Practical guide
Rendering around bay windows and shallow reveals
A bay window turns one flat wall into several small faces, each with its own corners, reveals and sill. On a rendered house it is where the best and worst workmanship shows. Shallow reveals, where the window frame sits close to the face of the wall, add a second problem: there is little room for the render to return neatly to the frame, and almost no room if the wall is being insulated.
On this page
The anatomy of a bay
A canted bay has a front face and two angled side faces, so it has at least four external corners, plus the reveals at each window, a sill or sills, and a roof or a head above. Hull City Council's appraisal for Ella Street describes two-storey Victorian terraces with canted bay windows as a common feature, so on many brick terraces these details are part of the front elevation whether or not it is rendered.
Every external angle needs a corner bead to give a straight, strong arris. Every place where render meets a window frame needs a stop bead set tight to the frame. Without beads, the narrow faces of a bay end up with wavering edges and thin render at the corners, which is where cracks start.
Beads, reveals and frames
Renderplas's installation guide for its beads shows stop beads set into render dabs and pushed into the reveals against window and door frames, and corner beads pushed onto external corners and tapped level. The render is then pressed into the edges of the beads so no air pockets are left, and finished level with the exposed bead edges.
Large irregularities in the old wall are dubbed out with mortar and left to harden before the beads go on; Renderplas's guide puts this first. On an old bay, where the angles are rarely true, that step decides whether the corners end up straight.
Sills and the bay roof
Each sill must project beyond the finished face of the render and have a drip underneath, so water falls clear rather than running down the wall below. When a bay is re-rendered, the new render can sit at a different depth from the old, so check the sill projection before rendering. Short sills are a common cause of stains and cracked render under bay windows.
At the top, many bays have a small roof covered in lead, slate or felt, with a flashing where it meets the main wall. The render above should stop on a bead clear of the flashing, not be packed down onto it. Render packed onto a flashing holds water and cracks.
Treat the bay as one piece of the wall rendering, not a set of separate patches: all its faces should be finished together in one session to avoid lap marks between them.
Shallow reveals
Where the window frame sits nearly flush with the face of the wall, the reveal is only a few centimetres deep. A stop bead at the frame and a corner bead at the outer edge leave very little render between them, so the work has to be precise. If replacement windows are planned, fit them before the render so the beads go tight to the new frames.
Shallow reveals also make it hard to add anything to the wall. With external insulation, the wall face moves outwards. JUB's installation overview for its system says that, where the frame is large enough, a 20mm insulation board is returned to the frame and rendered. If the frame is set in line with the outside of the wall, the insulation oversails the edge of the frame. If there is no room for a board, the reveal is rendered only, with a double layer of mesh, and remains a cold strip around the window.
Finishes on a bay
Coloured finishes are less forgiving on bays than on flat walls. A thin silicone finish has to be worked to a wet edge across all the narrow faces at once, or the joins show. A scraped monocouche has to be scraped at the same stage on every face, because early scraping gives a darker shade and late scraping a lighter one, as Parex's data sheet for its Monorex GM explains. Painted sand-and-cement render is the simplest to keep even.
The side faces of a bay are in shadow for much of the day, so the same colour can look different on each face. That is geometry, not poor workmanship.
Brick bays in conservation areas
On a brick terrace, the bay is often the most decorated part of the front, with moulded or contrasting brick, stone sills and lintels. Rendering over it changes the character of the house. In a conservation area, the government's permitted development guidance treats rendering any part of a house as cladding that needs a planning application, and Hull City Council's leaflet for Hessle Road lists external rendering of elevations among that area's weaknesses. If a bay is already rendered, repairing the render like for like is a different matter from rendering bare brick, but it is still worth checking the address before changing its finish or colour.
A worked example
Take a hypothetical Victorian terrace with a canted ground-floor bay, cracked render on the bay's corners and stained render under its sill. The renderer cuts the render back across the bay, dubs out the uneven brick, fixes corner beads on all four angles and stop beads at the three window frames, extends the sill so it projects clear with a drip, stops the render on a bead below the bay roof flashing, and finishes all the faces in one session. The example is illustrative.
Checking the finished bay
Before the scaffold comes down, look along each corner for a straight line, check the sealant where render meets the frames, and make sure each sill throws water clear. Look again after the first heavy rain: water should drip clear of every sill, and no dark streaks should run down from the ends of the bay roof. If they do, it is easier to put right while the render is new. Keep a photograph of the finished bay and a note of the finish used. For other rendering work, the Hull rendering homepage sets out the options.
See housing and planning information for Hull and nearby places, including the source and geography behind each figure.
