bottom of the frame. Shim with plastic packers to level and plumb the frame before any fixing is made. The packers should sit at fixing positions and at the corners; do not leave the frame unsupported between fixing points.
Check that the frame is square by measuring both diagonals — they should match within 2mm. Check that it is plumb on both jambs. Once you are satisfied with the position, mark the fixing points before removing the frame to drill.
Fixing
PVC-u frames are fixed through the frame into the surrounding structure. Use the appropriate fixing for the substrate:
- Masonry — frame fixings into plugged holes, or purpose-made masonry anchors through the frame
- Timber lintel or structural timber — wood screws of sufficient length to get solid bite, minimum 50mm into the structural member
- Strap anchors — used in new-build or where fixing through the frame face is not possible; strap into the cavity or to the inner leaf
Fixing centres should not exceed 600mm and must be within 150mm of each corner. Fixing screws should pass through the reinforced sections of the profile — check the frame drawing from your supplier to confirm reinforcement positions. Do not overtighten fixings; PVC-u will deform under excess clamping load.
Sealing
Sealing is the difference between a weathertight installation and one that fails within months. There are three distinct sealing zones on a casement installation:
- Expanding foam — fill the gap between the frame and the reveal all the way round. Use low-expansion window foam (not standard building foam which expands aggressively and can bow the frame). The foam insulates, eliminates air infiltration, and provides a degree of draught sealing
- Internal sealant — once the foam has cured and been trimmed, apply a continuous bead of paintable decorator’s caulk or silicone at the junction between the frame and the internal plaster or reveal. This seals against air movement internally
- External sealant — apply a continuous bead of external-grade silicone between the frame and the outer masonry face. Tool off cleanly and ensure full coverage — any gap allows water to track behind the frame and into the reveal
Hardware Adjustment
Once the frame is fixed and sealed, hang the sashes and check operation. Adjust the friction stays so the sash swings and holds position correctly. Check the locking points engage without forcing — adjust the keeps if the sash has moved slightly from its factory position. The sash should compress the perimeter weather seal evenly when closed.
Compliance
For full details of Scottish compliance requirements, see our guide to Part L building regulations in Scotland. For guidance on sourcing the right windows for your jobs, read our article on sourcing trade windows as a joiner. For help pricing your fitting work, see our window fitting pricing guide.
In Scotland, replacement windows must comply with Section 6 of the Technical Handbooks (energy). If your installation is a replacement in a dwelling, it requires a completion certificate unless the work falls under a competent person scheme. Check whether your supplier can provide the required U-value documentation, and retain a copy for the building warrant or notification record. Rooms used for sleeping require trickle ventilators in the replacement frames — confirm this with your supplier at the ordering stage.