In short
- New blockwork should be allowed to dry and settle before rendering.
- Timber-framed extensions need a render-board system with a ventilated cavity, not render straight onto timber.
- Movement joints are needed where new walls meet old.
- Matching colour needs the same system and a sample panel; finish to a natural break.
New blockwork
Fresh blockwork contains water and shrinks as it dries. Rendering too early risks cracking. Follow the manufacturer's guidance on substrate readiness.
Timber frame
Timber frame needs a proprietary render-carrier board fixed to battens over a breather membrane, creating a drained, ventilated cavity. The render system for boards differs from that for masonry.
Where old meets new
Old and new walls move differently. A movement joint or bead at the junction, or reinforced mesh bridging it, prevents a crack forming.
Matching the existing house
- Find the original system from building paperwork if possible.
- Prepare a sample panel on the new wall.
- Finish to a corner or joint.
- If the existing render has weathered, consider a thin-coat refresh of the whole house.
Building control and the extension
Extensions are built under Building Regulations approval, and the wall specification, including any render system and insulation, forms part of that. Make sure the renderer works to the approved specification, particularly for timber-framed and insulated walls.
Colour matching in practice
Through-coloured render is manufactured in batches, and even the same colour reference can vary slightly between batches. Weather during application, the suction of the substrate and the finishing technique all affect the final shade. Practical steps:
- Order enough material from one batch for each elevation.
- Keep a sample panel from the start as a reference.
- Finish each elevation in one continuous pass.
- Place any unavoidable joins at corners, downpipes or beads.
When re-rendering the whole house makes sense
If the original render has faded, stained or been patched over the years, a new extension in the same colour may still look different. A thin-coat silicone over the existing sound render, in one colour across old and new, often gives a better result than trying to match. It can also be the moment to address Part L. See Building Regulations for rendering.
New-build defects
If your home is under a new-home warranty and the render cracks or fails, report it to the builder and, if needed, the warranty provider before commissioning repairs, so you do not lose cover.
Questions people ask
Can render be applied straight onto new blocks?
Yes, once dry and prepared, as part of a proper system.
Why does my new extension render look different?
Weathering, batch variation and application conditions all affect colour.