Joining an extension to an existing rendered house
Joining an extension to an existing rendered house
When an extension is built onto a rendered house, the new wall and the old one have to meet somewhere, and that line is where cracks, leaks and mismatched colour show first. This article explains what happens at the junction, how each side is prepared, the order of work that protects it and how a match with the original render is judged.
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What happens where new wall meets old
An extension wall and the original house are two different walls that happen to touch. The extension is usually new blockwork, even and dry once it has stood for a while; the original may be brick, stone or old render with its own history. They can move differently as the new wall settles and the seasons change, so a render skimmed straight across the junction tends to crack along it.
The usual answer is a bead on the junction line: a movement or stop bead that lets the two renders meet at a clean, flexible joint instead of a random crack. Each wall then gets the preparation it needs. The new elevation can take a base coat with mesh straight onto the blocks; K Rend's HP12, as one example, goes on at a nominal 4–6mm with mesh at stress points such as reveals. The old wall may need stripping, repair or a different system altogether.
Thickness and levels need checking at the same time. If the house keeps its old render and the extension gets a new one, the two faces may sit at slightly different depths, which shows at the corner. The DPCs may also be at different heights, and each wall needs its bell-cast bead above its own DPC; Catnic's bell-cast bead is designed for use at DPC level so water drains clear of the wall below.
If you are considering insulation on the original house later, say so before the extension is rendered. Insulation would bring the old face forward, and a junction designed for two thin renders will not suit a thick insulated wall beside a plain one.
| Element | What to settle |
|---|---|
| Backing | New blockwork beside old brick or stone |
| Joint | Movement or stop bead on the junction line |
| Thickness | New render face against the existing face |
| Base | Bell-cast beads at each wall's DPC level |
| Top | Flashing and verge where the roofs meet |
Preparing each side of the junction
On the extension side, the blockwork should be complete and dry, with windows, sills and pipe holes finished, before any beads go on. A base coat with mesh is then applied up to the junction bead. K Rend's HP12, for example, needs mesh at stress points such as reveals, and the inside corner where the two walls meet is another place cracks start, so it is reinforced too.
On the house side, tap the old render for a metre or so back from the junction. If it sounds hollow, cut it back to a sound, straight edge and re-render that strip up to the bead, so the new work is never butted against loose material. If it is sound but dirty or faded, it can be cleaned and recoated to sit better beside the new wall.
If the house itself is to be re-rendered later, the junction bead becomes the temporary edge of the extension's render and is sealed so water cannot get behind it. When the house work happens, its new render meets that bead rather than being lapped over the extension's finished face.
The order of work at the junction
Rendering is a late trade, and the junction suffers most when it is not last. A flashing fitted after the render means cutting a chase through new work. A downpipe moved after the finish leaves plugged holes that never quite disappear. A sill that turns out to finish flush with the new render will streak the wall within a winter. Getting the roof, windows and services finished first means the renderer can work the beads and finish round fixed points.
Agree who does each of those items. The builder may own the flashing and the sills, an electrician the outside lights and the renderer the beads and coats. Where the extension has a different finish from the house, the stop bead at the junction is also where one trade's work ends and the other's begins, so it is worth marking it on the drawing.
Finish the extension roof, flashings and verge.
Fit windows and doors, with sills that will project past the finished render.
Complete cutting for pipes, vents and cables.
Fix the junction bead, corner beads and bell-cast beads.
Base coat and mesh both walls, or the extension only if the house keeps its finish.
Apply the finish to each elevation in one session.
Refit downpipes and gutters and seal the junction bead.
Judging a match with the old render
New render beside old rarely matches on day one. The existing finish has weathered, collected dirt and may have been painted, while the new one is clean and bright. Through-coloured renders such as silicone and monocouche keep their colour but still weather slowly, so even a close match at handover changes over the years.
There are three honest routes to a consistent look. Recoat or re-render the old wall at the same time so both parts share one fresh finish; make the difference deliberate with a contrasting colour or a brick panel on the extension; or accept a visible difference on a side or rear wall. Matching a weathered wall exactly with a new product is the hardest of the three.
If you do want a close match, clean a small area of the old render first so the sample is compared with its true colour, then set the new sample beside it and look in daylight once it has dried. Compare texture as well as colour: a fine silicone grain beside an old scraped or roughcast finish will look different even in the same shade.
For more detail, the individual-wall service page covers rendering a single face such as an extension wall, and the whole-house page covers re-rendering the original house so both parts share one finish.
Related services
Whether the original house needs a repair, a recolour or a full re-render decides how the junction is finished. The Wakefield rendering homepage explains how to tell which applies.
Common questions
Can the extension render simply be carried across onto the house?
It can look neat on day one, but if the two walls move differently the render will crack along the junction. A bead on the junction line gives that movement somewhere controlled to go.
Should the extension be rendered before or after the house?
If both are planned, rendering them together, or the house first, avoids lapping new render over a finished extension. If the extension has to come first, its render stops on a sealed bead at the junction, ready for the house work to meet it later.
What if the extension's DPC is at a different height?
Each wall gets its own bell-cast bead just above its own DPC, and the bead line steps at the junction rather than one line being forced across both walls.
See housing and planning information for Wakefield and nearby places, including the source and geography behind each figure.
