The two thatched timber-framed terrace cottages sat serene and virtually unchanged in the village of Brockhall, in rural Northamptonshire, for well nigh four centuries. But one day in March last year disaster struck. The fire in the Grade II-listed building is thought to have with a spark from a chimney landing in the thatch. Bar a few charred timbers and stone walls, the roof and upper floor internal structure went up in smoke.

While it looked a mess, however, once the remains of Rose and Western Cottages had been assessed, it was decided they could be salvaged. The aim was to return the internal timber structure and external façade to its original glory, using, as far as possible, the same materials and construction methods. But the restoration team also took the opportunity to add a few modern fire prevention tweaks.

Little remained of the upper floor structural timber in Western Cottage after the fire

The restoration strategy for each of the cottages was pretty similar, but the owners chose their own teams for the work. The contract for Western Cottage went to architects Barker and Associates and the restoration specialist arm of building contractors Haymills – Haymills Conservation.

The first step was to encase the whole structure in a 30ft scaffold cage covered with Monarflex sheeting to protect the remaining fabric and allow work to continue in all weathers. Next the interiors were stripped out, which necessitated some demolition to loosen what remained of the structural timber for reuse. It didn't add up to a great deal. “A charred section of the principal trusses of one of the A-frames was one of the few elements left,” said Haymills Conservation's site manager Dave Booth.

The first stage of restoring the timber structure was to install three 200x100mm green oak beams, each weighing in at 500kg, across the width and length of the house. These take the weight of the upper floors and had to fit exactly in to the original openings in the walls. It was a fiddly process.

Beneath the roof lathes a layer of Envirograf FB70 should help prevent a repeat of the catastrophic fire

“Because the building was enclosed in the scaffolding frame, we couldn't use a crane, and the scaffolding itself couldn't support a block and tackle, so we had to slide the beams in from a flat-bed hiab, through two windows using scaffold poles as rollers,” said Booth. “It was also a tense moment for our carpenters doing the final cutting to fit on site. These were expensive timbers and were on a fairly long lead time. It was a measure twice, cut once job!”

The two beams that met at the corner of the cottage's living room and dining room also had to be fitted together using a traditional half-lap joint to minimise movement. Once they were in place, 50 green oak floor joists were fitted across the beams. Like the latter, these were supplied by restoration timber specialist Whippletree Hardwoods of Royston, in a mix of English and European varieties. They linked into the beams using half-lap joints, with chipboard laid over the top.

Ideally the heavyweight replacement green oak A-frames would have been prefabricated and put in place in one piece. But again the scaffolding was an obstacle. “We had to winch up one side, then the other, bracing the weight on the wall, and finally sliding the collar over before clamping them with cloth ratchets and dowels,” said Booth.

Heavyweight oak beams were installed the length and width of the house

To cope with the loads, the A-frames were made with double mortice and tenon joints, the carpenters using traditional tools, including adzes, to cut the timber. “We've got some real craftsmen in-house and they played a key role in deciding how the structure fitted together,” said Booth.

Once the A-frames were fixed, six green oak purlins were fitted, running the length of the house, and fixed to the principal trusses with heavy-duty coach screws. These provided the base for the oak rafters which sat off the wall plate and were notched over the purlins and half lapped where they met at the apex forming the roof ridge.

Among the modern fire prevention additions was a layer of FB70 fabric from Envirograf. This comprises foil reinforced glass cloth impregnated with a red intumescent and, in the version used by Haymills, is water as well as fire resistant.

“The fabric handles like a thick roof felt and we attached it using the special adhesive supplied on to a layer of plywood nailed to the rafters,” said Booth. “It's not cheap – the 180m2 we needed cost £3,000 – but it gives you at least a 70-minute safety window in a fire. If the thatch catches, the rest of the building is protected and vice versa.”

The oak rafters link into one of the main 500kg beams

To reduce the fire risk further – and meet modern Building Regulations – the chimneys were also raised to a minimum 1.8m above the thatch ridge line. The single chimney was also lined using the Cico system, where a 'balloon' mould is inserted the length of the flue and light, quick-drying aggregate poured around it, leaving a smooth surface to minimise tar and soot build-up. In the other chimney, Lytag (volcanic rock) was poured around the existing flue and set in place with cement.

The modern fire precautions are hidden in the fabric of the building, so visually, inside and out, it is as good as old. The doors and windows damaged in the fire have been replicated in kiln-dried English oak and the new edge has been taken off the exposed structural timbers with a light sand-blasting. The beams have also been given a curved “stopped chamfer” detail which was prevalent in the 17th century.

Finally the walls were given a three-layer lime render, the first comprising sharp sand, lime, water and horse hair, the second softer building sand and lime mix, the finishing coat silver sand and lime.

The restoration took 22 weeks and cost £160,000, but everyone involved thinks it was worthwhile. “It has been a joy to see the building come back from four walls to a home and it's had a lot of compliments,” said Booth. “Perhaps the most pleasing was from the owner of Rose Cottage. She said she might put her house on the market and buy this one!”