Thinking of Rendering over Pebble Dash?
Rendering over pebble dash or other render finishes for that matter can present problems. Before you consider the possibility of applying render over pebble dash you have to consider a few things.
Advice for rendering over pebbledash, consideration needed.
Firstly how solid is the current render, If the backing render coats are not solid and they fail, then whatever is applied to it will also come away. Hammer test the render to establish how much is loose and then make a judgement if it’s worth hacking off the loose and repairing before rendering over the top if you find large areas have blown (come away from the wall, lost adhesion) and are defective then it may not be worth patching. Removing the pebbledash and render and starting from a new stripped base nearly always provides a much better finish.
Has the render or pebbledash been painted?
If the pebbledash has been painted a good key may not be possible. Bonding the surface with a latex SBR OPC mix may work after high-pressure water jetting may work with a polymer render. All too often we see delaminated render that has failed due to the inadequate bond between paint finishes, the render should be ‘sucked into the wall” not just sat on a film of paint. Many inexperienced builders recommend bonding the walls with PVA glue, often this emulsifies and breaks down, again the adhesion key is lost.You also have to consider what type of paint you are going over, if its oil based, or pliolite such as the textured coatings this may well cause problems
Also if the walls are solid build (mostly pre 1919) you may be better with a lime based render, applying lime based renders over concrete renders (sand & cement) negates any benefits of having a lime render as the surface you going over does not breathe.
Cracks can also form in the old pebble dash layer and appear through the new finish. Definition and neat angles can be difficult or impossible to form, detail such as corners, window frames, stone or brick features etc can get swallowed up and buried in the new render finish. Old metal rusting corner beads that may have been used in the original dash render can quickly spoil any new render or paint you apply. As mentioned above If the pebbledash is of a concrete material (which it mostly will be) it will not breathe, on solid walls, this may cause problems. So the main things to consider are, do you want a breathable finish, can the wall take another layer of render without encroaching onto window frames etc?, Has the pebble dash been painted?, Is the render mostly solid? How thick will the render need to be to cover the undulations and high points of the pebbledash or roughcast?
If after considering the facts you decide on rendering over pebble dash roughcast or render, below are some tips on how best to prepare it and what material to use. Firstly remove and replace (with stainless or PVC-u ) any rusting galvanised angle beads, they will quickly spoil any new render. Next, remove any loose render back to a sound base, large cracks should be ground back.
If the pebbledash is very rough it can be raked back, the pebble that is, a shovel turned the other way and some elbow grease works well, basically, the outer layer of pebbles are raked off, this only works with a dry dash finish. This will provide a less undulated textured surface.
Also as mentioned, if the current render up to the window/door reveals may not allow for another layer of render, you may be best to remove this strip of render otherwise you may bury the window/door frames. The chances are that the existing dash and render around the frames, will be wavy and won’t be a nice neat finish, ( this is the return up to the windows that most houses have, this could be 20-200 mm deep)
High pressure wash the walls, very important to remove friable loose paint, if you are going over a painted surface and apply a disinfectant, to kill any lichen, algae, etc.
Patch the areas in a polymer render such as HP12 or make your own using latex SBR sharp sand and OPC, 4-1 if you wish you could add some chopped glass fibre strands to stop slumping and give strength against cracking
Render the areas you have removed and scratch the surface to form a key, finish up to around 5mm below the other render you have left in place. Next, apply your base render, about 5mm thick this can be the polymer render such as HP12 or your own accurately gauged mix, lightly key this render when it has partly set. When ready apply your finish coat this could be a through coloured render such as K rend or Parex or a finish coat of plastering sand, OPC (no chopped fibre) with a splash of latex SBR. finish flat, leave to dry then paint your new render or scrape back in the case of K rend.
There is an easier way to tackle your rendering project, why not ask All4Walls to conduct a survey at your house?
We can offer you a report on the condition of the existing render, and installation with options to either remove and replace it with an appropriate render finish or if it is feasible we can render over the finish in a suitable render finish.