Monocouche rendering in Hull

One coat, but a thick one

Parex's data sheet for its Monorex GM is one example, and it shows how much material goes on. For a scraped finish on masonry, a compact, level coat of 18mm is applied, left for between 3 and 16 hours depending on the wall and the weather, and then scraped in small circles, taking off no more than 3mm. The minimum finished thickness after scraping is 15mm. For a sprayed texture, a level first coat of 13–15mm goes on, is allowed to dry, and the textured layer is sprayed over it.

The same sheet lists the finishes: a medium scraped surface, spray textures from a light Tyrolean to a heavy roughcast, and a smoothed spray texture made by pressing a float over the stiffened texture.

Backgrounds it needs

Monocouche suits new masonry and render that is clean, sound and well bonded. Parex lists concrete blocks, bricks, clay blocks, aircrete, stone and existing well-adhered render among suitable substrates. It rules out weak mortar or gypsum-coated walls and painted surfaces unless they are specially treated, and asks to be consulted for work below the DPC.

Mixed backgrounds are the trap. A rear extension in new blockwork joined to an older brick wall, or a block wall patched with brick, soaks up water unevenly. The data sheet recommends a key coat on very high or very low suction backgrounds, and on wet or patchy ones or where different materials meet, left to dry for at least 48 hours, to guard against block ghosting, where the pattern of the blocks shows through as shade differences. Reinforcing mesh may also be specified.

Application, step by step

Each elevation is normally sprayed in one session and scraped in the next, so the whole wall is finished at the same stage.

1. 1

   Clean the wall and remove anything that would stop adhesion; repair and dub out first.

2. 2

   Apply a key coat where suction is uneven or materials change, and let it dry.

3. 3

   Fix beads at the base, corners, reveals and movement joints to the finished depth.

4. 4

   Apply the render in one compact, level coat across the whole elevation and rule it flat.

5. 5

   Leave it to stiffen, then scrape the whole wall in small circles, removing only a few millimetres.

6. 6

   Brush off the dust and stand back to check the whole surface while faults can still be corrected.

7. 7

   Keep the fresh render damp in warm weather and protected from rain, frost and drying wind.

Shade, weather and exposure

Scraping exposes the coloured body of the render, so its timing sets the shade. Parex warns that early scraping gives a slightly darker colour and later scraping a lighter one, so each elevation has to be scraped at the same stage, and advises using the same batch number across a façade. A wall finished over two days, or with half of it in sun, can show a line.

Temperatures matter more for dark colours. The minimum for Monorex GM is 5°C for light colours and 8°C for dark ones, with special precautions above 30°C, and it should not go on frozen or thawing walls, in direct sun or in hot, drying winds. Very wet walls can cause discolouration.

For very severe wind-driven rain, high-exposure or coastal locations, Parex asks to be contacted because the render may need to be thicker or given an extra surface treatment. If your wall faces open ground or the estuary, ask what exposure the specification assumes.

Living with a monocouche wall

There is no routine repainting. Dirt and green growth can be washed off with a cleaner suited to the render. Repairs are the weak point: a patch will not match a wall that has weathered for years, so keep a note of the product, colour and batch.

When a monocouche has faded or been patched beyond hiding, the usual answer is a coating made for existing render, which turns it into a respray job. K Rend, for example, makes a coating for scraped and unscraped renders, applied after algae treatment and repairs.