Marianne Suhr: Replacing a timber frame

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Old buildings expert Marianne Suhr checks the progress at Watts Barn, where ancient beams are undergoing a health check and preparations are made for a sturdy new framework.

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: Repairing the ancient timber frame and laying a new concrete foundation plinth.

With all the necessary permissions in place, a detailed design to work to and a builder on board, it was all systems go for Watts Barn. It had taken a very long time to get to this point, and owners Heather and Graham were really keen to see the project get moving.

Site foreman Mike Staples, aka ‘Chico’, was running the job for the main contractor, Gates Builders. He recognised that the project was going to be a complicated one that would utilise some very new materials, such as Hemcrete, and that he and the team would be on an exceptionally fast learning curve.

A metal strap repair to an ancient barn timber; Richard and Mike closely inspect the old rafters; signs of beetle attack to the exposed end grain of a timber beam

ABOVE (clockwise from left): A metal strap repair to an ancient barn timber; Richard and Mike closely inspect the old rafters; signs of beetle attack to the exposed end grain of a timber beam.

Close inspection
The first task was to strip the existing elm boarding from the timber frame. This was done with great care, so that any boards in good condition could be set aside and reused. Richard Cain, the building surveyor, explained that the boarding was stiffening the frame, so as it was taken off certain timbers had to be propped up to make sure the structure didn’t collapse.

Each timber had to be inspected and assessed, and repaired wherever necessary. ‘We were really keen to keep as much of the historic structure as possible,’ Richard told me. ‘We particularly wanted to keep the wibbly-wobbly lines, rather than straighten the whole thing up.’

The new design incorporated an impressive living space to the centre of the barn, open from the ground floor right up to the rafters. At either end, first floors were inserted to accommodate the bedrooms. But to achieve enough height to put in a first floor, the builders had to dig down by nearly two metres, a part of the project that had to be done with great planning and care. The whole frame was propped up while a mini digger gently nibbled away at the earth beneath, a task which was pretty hair-raising at times.

The digger carefully nibbles away to lower the floor level beneath the barn; The concrete floor slab is cast

ABOVE (left-right): The digger carefully nibbles away to lower the floor level beneath the barn; The concrete floor slab is cast.

Support system
When owners Heather and Graham visited the site, they must have questioned what they’d let themselves in for. It resembled a jumble of rotten old sticks propped up on stilts in a sea of mud. By now, steel toecap welly boots were essential site attire; it took a huge leap of imagination to visualise a warm and cosy home one day replacing this mass of scaffold and acrow props.

Once the earth had been dug away, the bases of the props were boxed in and the whole floor was flooded with concrete to create a raft foundation. Once set, this would duly form the structural base and the floor slab.

The new plinth walls were built off the concrete to support the ancient timber frame above. At last the skeleton of the building was structurally sound and finally ready for conversion to a 21st-century home.

How good is your wood?

Old oak, provided it has been kept reasonably dry, will be incredibly hard and strong – even if it has a few woodworm holes on the outer surface it will usually outlive a new bit of treated softwood, so think twice before condemning it.

Use a screwdriver to probe any suspect timbers. If you can’t push it in, then chances are the timber is sound. Where they have been subjected to damp conditions for a long period of time, they may have rotted through, but this should be apparent.

If you need to assess timbers without damaging them, then use a non-destructive testing company with a micro drill. Although this might appear expensive, it could save you a fortune on unnecessary repairs. Try Demaus Building Diagnostics Ltd (01568 615662) and Environmental Building Solutions (01908 266522; ebssurvey.co.uk).