Repairing a 19th-century boathouse

The boathouse at Belton House is one of only two surviving examples in the UK of a rustic stickwork construction, and over the years has fallen into substantial disrepair. In 1984, it was put into the care of the National Trust when the organisation was gifted Belton House by the Brownlow family. Belton House is grade I listed, while the gardens and parkland and the boathouse are listed at grade I and grade II respectively.

The date of the boathouses construction is unclear, but a vernacular building survey estimates it at around 1840. It is constructed of rustic timber logs and stickwork timber infill panels over a brick plinth with ashlar coping, and was originally finished with a fish-scale-tiled roof. A contemporary painting by Sophia Cust – who painted many watercolours of the interior and exterior of her family home – has been used to inform the conservation and restoration.

The trust monitored the building and, on seeing an accelerated decline, decided to repair it. The condition of the boathouse reached a critical point as the foundations and footings were found to have been eroded by the river. Meanwhile the wing walls had collapsed as tree roots sought moisture, putting pressure on the supporting walls, and compromising the connection between the brickwork, plinth stones and timber posts, with resultant loss of the lateral and diagonal bracing.

The posts were connected to the stones with tenons into stone mortices, and the tenons had deteriorated, causing the structure to rotate. A labyrinth of bracing and raking shores was therefore introduced for support and to try to arrest any collapse. The corrugated iron roof had been added in the 1970s, replacing a bull-nose Westmorland slate roof that had given the impression of fish scales. A single panel of the original rusticated stickwork side walls remained in situ.

The heritage impact assessment then evaluated the proposed repairs and their impact on the boathouses significance. With the evidence of the original materials and the arrangement of construction quite clear, it was decided that conservation that kept modern interventions hidden but well documented would be of the most benefit.

Although the boathouse construction seems simple, the quality of the original work is obvious; the challenge was therefore to replicate this and the subtle detailing in all the interventions, while also ensuring structural stability. Several proposals were discussed before the team decided on invisible structural repairs that would respect the original constructions simplicity, rather than introducing visible steelwork and straining wires that would detract from it.

The project team included the National Trust's direct labour craft teams, who would undertake the work, while staff from Clumber Park repaired the carpentry and Hardwick Hall Masons the stonework and brickwork. Selected external contractors then installed the coffer dam, a temporary structure used to exclude water from the working area, and removed the silt from the boathouse while others carried out slating and blacksmithing.

Once the boat dock was dewatered some 45 tonnes of silt was removed, having accumulated over many years. This revealed a clay floor and exposed the extent of the brickwork repairs required. Trenches were dug by hand around the perimeter of the building to relieve the pressure on the walls, identifying numerous tree roots that were pushing against them and fibrous roots in search of moisture.

Following consultation with the arboriculturist, the roots were protected by wrapping them in hessian and keeping them damp while they were exposed during installation of a barrier behind the walls.

The next task was to provide a structural scaffold from which to lift the roof vertically and allow it to slide back into a vertical position and be lowered on to the repaired and reset posts. The scaffold not only provided the structural elements to lift the roof but also beams allowing the stone copings to be lifted and placed in the boat dock for repair and resetting.

Repairs to the posts were assessed and new timber was introduced where necessary being profiled to each post. To ensure structural integrity between the posts and plinth stones, the former were worked to accept a 316-grade stainless steel shoe, cruciform in profile, secured with four 500mm stainless steel-threaded bar fixings, resin fixed to the brickwork and stone copings. This provided the structural connection between the brickwork, stone copings and the timber posts, an intervention designed so that none of the stainless steel supports would be visible.

The end grain of the base of each timber post was sealed to prevent water ingress, which had contributed to the previous decay. The posts were found to be connected to the roof wall plate with pegged mortice and tenon joints; these connections were strengthened with a hidden metal shoe, and those on the water edge also had large coach bolts introduced.

The roof was in sufficiently good condition that it did not need dismantling. The structural engineer advised, however, that the two collared trusses required strengthening, which was done by introducing supplementary rafters and collars that matched the existing ones.

Leaving the roof in place was beneficial as it protected the work beneath from the elements. Once the brickwork and copings were reset and the posts were repositioned and set true, the roof was manoeuvred back into the vertical position and lowered back on to the posts.

The next stage was to fit the horizontal rails and reintroduce the stickwork, which gave substantial lateral and diagonal restraint to the structure. Indeed, one of the main reasons that the original structure had failed was that these elements had been removed, causing the posts to rack on the foundations – that is, the structure pushed out at the roof level – so the top of the posts had to be brought back to a vertical position with the base stone copings.

When dealing with the stickwork we followed the natural flow of each piece, which required accurate execution to ensure the stickwork met and continued through the posts and rails into the next panel.

Each joint was scribed with a saddle joint where it intersected with its corresponding stickwork. If timbers passed each other they were secured with stainless steel fixings.

The most difficult elements of this work involved introducing the curved stickwork to the curved wing walls forming the quadrant that curved the opposite way and making them secure.

The corrugated iron roof was removed and replaced with green Vermont bull-nose slates to give a fish-scale appearance with mitred hips. The ridge was finished off with scalloped leadwork to match the slates.

The two weathervanes were commissioned based on both the design in the Cust painting and other vanes on the Belton estate. The vane, pointer and sphere were gilded with the remainder of the ironwork, which had been painted matt black. Several visits were made to the blacksmith's shop during fabrication to ensure that the detailing to the weathervanes was correct and that each was perfectly balanced.

The joiner in the team was an apprentice from the National Trust, and the work he did on the project enabled him to understand the philosophy behind the repairs by taking part in discussions –  conservation work can be very subjective, and it is vital that all project workers can express their views as well as allowing him practical experience.

The project emphasised the importance of providing training that allows apprentices to develop a broad range of conservation skills. One option may be to provide longer apprenticeships than are currently offered by the sector, giving apprentices an opportunity to learn conservation principles and techniques.

Such schemes would enhance the strength of qualifications and skill sets, and employers should offer suitable financial remuneration - the kind of remuneration that complements the privilege of working on some of the finest traditional buildings in the country. 

Paul Wankiewicz is a senior building surveyor at the National Trust paul.wankiewicz@nationaltrust.org.uk

Related competenices include: Conservation and restoration

Building conservation accreditation competencies include: Construction techniques, Diagnosis of defects