Applying lime plaster

ABOVE (click on gallery image to view larger picture): The conversion of Watts Barn into a family home is nearing completion.

Old buildings expert Marianne Suhr revisits the conversion project at Watts Barn and admires the way the team solve the problem of applying lime plaster between the ceiling rafters.

The project: The sympathetic conversion into a four-bedroom house of Watts Barn, a Grade II listed timber-framed building, dating from the early 1700s
This month's challenge: Lime plastering and painting inside Watts Barn.

It was nine months since Gates Builders had started on site at Watts Barn. Their first job was to repair the ancient timber frame and stabilise the base of the building. They then concentrated on filling the walls with Hemcrete – an ingenious new building material made of hemp and lime – and putting the roof back on. Finally, with all the structural works complete, it was time to start the plastering. But as with everything on this project, it was not a simple job.

It had always been intended to make a feature of the ancient frame by exposing the rafters, the sloping timbers that form the roof structure. Usually this is done by cutting plasterboard to fit in between the rafters, and skimming it with modern gypsum plaster. But the barn’s roof structure was formed of such crooked ancient timbers it would have been extremely difficult to cut the plasterboard to fit.

Another factor to consider was that the owners, Heather and Graham, wanted to use traditional lime plaster and this wouldn’t stick to plasterboard.

ABOVE (left-right): The lime plaster walls are painted with breathable clay-based paints to allow moisture to escape easily into the outside air; once the reed matting was installed on the rafters, the ceiling and Hemcrete walls were ready for plastering with traditional lime plaster.

Plasterboard alternative
The solution was agreed early in the build after various tests with different materials. Before the roof went on, it was decided to use rolls of reed matting fixed over the rafters to form a substrate, or backing, for the lime plaster. Unfortunately, this decision clashed with a countrywide shortage of reed matting, usually available as stock from lime suppliers.

Richard Cain, building surveyor for the project, was determined to track some down and realised similar matting was used in gardens as screening. Having visited numerous garden centres to look at various types of screening, he eventually found something he thought might work and took it back to the site for further testing.

After fixing a small area to the rafters, a panel was plastered to check it was rigid enough for the job. The trial got the thumbs up from Mike Staples, aka ‘Chico’, the project foreman, and the material was ordered in bulk and fixed to the roof. Months later, after the roof had been completed, the matting panels were ready for plastering.

The main walls had been built with Hemcrete, a breathable material that absorbs and releases water vapour. It was crucial that this was also finished with lime plaster and breathable paint to allow it to function properly and to prevent the ancient timbers in the frame from rotting.

Fixing the reed matting and applying the lime plaster

ABOVE (left-right): Site foreman, Chico, fixes trial panels of reed matting to the rafters to create a substrate for the lime plaster; the team apply several coats of lime plaster to the walls and ceiling.

Find lime render specialists in the Period Living Directory...

Sourcing expertise
Chico had the tricky job of finding a specialist plasterer who was familiar with traditional materials. Finally, he tracked down Clive Miners of CM Plastering, who relished the challenge of such an interesting project.

Once the plastering had been completed, all the timbers had to be scrubbed to remove several centuries of dirt, along with splashes of lime plaster. The rafters and beams were then finished with boiled beeswax, to bring out the natural colour of the wood. This took several weeks. Finally, when the plaster had dried, a breathable clay-based paint was applied. Next month Building snags are sorted out at Watts Barn as the project nears completion.

The lime plastering is completed but the timber rafters still have to be cleaned and polished

ABOVE: The specialist plastering is finally completed, but the timbers need scrubbing to remove layers of ancient grime. Boiled beeswax is then used to bring out the natural colour. This is a time-consuming job, necessitating the skilled use of some very long ladders.