Case Study - Government House, Wellington, New Zealand

 

 

Clay Roof Tile: Dreadnought plum Red and Brown Antique Tiles

Client: Department of Prime Minister and Cabinet

Architects: Athfield Architects

Main Contractor: L T McGuinness

Roofing Contractor: ARFI

 

 

 

Background

Government House Wellington, the official residence of the Governor General, is an historic, 100 year old building in an award-winning landscape and forms an important part of New Zealand’s heritage.

Athfield Architects were engaged along with a number of other consultants to provide an analysis of the existing building fabric and review its functionality. The result of this analysis was a concept plan to meet the future needs of Government House while retaining and conserving the heritage features of the building.

The concept design drew on the strengths and beauty of the existing spaces and structures of Government House and introduced subtle interventions to improve the experience and functionality of the environment for both visitors to the house and grounds, and those who live and work on site.

Wellington the capital of New Zealand sits in an earthquake zone at the southern end of New Zealand’s North Island and for good reason is nicknamed the windy city. The changes to the structure and fabric of the building needed to take both these factors into account and integrate a high degree of structural and infrastructural upgrade to bring it up to modern specifications.

Challenge

The original roof tiles on the roof of Government  House were supplied in 1910, they were manufactured in the UK by Tunstall Tileries of Thomas Peake Limited, a North Staffordshire manufacturer that ceased trading over 50 years ago.  A sample of these existing tiles from the roof was sent to Dreadnought by Athfield Architects, where it was established that the tile was single cambered, made from Etruria marl clay with a predominantly deep red colour.   However,  the tiles also exhibited a limited amount of subtle colour variation from tile to tile and a bluer colouring around the tail and the bottom of the sides of the tiles, a phenomenon known as  “picture framing”.  This occurs when the iron rich Etruria marl clay is exposed to a mild reduction atmosphere in the kiln, particularly in the areas where the tiles were in contact with one another.  It also confirmed that the tiles were naturally coloured without the use of any surface pigments or stains.

Salt damage had affected a significant proportion of the original roof tiles, the strong Wellington winds had driven salt laden air under the tiles, which over time caused  some of the tiles to weather all the way through leaving rather large holes.  Paul Cummack of Athfield Architects commented that salt damage is proportional to the porosity of the clay and to make sure that the replacement tiles were of low porosity to minimise a repeat of this damage and to confirm the test results provided by Dreadnought Tiles, he carried out his own water absorption tests in his kitchen by boiling the tile samples for 24hr, weighing them, drying them in his oven and then weighing them again to measure the percentage change.  This confirmed the results he had been supplied.

Having studied the photographs of the original roof in detail it was observed that there had been an improper use of certain standard tile fittings, for example valley tiles had been used upside down on a covering of a section of the ridge and the use of what appeared to be arris hip tiles not designed for the roof pitch on which they had been used.  Also certain hips had been mitred over lead soakers using special double tiles to accommodate the angle of cut and leaving space for a nail hole. The photographs suggest that these large clay sections had been difficult to secure and became easily dislodged.

The roof pitch on Government House is 35° and a probable reason for the incorrect use of arris hip fittings at the time and the mitring of some hips was that only in 1990 was the recommended roof pitch for plain clay tiles reduced from 40° to 35°, as a result of the introduction of a frost standard into the British Standard. Prior to this date tile manufacturers did not manufacture fittings such as hips and valleys for roof pitches below 40º and therefore it is likely that 35° arris hips were not available in the early 1900’s from Thomas Peake Limited.

Solution

To reproduce the original colouring of the roof as closely as possible it was suggested that a mix of Plum Red tiles together with a small proportion of Brown Antique tiles would best match the original appearance of the roof.  To test this on site a selection of samples were sent to Wellington to establish the right proportions of the mix. Following the creation of this mock up a ratio of 80% Plum Red and 20% Brown Antique was settled on to introduce the original subtle colour variation on the roof and  recreate a limited element of blue around the tails of some of the tiles.

Dreadnought advised against the repeat of the inappropriate use of fittings on the original roof.  Arris hips for a 35º roof pitch were recommended and adopted for the re-roofing; they provided the same visual roof profile, were greatly more robust, and were easier and more economical to fit.  To retain some of the appearance created by the use of valley tiles on the small section of ridge on the original roof, it was suggested that capped angle ridges could be used to give some of the same castellated profile but would introduce a much greater degree of integrity to this area of ridge, and again this suggestion was adopted.

An earthquake can generate a vertical force equivalent to 2G which would have the potential to lift all the tiles off the roof if they were not appropriately secured.  Each roof tile was therefore fixed to its batten with 2 screws.   Furthermore screws were preferred to nails because the New Zealand pine battens were found to have a bit more bounce than typical battens used in the UK.  New Zealand pine battens exhibited fewer growth rings which was felt to be the reason behind this extra bounce.  A special electric saw extension was devised with an arm and a blade that could travel under the tiles to cut the heads of the screw in the event that tiles needed to be replaced in the future.  To secure the ridges a mechanical fix was required in addition to the mortar due to the earthquake risk.

Result

The painstaking specification by Athfield Architects and the high level of workmanship by the main contractor and roofing contractor have yielded a refurbishment of exceptional quality which is not only  faithful to the buildings historical context but which has also introduced  the high degree of structural and infrastructural upgrade required in many areas.  The refurbishment carefully balanced the responsibilities of reinforcing and celebrating the heritage attributes of the house and grounds while incorporating a level of contemporary amenity, and acknowledgement of contemporary cultural place and time in New Zealand. With a 100 year design life this conservation project has successfully drawn on the strengths and beauty of the existing spaces and structures of high profile building and introduced the subtle interventions that will improve the experience and functionality of the environment for both visitors to the house and grounds for generations to come.

Comments

The roof speaks for itself, it clearly enhances the whole building and the restoration project was a real challenge but delivered in every respect.