Kedleston Hall - Stone Stairs
Location: Kedleston Hall, Near Derby
Building period: late eighteenth century
Listing Status: Grade I
Construction value: c.£150,000
Kedleston Hall is composed of a central block with two flanking pavilions. All of the vertical circulation within the house is on stone ‘cantilevered’ stairs. The stairs include the Great Stair, Blue stair, Pink stair and Orange stair.
Background
Concern over the Great Stair was originally raised 20 years ago following which works was carried out to try to stiffen the landing at piano nobile level, it being on the main visitor route. The strengthening took the form of introducing tie rods within the thickness of the landing. Over the years this has induced some secondary cracking. The area of greatest concern was the top landing to the Great Stair. This has wide and slender treads and rises the full height of the building.
The top landing of the stair is wide and thin and noticeably distressed, in the structural engineer’s terms it was considered highly defective and of concern with regard to useage. In view of this a support scaffold was put in place, carefully designed and constructed not to apply forces to the stairs but to be present if needs be.
Great Stair
The top landing of the Great Stair was important to the Property because it was the only access route to several of the top floor rooms. In view of the fragility of the landing discussions were held to consider if there were means to make it perform to give access to these rooms. Consideration of a variety of options involving provision of support brackets, tension rods a structure built above and reconstruction with new stone and the like were all dismissed as to invasive not only physically but also visually into what is in some respects a work of art. An alternative strategy was developed whereby opening up a route through other to floor rooms could mean the Grate Stair could be treated with a light hand. The new route involved the creation of a new jib door in a partition that, from archive research, was known to be a later introduction. With the new route in place, work to the Great Stair was then able to take the form of traditional masonry repairs by pining with stainless steel dowels. The stair is generally not in use but the functionality of the house from the operational point of view has been maintained.
Blue Stair
In conjunction with work to the Great Stair a functional service stair needed to be in place to serve each of the east and west sides of the main central block. Of the two stairs the Pink Stair was considered generally to be in serviceable condition only requiring some small additional support steels under a landing, concealed in existing service ducts.
The Blue star was more of a challenge. Many of the treads had cracked in a manner typical of failure to cope with the torsional loads imposed on them. In discussion with the structural engineers the initial approach that was tried was to resin inject the cracks to reinstate some of the lost strength - this was a technique that had been successfully used by Alan Baxter and Associates on the Dean’s Stair at St Paul’s Cathedral. The resin injection process required sealing around the cracks so that a vacuum could be created to help draw the resin through. Different tapes and sealants were trialled to determine which would not leave any unwanted residue on the stones. With a sealing system in place trails were carried out using both polyester and epoxy resins of different viscosity. The resins were put under varying pressure to see if they could run through the cracks. In spite of a number of trials it was determined that the crack size was too fine for the resins to be drawn through.
A new strategy needed to be developed. This type of work took different forms depending on which part of the stair was under consideration. For the main flights use of stainless steel pins set in resin across he various cracks was adopted. A detailed assessment of the cracks was carried out by the structural engineers (this was done on all of the stairs so a good condition base line was established) so the efficacy of the pins could be assessed. On the top landing, (which is ‘L’ shaped) one leg needed to perform well for general everyday use. This section of top landing was strengthen by the introduction of a single steel. The pros and cons of encasing the steel were debated both within the region and with the National Trust Architectural panel and it was concluded the steel should be left exposed, keeping its physical presence to a minimum. By careful selection of steel profile and weight a slim section was introduced whose lines coincided with the plaster cornice.
The other leg of the landing did not need to work for general everyday use but did need to be able to perform in event of emergency. The solution adopted was not to use a further steel – which would have been visually unsatisfactory and would have reduced the head height on a quarter landing below to an unacceptable dimension - but to fix a stainless steel strap to the underside of the landing which would prevent any catastrophic collapse should the landing become overloaded in an emergency.
On both the Great and the Blue Stairs those landing stones that had their tails in doorways were provided with restraints to stop overturning. To achieve this the stone thresholds in the doorways were lifted and stainless steel angles were introduced to hold down the tails.
The end result has been one of relatively low physical intervention with little visual, impact but nevertheless leaving the property with functional serviceable staircases.