Replacing windows: when you need a scaffold and how to set it up
Whether a window replacement needs a scaffold usually comes up when the window company's quote arrives. Some firms replace every window in a house from inside without any scaffold; others insist on one. It depends on how the new windows are fitted, how heavy they are and what happens on the outside of the facade. This guide helps you answer the question for your building and, if a scaffold is needed, plan it so that installation goes smoothly.
From inside or from outside?
In a conventional replacement the old window is removed from inside, and the new one is set into the opening from inside, aligned, fixed and sealed. The fitters stand in the room and the unit comes up the staircase. If the outside of the opening stays intact and the facade is not touched, no scaffold is needed; small render repairs on the ground floor can be done from a ladder or a small tower. Replacing sash windows in a Victorian terrace, for instance, is often done entirely from inside.
A scaffold becomes likely when one of the following applies:
- External sealing: the outer weather seal, membranes or tapes, is fitted from outside and joined to the facade.
- Render or insulation junction: removing the old frame damages the render at the reveal, which then has to be made good.
- Installation in the insulation layer: in an energy retrofit the new window sits in front of the old wall, in the future insulation zone, and is fitted largely from outside.
- Shutters and blinds: external boxes, guide rails and sun shading are fitted from outside.
- Large or heavy units: floor-to-ceiling windows, sliding doors and big fixed glazing may not fit through the staircase or be safe to handle indoors.
- Facade work at the same time: if a scaffold is up for render, insulation or paint anyway, the windows are scheduled into that phase (see facade renovation scaffolding).
How heavy are new windows?
New windows are noticeably heavier than old ones, mostly because of the glass. Glass weighs about 2.5 kg per square metre for each millimetre of thickness, so triple glazing with three 4 mm panes has 12 mm of glass and weighs about 30 kg/m² before frame, sash and hardware are added.
| Unit | Size (approx.) | Glass area | Glass weight | Total (rough) |
|---|---|---|---|---|
| Single-sash window | 1.00 × 1.25 m | 0.9 m² | 27 kg | 40–50 kg |
| Double-sash window | 1.25 × 1.50 m | 1.4 m² | 42 kg | 60–75 kg |
| Floor-length unit | 1.00 × 2.20 m | 1.6 m² | 48 kg | 70–90 kg |
| Sliding door | 3.00 × 2.30 m | 5.5 m² | 165 kg | well over 200 kg |
These are rough values for triple glazing with 4 mm panes; acoustic or safety glass is thicker and heavier. For the scaffold, two fitters at about 100 kg each with tools plus a 70 kg window put about 270 kg on one bay measuring 2.57 by 0.73 m, just under 1.9 m². That is about 1.4 kN/m², already close to the 1.50 kN/m² of load class 2, and setting the unit down adds impact. A class 3 scaffold is therefore advisable (see load classes and width classes). Large sliding doors are not carried over the scaffold but brought in by crane, vacuum lifter or handling device.
The right working height
The fitter has to set the window into the opening, place it on packers, align it and seal all round. That is easiest with the bottom of the opening at about knee height, 400–600 mm (16–24 in) above the deck: the sill is reached with slightly bent knees and the head with outstretched arms.
Example: the first-floor level is at 2.95 m and the inner sill height is 0.90 m, so the bottom of the opening is at 3.85 m. With base jacks at 0.3 m and 2 m frames, the decks lie at 2.3 m and 4.3 m. From 2.3 m the sill is 1.55 m up, chest height and unsuitable for setting a heavy unit. From 4.3 m it is 450 mm below the feet, and the bottom joint is hard to reach. Starting the scaffold with a 1 m high compensation frame lifts the decks to 1.3 m, 3.3 m and 5.3 m; from 3.3 m the first-floor sill is 550 mm up, right at knee height. Ground-floor windows are then fitted from the ground or from the 1.3 m lift.
Reveals, wall distance and ties
There must be room in front of the window to lift a unit in and to work at the sides, while the gap between deck and wall is generally limited to 300 mm. Both fit together: at 200–300 mm the fitter has space and fall protection towards the wall is maintained. Deep reveals and installation in the insulation layer are harder. A window set 150 mm in front of the old wall comes that much closer to the deck, and insulation later moves the face forward again. Then the scaffold is set further out and the gap closed with inside brackets that can be moved as needed. External sills project a few centimetres beyond the finished facade and are fitted last; a sill that someone leans on from the scaffold is quickly bent.
Ties belong in load-bearing masonry, never in window frames, shutter boxes or reveal render. That matters especially here, because the substrate around the opening changes during the work. Place ties in the masonry between the window columns, far enough from the reveal that nothing breaks out during removal. Inside, every removed window leaves an opening to the outside. With normal sill heights that is not critical while the scaffold stands in front; with floor-length windows on an upper floor the opening is a fall edge from inside. Protect it until the new unit is in and tell all occupants, especially when there are children in the house.
Getting the units up
Nobody carries a 60 kg window up a scaffold ladder. There are three routes:
- Through the house: the unit goes up the staircase into the room and is fitted from there; the scaffold is only the outside workplace.
- Via a stair tower: with many units and a wide scaffold, two people can carry a window up a stair tower. The route along the deck to the opening must be clear. Access options are compared in access to the scaffold.
- By hoist or crane: for large or very heavy units, delivered straight into the opening or to a designated area.
Which scaffold for which situation
| Situation | Scaffold | Note |
|---|---|---|
| Replacement from inside, facade untouched | usually none | ground-floor repairs from ladder or tower |
| Single upper-floor windows with outside work | short scaffold at the affected window columns | still needs ties and edge protection |
| All windows with external sealing | facade scaffold on the affected elevations | match lifts to sill heights |
| Installation in the insulation layer | facade scaffold all round | larger gap, inside brackets |
| Ground floor, level ground | mobile tower or trestles | quick to move, lock the castors |
For a few windows, a short scaffold two or three bays wide in front of the window column, up to just above the head, tied and with edge protection on all open sides, is economical. It goes up in half a day and can be moved to the next column, but every move means dismantling, re-erection, new ties and a fresh inspection. From three or four window columns on one elevation, a continuous scaffold along the whole side is often quicker and leaves fewer holes in the facade. Used frame scaffolding suits both approaches.
Between removal and installation the opening stands open: usually a few hours with inside fitting, longer with outside installation or late deliveries. Keep sheeting and battens ready to close it from outside, which is quick and safe from the scaffold. Clear the deck after every window: foam cans, wedges and screws are trip hazards, and anything rolling off lands in front of the house. In winter, allow extra time for the joints, since many sealants and tapes have minimum application temperatures.
Ask the window company early about the fitting method and the weight of the units, then decide on the scaffold. When several trades are involved, schedule the windows before insulation and render. To size the scaffold, use the scaffolding calculator; once the job is done, well-kept material can be sold back to CETRAC.