Repairing Rendering on Brickwork

[JR]

X-No-Archive: Yes

Hi

The front of the house is rendered in (I assume) concrete, but it's got a few cracks, a leak or two (to be fixed!), and a few patches which have "blown" away from the brickwork, but are still held there by the rest of the rendering. (You can tell the areas are loose because when I tap them they sound different)

Would I be better off chiselling the bits out where they're not stuck to the wall any more (they might be quite big) and trying to re-render those patches after I've fixed any cracks in the brickwork?

There's a light "dimple" pattern on the rendering that you can only see from close up, but it's only the first floor (above a flat roof) that's actually rendered. Will I need to match this, or will masonry paint cover a multitude of sins?

Thanks in advance.

Jason

[The Medway Handyman]

Part answer. The rendering will be a sand/cement mix not concrete. The pattern is done using a tyrol machine

which all hire shops will have.

[Stuart Noble]

IME ordinary masonry paint would be a waste of time on blown render, and may well make things worse. Johnstones and others do solvent based products which are much more effective.

[mail]

Dear Jason First, determine if it is a sand:cement or a lime render. It is likely to be the former if it is a building from the 20th C onwards. If, as is likely it is sand cement, estimate the percentage area you are likely to need to replace by judicious removal of selected areas that appear to be unsound v (post-removal) the actual area you had to remove to get to a sound bit of render. From these data you are able properly to estimate the likely removal rate over the whole area. If it is more than, say, 40% consider re-rendering entirely. If less then repair. The ultimate document for lime mortar repairs is by John Ashurst "Mortars, Plasters and Renders in Conservation" but for a simple sand:cement repair the BRE have reasonable data and Defect Action Sheets. If I were to do this I would

1) cut out the defective mortar using an angle cut like a dentist so that the mortar is like a dental filling bigger by the base at the brick than on the face 2) rake out the pointing 20 mm min 3) wet it down and apply a backing scratch coat 10 to 12 mm of 3 parts sand 1 part Portland cement with a) a water-proofing additive such as SIKA No 1 (slicatious not hydrophic) 4) dub out to the final layers level with the rest and finish off in a matching rough finish with the machine suggested IF that is the finish explanation Bear in mind you need to finish to the same level as the original plaster not the final paint layer! Allow it to set for some weeks before you try to paint it Use an appropriate paint

Chris