How to Shim a Door Frame Correctly: A Trade Guide
09/06/2026
Frames rack. Openings are rarely perfectly square, plumb, or level. The gap between a rough opening and a finished frame needs to be filled precisely if the door or window is going to hang, operate, and seal correctly. Getting that right is what shimming is for, and done properly it is one of those details that separates a quality installation from one that causes problems for years.
This guide covers the practical process: which shims to use, where they go, how many you need, and what to watch out for.
Why Metal Shims Rather Than Plastic or Timber
Timber packers have been used for decades and still appear on many sites. The problem with timber is that it moves. It absorbs moisture, swells, dries out, compresses under load over time, and can rot in damp conditions. A door frame that was perfectly square on installation day can drift out of true as the packing behind it changes dimension.
Plastic packers are better than timber in damp conditions but they creep under sustained load, particularly in warmer environments or where the frame is carrying significant weight.
Metal shims do none of these things. Laser cut mild steel or stainless steel shims are dimensionally stable, do not compress, do not rot, and do not move. The thickness you install is the thickness that remains. For external frames in particular, stainless steel shims are the correct specification: they will not corrode, stain the surrounding masonry, or degrade behind the frame over the life of the building.
Which Shims to Use for Door and Window Frames
Timber packers have been used for decades and still appear on many sites. The problem with timber is that it moves. It absorbs moisture, swells, dries out, compresses under load over time, and can rot in damp conditions. A door frame that was perfectly square on installation day can drift out of true as the packing behind it changes dimension.
Plastic packers are better than timber in damp conditions but they creep under sustained load, particularly in warmer environments or where the frame is carrying significant weight.
Metal shims do none of these things. Laser cut mild steel or stainless steel shims are dimensionally stable, do not compress, do not rot, and do not move. The thickness you install is the thickness that remains. For external frames in particular, stainless steel shims are the correct specification: they will not corrode, stain the surrounding masonry, or degrade behind the frame over the life of the building.
Where to Place the Shims
Timber packers have been used for decades and still appear on many sites. The problem with timber is that it moves. It absorbs moisture, swells, dries out, compresses under load over time, and can rot in damp conditions. A door frame that was perfectly square on installation day can drift out of true as the packing behind it changes dimension.
Plastic packers are better than timber in damp conditions but they creep under sustained load, particularly in warmer environments or where the frame is carrying significant weight.
Metal shims do none of these things. Laser cut mild steel or stainless steel shims are dimensionally stable, do not compress, do not rot, and do not move. The thickness you install is the thickness that remains. For external frames in particular, stainless steel shims are the correct specification: they will not corrode, stain the surrounding masonry, or degrade behind the frame over the life of the building.
How Many Shims and What Thickness
The gap between the frame and the structural opening varies around the perimeter, which is the whole reason shimming is necessary. Measure the gap at each packing point before you start. A combination of shim thicknesses is usually needed to build up to the exact dimension required.
Standard shim thicknesses run from 1mm up to 6mm. The most useful sizes to have on site are 1mm, 1.5mm, 2mm, 3mm and 5mm. For most frame installations the gaps you are filling will be between 2mm and 15mm depending on how true the opening is. Start with the thickest shim that fits within your target gap and use thinner shims to make up the difference. Three shims to reach a target is better than ten: keep the stack compact and it will be more stable under load.
At each packing point, build the shim stack up until the frame is sitting plumb, level, and square. Check with a spirit level as you go. Add or remove shims until the reading is correct, then drive the fixing home to lock the position. Work around the frame methodically rather than tightening one side fully before packing the other: tightening one side changes the loading on the rest of the frame and you will be chasing your tail if you do not keep everything roughly in balance as you go.
Checking for Square as You Go
A door frame that is plumb on both jambs but not square will still bind. Check diagonal measurements across the frame opening at each stage: both diagonals should be equal. If they are not, the frame is racked and needs adjusting before fixings are tightened.
The standard tolerance for a well-installed door frame is 2mm across the diagonal measurement. Beyond that and you will start to see binding at the latch or hinge, gaps in the seal at the head, and difficulty achieving a consistent reveal around the door leaf.
Window frames are less forgiving than door frames on squareness because the gap between the sash and the frame is smaller and the tolerances on the weather seal are tighter. Take your time on the diagonal check and do not move on until it is right.
A Note on Fire Doors and Certified Installations
If the door frame is part of a fire door installation, the specification for the frame packing may be dictated by the door set certification. Some fire door manufacturers specify intumescent packers at certain positions, or restrict the materials that can be used in the cavity around the frame. Check the installation instructions for the specific door set before specifying shim materials, and keep a record of what was installed at each position for the O&M documentation.
For standard residential and commercial installations not subject to fire door certification requirements, laser cut metal shims in the appropriate material grade are a straightforward and reliable solution.
A Note on Fire Doors and Certified Installations
Horseshoe shims for frame packing are available in mild steel and stainless steel across the standard thickness range. Stainless steel is recommended for all external applications. Square packers are available in the same materials and suit sill and threshold packing before fixings go in. If the gap you are filling calls for a non-standard size or a thickness outside the standard range, custom parts are cut to order with a lead time of 3 to 4 working days.
For standard residential and commercial installations not subject to fire door certification requirements, laser cut metal shims in the appropriate material grade are a straightforward and reliable solution.
All parts are manufactured at our facility in Braintree, Essex. Free delivery on orders over £100 ex VAT.