Types of Sash Windows: Complete UK Guide
Sash windows come in several distinct types, each with different balancing mechanisms, period characteristics, and suitability for repair or replacement.
Last reviewed: 2026-09-21 | 8 min read | Verified against 3 sources
- Box sash frame depth
- 130-150mm (to house weights)
- Spring sash frame depth
- 85-95mm (no weight boxes needed)
- Most common Georgian pattern
- Six-over-six panes
- Most common Victorian pattern
- Two-over-two panes (after 1840)
Table of Contents
What is a box sash window?
A box sash window is the traditional British sash window design, dating from the late 1600s and dominant throughout the Georgian, Victorian, and Edwardian periods. The name comes from the hollow box sections built into the frame sides.
Inside each box sits a cast-iron weight (typically 3-5kg per sash) connected to the sash via a cord or chain running over a pulley wheel at the top. When you lift the bottom sash, its weight pulls down on the cord, which lifts the counterweight inside the box. This balance means the sash stays put wherever you leave it.
Box sash frames measure 130-150mm deep to accommodate the weights. This makes them visually chunkier than modern alternatives, but for listed buildings and conservation areas, this authentic depth is usually required.
The key components are:
- Outer frame (casing): fixed to the building opening
- Box sections: hollow vertical sides housing the weights
- Sashes: the two moving frames that hold the glass
- Pulleys: cast-iron or brass wheels mortised into the frame head
- Sash cords: historically hemp or cotton, now often nylon or pre-stretched polyester
- Weights: cast iron, sized to match each sash
- Parting bead: the thin vertical strip separating the two sashes
- Staff bead: the removable inner strip holding the bottom sash in place
When sash cords break (typically after 15-25 years with traditional cotton cord, 40+ years with modern pre-stretched cord), the window becomes inoperable. Cord replacement requires removing the staff bead and tilting out the bottom sash to access the weight pockets.
What is a spring sash window?
Spring sash windows use coiled springs instead of weights to balance the sashes. This eliminates the need for the thick box frame, allowing a slimmer overall profile of 85-95mm.
The springs sit in recesses routed into the frame sides, either as helical coil springs (twisted metal coils, similar to a car suspension spring) or as flat spiral springs (wound metal ribbon). When the sash moves, the spring compresses or extends to provide the counterbalance force.
Spring sash windows became common in the 1920s-1950s as a more economical alternative to box sash, particularly for speculative housing and commercial buildings. They were marketed as "maintenance-free" (no cords to replace), though in practice the springs can fatigue and lose tension after 30-50 years.
Advantages over box sash:
- Slimmer frame (more glass area for the same opening size)
- No sash cords to replace
- Slightly cheaper to manufacture
Disadvantages:
- Not period-authentic for pre-1900 buildings
- Spring replacement requires specialized tools and knowledge
- Fatigued springs can make sashes drop suddenly (safety risk)
- Conservation officers will not permit spring sash on listed Georgian or Victorian properties
What is a spiral balance sash window?
Spiral balance sash windows use a twisted metal rod inside a tube as the balancing mechanism. As the sash moves, the rod twists tighter or looser, providing the counterforce. This design allows the slimmest frame profile (75-85mm) and became popular from the 1960s onwards.
The spiral balance unit is a complete sealed cartridge, typically mounted in grooves milled into the frame jambs. When a balance fails, you replace the entire cartridge rather than trying to repair the internal rod.
Spiral balance windows are common in:
- Post-1960s housing developments
- Modern replacement windows (especially uPVC and aluminium)
- Commercial buildings where slim sightlines are preferred
They are not appropriate for period properties and will not receive conservation consent in listed buildings or designated conservation areas.
How do period styles differ?
Georgian sash windows (1714-1830): Almost exclusively box sash. Characterized by six-over-six glazing (six panes per sash) with slim glazing bars (12-25mm wide). Frames often flush with the wall face (pre-1774) or set back 100mm (post-1774 Building Act). Timber sections deliberately slim to show off joinery skill. Hand-blown cylinder glass with slight distortions.
Victorian sash windows (1837-1901): Box sash remained standard. Larger panes became fashionable as glass manufacturing improved. The two-over-two pattern (two large panes per sash) became common after 1840. Some grander properties used one-over-one (single large pane per sash). Horns (projecting lugs on the meeting rail) appeared from the 1860s to strengthen the joints for heavier glass. Frames typically 140-165mm deep.
Edwardian sash windows (1901-1910): Box sash still dominant but early spring sash starts to appear in speculative housing. Glazing patterns revert to smaller panes in some Arts & Crafts influenced designs. Coloured glass (stained or leaded lights) common in top sashes. More decorative mouldings than plain Victorian equivalents.
What are the common glazing patterns?
Glazing patterns describe how many panes of glass each sash contains, written as bottom-over-top. A six-over-six window has six panes in the bottom sash, six in the top.
Common UK patterns:
- Six-over-six: Georgian standard, each pane roughly 250mm × 350mm
- Eight-over-eight: Grander Georgian properties, slightly smaller panes
- Two-over-two: Victorian standard from 1840s, large panes 600mm × 900mm+
- One-over-one: Late Victorian/Edwardian, very large single panes
- Six-over-one: Victorian compromise, decorative top sash over plain lower
- Two-over-one: Common in 1920s-1930s suburban housing
The pattern affects both the visual character and the ease of repair. More glazing bars mean more joints to maintain but easier glass replacement (smaller panes are cheaper and easier to source). Large single panes are visually cleaner but expensive to replace if broken (£180-350 per pane vs £25-60 for a small Georgian pane).
Which material should I choose?
Timber (softwood or hardwood): The traditional choice and the only acceptable material for listed buildings. Softwood (typically Scandinavian redwood pine) is cheaper (£800-1,200 per window supply-only) but needs repainting every 4-6 years. Hardwood (oak, sapele, accoya) costs more (£1,200-2,000 per window) but lasts 60-80+ years with minimal maintenance. Accoya (acetylated pine) offers softwood workability with near-hardwood durability and comes with a 50-year guarantee against rot.
uPVC: Cheaper upfront (£400-700 per window fitted) and sold as "maintenance-free." Realistic lifespan is 20-25 years before seals fail and plasticizers degrade. Not permitted on listed buildings or in many conservation areas. Cannot be repaired like timber (when it fails, the whole unit must be replaced). Poor thermal performance compared to well-fitted timber (uPVC frames conduct heat, timber insulates). Resale value studies show uPVC windows reduce property value by 5-8% on period homes.
Aluminium: Slim sightlines and good durability (40-50 years). Expensive (£1,400-2,200 per window). Common in commercial buildings and modern high-spec residential. Not period-authentic. Thermally broken aluminium (with insulating plastic strip between inner and outer frame) performs reasonably well, but solid aluminium is a thermal bridge.
For any building pre-1919, timber box sash is the appropriate choice. For post-1960s buildings without heritage constraints, the decision is primarily budget vs longevity.
Related guides
- Historic England, "Traditional Windows: Their Care, Repair and Upgrading" (2017), accessed 21 September 2026
- SPAB, "The Repair of Wood Windows" technical guidance (2021), accessed 21 September 2026
- Georgian Group, "Sash Windows" conservation note (2019), accessed 21 September 2026