Bay Windows

A bay is structure with windows in it

On most period houses the bay is holding something up. The wall above it, the roof over it, sometimes a second bay on the floor above — the load comes down through the bay’s own corner posts and mullions and into its base. That is the difference between a bay and every other window on the elevation: take a casement out and you have a hole; take a bay out and you have removed a piece of the building’s structure.

It is also why a bay is the window most often replaced badly. A unit ordered to the opening size and dropped into a structure nobody surveyed will carry load it was never sized for, and it shows within a few winters — dropped heads, sashes that stop running, cracking in the brickwork above. The structure has to be understood before the joinery is drawn.

How a bay replacement actually runs

The structure above is propped before anything comes out. Acrow props take the load of the wall or roof bearing on the bay, and they stay in while the old bay is extracted and the new one is set in and made good. None of that is improvised on the day: it is planned before the job starts, because the propping decides the sequence and the sequence decides how long the elevation stands open.

The mouldings and trims are part of the job rather than something fitted afterwards. Brackets, cornice, the panelled or rendered base, the internal architraves and the window board are restored where they survive and remade to match where they do not. Where the brickwork or the bay roof has gone past restoring, that work can be taken on with the joinery rather than left for somebody else to reconcile.

A bay is programmed as a small project for that reason — closer to a piece of building work with joinery at the centre of it than to a window order.

Sash bays and casement bays

The elevation decides which. A Victorian or Edwardian bay almost always wants sliding sashes in the proportions the elevation already carries — the centre light wider than the splays, the meeting rail sitting where the neighbouring windows put theirs. A cottage, farmhouse or Arts and Crafts elevation is as likely to want side-hung casements, sometimes with leaded lights or tracery to the centre.

What does not change between them is the geometry. A bay is made of angles: the splays of a canted bay, the returns of a square one, and the mullions where the planes meet. Those mullions are the slimmest members in the window and the ones carrying the head, so the specification hangs on getting their sections right — slim enough to read correctly, sized to hold up what is above.

Listed and conservation work

On a listed building or in a conservation area the bay is usually the most conspicuous window on the front of the house, so it is the one looked at first. The specification is produced accordingly — sections, mouldings, glazing and ironmongery set down before manufacture and matched to what survives, written as the document consent is granted against rather than drawn up after the fact.

Material follows the same rule as the rest of the studio’s work: oak where the timber is meant to be seen, Accoya where the bay is to be painted and the paint line has to hold. On a painted period bay that is nearly always Accoya, for the reasons set out on the oak page.