If you're restoring a period property, building a garden office or just replacing a rotting frame, getting the timber right makes all the difference. Timber window frames have been the standard in UK construction for hundreds of years, and they remain the best choice for authenticity, workability and repairability.
This guide covers everything you need to know: the difference between frame types, which timber sections to use, how standard sizes work, and what components make up a complete window.
Casement vs sash: which frame type do you have?
Most UK homes have one of two window types, and the timber sections required are different for each.
Casement windows
Casement windows open on a hinge, either side-hung or top-hung. The frame is built from four pieces of timber: two vertical stiles, a top rail and a bottom rail. The joints are typically morticed and tenoned. For a single-rebated casement, you'll usually work with 44mm or 56mm rebated sections. Double-rebated profiles are used where a second pane or draught seal sits in the frame.
Most DIY and restoration casement projects use softwood, either redwood or whitewood, planed to a rebated profile. If you're painting the frame, the species matters less than getting straight, well-seasoned timber. For external frames left with a clear finish, European oak or accoya are worth the premium.
Sash windows
Box sash windows are more complex. They consist of two sliding sashes that run in a boxed frame, held in balance by weights and cords. The components include the outer linings, inner linings, parting beads, staff beads, sill, head and the sashes themselves (each built from top rail, bottom rail, stiles and glazing bars).
Sash windows are common in Victorian and Edwardian terraces. If you're repairing rather than replacing, you'll often need specific sections: 32mm x 19mm parting bead, 25mm x 32mm staff bead, and glazing bars milled to the correct ovolo or lamb's tongue profile.
Standard timber window frame sizes
Timber window sections are sold by their finished (planed) size or their nominal (sawn) size. A nominal 50mm x 75mm piece will finish at approximately 44mm x 69mm after planing. Always confirm whether you're ordering finished or nominal dimensions before you buy.
Common sections for window frame construction:
- Stiles and rails: 44mm x 69mm or 44mm x 95mm finished
- Sash stiles: 44mm x 44mm or 44mm x 57mm finished
- Glazing bars: 32mm x 44mm or custom milled profiles
- Parting bead: 10mm x 20mm or 12mm x 20mm
- Staff bead: 15mm x 32mm
- Window sill: 44mm x 145mm weathered and throated
If you're working to existing openings in a period property, measure twice before ordering. Openings are rarely perfectly square and the frame needs to be built to suit the actual dimensions, not a standard sheet size.
Choosing the right timber for window frames
External joinery takes a beating from rain, UV and temperature changes. The timber needs to be stable, take a finish well and resist rot.
Redwood (Scots Pine / Baltic Pine) is the most widely used timber for painted external joinery in the UK. It's stable, widely available and takes paint and primer well. For external use, it should be primed all round before installation and kept well painted. Untreated redwood will deteriorate if left exposed.
European Oak is the traditional choice for unpainted external frames. It's naturally durable, hard-wearing and ages well. It also costs significantly more than softwood and requires pre-drilling for fixings due to its hardness.
Accoya is a modified softwood (usually radiata pine) that has been acetylated to make it dimensionally stable and highly rot-resistant. It's increasingly used in high-specification external joinery and carries a 50-year guarantee against rot above ground. It machines and paints like softwood but performs like hardwood.
For internal window boards and linings, most contractors use MDF or softwood. MDF gives a cleaner painted finish and is less prone to movement, though it won't tolerate moisture in areas where condensation is a concern.
Window frame kits
If you need a complete frame for a shed, outhouse, garden room or agricultural building, a wooden window frame kit is usually the fastest option. Kits come pre-cut with a rebate for glazing, morticed and ready to assemble. Standard sizes cover most openings from 450mm x 450mm up to 1200mm x 900mm.
For made-to-measure sizes, the frame sections are supplied cut to length and you assemble on site. This suits awkward openings or where you want to match a specific bar pattern.
Sash window parts and glazing bars
Restoring a sash window often means replacing individual components rather than the whole frame. The most commonly needed parts are glazing bars (which hold the individual panes), beads (which retain the glass in the rebate) and sash horns (the decorative extension at the bottom corners of the upper sash).
Glazing bars are milled to a specific profile: ovolo is the most common in Victorian properties, with a rounded edge facing the room. Lamb's tongue is found in earlier Georgian windows and has a more pronounced S-curve. If you're replacing a single bar, take a cross-section to match the profile accurately.
Browse our full range of window and door timber sections, including glazing bars, beads, sash components and frame sections.
Installation tips
A few practical points before you start:
- Prime all end grain and all faces before installation, not just the visible surfaces. End grain is where moisture gets in first.
- Leave a 3mm gap on each side of the frame in the opening for adjustment and allow for mortar packing.
- Use stainless steel or galvanised fixings. Bright steel will rust behind the paint and cause staining and frame failure within a few years.
- Silicone the external perimeter seal after painting, not before. Paint won't bond to cured silicone.
- In exposed locations, consider a drip groove on the underside of the sill to prevent water tracking back under the frame.
For profiles, beadings and mouldings to finish the frame internally, take a look at our beadings and mouldings range.