How to pack a window frame
Updated: 23 October 2026
The frame looked square when you offered it into the opening, and it bowed as soon as the fixings came tight. That usually means a packer was missing beside the screw, or the screw was used to pull the profile back to the wall. This page is where rigid packers go, how tight is tight enough, and the point where you stop. It assumes you already have flats or horseshoes, not a bag you still need to choose. Packs are on the [shims comparison](/hub/diy-supplies/shims). A qualifying purchase can earn a commission at no extra cost to you. No lab test was run. Follow the frame maker's sheet if it disagrees with the example distances below.
The frame looked square when you offered it into the opening, and it bowed as soon as the fixings came tight.
The frame looked square when you offered it into the opening, and it bowed as soon as the fixings came tight. That usually means a packer was missing beside the screw, or the screw was used to pull the profile back to the wall. This page is where rigid packers go, how tight is tight enough, and the point where you stop. It assumes you already have flats or horseshoes, not a bag you still need to choose. Packs are on the shims comparison. A qualifying purchase can earn a commission at no extra cost to you. No lab test was run. Follow the frame maker's sheet if it disagrees with the example distances below.
Our top picks
| Badge | Product | Type | Piece size | Slot for a screw | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  EVOGES EVOGES flat glazing packers, 42 mixed | Flat packer | 28 × 100 mm | No | 8.8 | about £6.99 | View offer |
| Alternative |  EETU EETU horseshoe shims, 200 mixed, Broadfix | Horseshoe | 55 × 43 mm | Yes | 8.4 | about £18.97 | View offer |
| Best value |  HuangXinyu HuangXinyu flat glazing packers, 72 | Flat packer | 100 × 28 mm | No | 8.4 | about £5.99 | View offer |
On this page
Two different gaps
A window has two gaps. The frame-to-reveal gap is where the fixings go, and packers there stop the screws bending the frame. The glass-to-rebate gap is inside the frame, where setting blocks and toe-and-heel packers sit. Using one diagram for both is how people pack the middle of a cill and then wonder why the sash drops.
An EVOGES review is the warning. The packers were not enough because the window was the wrong size, and the job needed build-up bars. A 1–6 mm flat does not lengthen a short frame. A soft wedge such as Pe&Hui is the wrong permanent packer.
Where the frame packers sit
Put rigid packers next to the fixings, not in the empty middle of a side. The Glass and Glazing Federation treats them as part of the fixing: beside the screws, left in, and able to resist compression, rot and corrosion. Sidey's 2024 guide includes hinge and lock points. A common pattern, also in the British Woodworking Federation note, is within about 150 mm of a jamb and no more than about 600 mm apart. Your frame sheet wins if the numbers differ.
The packer should bridge the frame depth. A 100 × 28 mm flat is the usual piece. A horseshoe is for a screw already in. NHBC says foam does not square the frame. Once the screws are home, the packers stay.
Tightening without a bow
Offer the frame, pack beside every fixing, and tighten until the frame is held, not until the profile kisses the wall. The GGF guide and the Sidey guide both warn against over-tightening. If the frame moves towards the wall as the driver turns, the packer is missing or too thin. Add thickness. Do not keep driving.
Check the jamb and the cill with a straight edge before the next screw. A rubber wedge squashes, so the thickness you set is not the thickness you keep. A sealed reveal wants polypropylene rather than wood, which is the split on plastic packers or wooden shims. Place a horseshoe carefully the first time. UK reviews of the EETU bag say the barbs can grip hard.
Toe-and-heel, then stop
Toe-and-heel is inside the rebate, not a second copy of the frame packing. Wickes puts glass packers about 100 mm from the corners, with extra support if the unit is wider than about 1,200 mm. The diagonal pair runs from the bottom hinge corner to the opposite top corner so a side-hung sash does not drop. SAJE says bridge the bottom, then toe-and-heel from hinge-side bottom to lock-side top. The block should cover both panes and stay clear of the drainage. Do not work alone with a large unit.
Stop if the lintel is damaged, the unit is too heavy, the sash still drops, the building is listed, or the door is fire-resisting. Replacement windows are building work, notified through FENSA or Building Control. This page cannot issue a certificate. If the hole and the window disagree by more than a stack of known packers, call a fitter.
Common questions
Do I take them out after the screws are tight?
No. Frame packers stay in. The GGF guide treats them as part of the fixing, so the screw does not pull the profile back to the wall. You remove a packer only to reposition it before the final tightening. Foam is not a substitute. NHBC says not to rely on foam to square the frame. Foam can close the joint afterwards.
How is toe-and-heel different from frame packing?
Frame packing is outside the frame, between frame and wall. Toe-and-heel is inside the rebate, under and beside the glass, so a sash does not drop on its hinges. Wickes describes packers about 100 mm from the glass corners, with the diagonal pair from the bottom hinge side to the top lock side. Do both jobs. Do not use one diagram for the other.
How far from the corner should a fixing be?
Use the sheet for the frame you bought. As an example, BWF timber windows ask for durable packings within 150 mm of each jamb, and other UK notes put fixings about 150–200 mm from corners and no more than 600 mm apart. A packer sits next to each fixing. If the drawing says something else, follow the drawing.
What happens if I over-tighten?
The profile bows towards the wall, the sash starts to scrape, and the seal gap goes uneven. Sidey and the GGF both say not to over-tighten. If the frame moves as you drive the screw, the packer is too thin or missing. Add a known thickness and ease the screw until the jamb is straight. A rubber wedge will squash and hide that mistake.
Can I pack a window that is simply too small?
Only within the few millimetres a stack of rigid packers is meant to fill. An EVOGES review still needed build-up bars because the window was the wrong size. Packers do not resize a frame. If you are building a tall stack to chase the lintel, measure the unit against the opening and stop. That gap is a survey problem.
Do I need Building Control or FENSA for a replacement?
Replacing a window is building work, normally notified through a competent person scheme such as FENSA or through Building Control. Packing a frame does not remove that duty, and this page cannot issue a certificate. Also stop if the lintel is damaged, the unit is too heavy, the sash still drops, the building is listed, or the door is fire-resisting.
Related pages
Sources
Public UK sources and the amazon.co.uk listings compared on this page. Scores dated 22 October 2026. Updated 23 October 2026.
- BPF Online — How to choose the right plastic packer (retrieved 22 October 2026)
- Fixfast — Plastic packing shims data sheet (retrieved 22 October 2026)
- Glazpart — Glazing accessories (retrieved 22 October 2026)
- NBS — BS 6262-1:2017 (retrieved 22 October 2026)
- Fix Direct — how glazing packers are used (retrieved 22 October 2026)
- SAJE — How to toe and heel (retrieved 22 October 2026)
- Wickes — fixing distances (retrieved 22 October 2026)
- NHBC — Section 5, first fix operations (retrieved 22 October 2026)
- Approved Document 7 (retrieved 22 October 2026)