On an ordinary uPVC casement the opening sash stands proud of the outer frame by a centimetre or so. You can see the step from the pavement and it is the detail that makes a plastic window look like a plastic window.
A flush sash sits level with the frame instead, the way a joiner made timber windows before any of this existed. From the street you see one flat plane of frame with a fine shadow line where the sash meets it.
That is the entire difference, and on the right building it matters a great deal.
Which houses want them
Stone frontages. Anything Victorian or Edwardian. Cottages and converted steadings. Streets where the neighbours still have timber and yours would stick out. Conservation areas, where the planning department will be looking at the sightline whether or not uPVC is permitted at all.
Also, increasingly, new build. A lot of developers have gone flush across the board because it photographs better.
If your house is a nineteen sixties semi with wide windows and a bay, a standard casement will look correct and a flush sash will cost you more for a difference nobody notices. We will say so.
How they are built
The sash and frame are designed as a matched pair rather than the sash being a smaller version of the frame. That means a different profile system, not a modification of a standard one, which is the honest reason they cost more.
The gasket arrangement changes too. On a standard casement the sash presses onto a seal in the frame rebate. On a flush design the seal has to work in a tighter, shallower joint, so the quality of the gasket and the accuracy of the manufacture matter more. A well made flush window seals every bit as well as a standard one. A badly made one whistles.
Hardware is conventional. Friction stays, espagnolette locking, standard handles. Which means that in fifteen years, when something wears out, we can get parts.
Finishes
This is where flush sash earns its keep. The point of the style is to look like painted or stained timber, and the foils have got good.
White for a painted look. Cream and heritage off whites for older properties. Anthracite grey, which has been the default for a decade now. Chartwell green, sage and duck egg for cottage frontages. Oak and rosewood woodgrains where you want to read as timber rather than as paint.
Two points worth knowing. Foiled finishes are available on one side or both, and white inside with colour outside is both cheaper and usually what people want. And a dark foil on a south facing elevation runs hot, which over many years is what ages a frame, so the substrate quality behind the foil matters more on a dark window than a white one.
Astragal bars and glazing detail
If you are chasing a period look, the bar detail does as much work as the flush sightline.
Bars inside the sealed unit are the cheapest and look it, because there is no shadow. Surface applied bars on both faces with a spacer between them inside the unit are the usual compromise and read convincingly from the pavement. A true astragal, where the sash is genuinely divided into separate panes, is the real thing, costs the most, and is the only one that will satisfy a fussy conservation officer.
There is more on matching existing glazing detail in our note on Georgian bars and leaded glass.
Planning and listed buildings
If the property is listed, or in a conservation area, do not order anything until you have spoken to the council. In some areas uPVC is refused outright regardless of how good the sightline is. In others a flush sash in the right colour with the right bar detail is acceptable where a standard casement would not be.
Scotland runs on its own building standards and its own consents rather than the English system, and your local authority is the only reliable source for what applies to your address. Our notes on the Scottish window rules cover the general position.
Where consent is going to be a problem, secondary glazing is sometimes the route that keeps everybody happy.
What we would tell you on the doorstep
Flush sash is worth paying for when the building deserves it and when you can see the frontage from the road. It is not worth paying for at the back of the house where nobody looks, and there is no rule saying every window has to match. Plenty of jobs we do are flush at the front and standard casement at the back, and the money goes where it shows.
That is not the advice a salesman on commission gives. There is nobody on commission here.
Other styles
Tilt and turn for inward opening and cleaning from inside, reversible for full rotation, vertical sliding sash for the traditional up and down Scottish sash, or the replacement windows page for how we quote.
Getting a figure
Send a photograph of the frontage on WhatsApp and we can tell you fairly quickly whether flush is the right call for your building. Or ring 01698 604 278, Monday to Friday, eight until five. Contact details here.
Written by the team at Wishaw Window Repairs. Over thirty years repairing and replacing uPVC windows and doors across Lanarkshire.