Lessons learned from building new round tower in heart of Norfolk

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Michael Knights, who was Norfolk County Council’s heritage and landscape manager, describes the project’s background which started almost 20 years.

It was quite a challenge to build the first “new” flint round church tower in the heart of Norfolk for almost 600 years.

In 1999, Norfolk County Council was a partner in a European project called the North Sea Viking Legacy.

This InterReg programme involved sharing cultural and historical links with more than a dozen partners in Scandinavia and north-east Europe.

Norfolk’s task was fourfold. First, to research origins of round tower churches, which had been carried out by Stephen Heywood in the 1970s.

Secondly, to design trails to encourage the public, especially children to visit these iconic buildings; thirdly, make a documentary film and finally, the most ambitious element – build a flint round tower.

Work began at the Eco Tech Centre in autumn 2000. Architect Dirk Bouwens supervised the construction, which used traditional materials including lime mortar and Norfolk flints.

It was found that flexible wicker shuttering, with hazel and willow, was essential for the exterior walls. It gave time for the lime mortar “to go off” or harden. Fortunately, a dozen wicker hurdles were already on site – used to build a “yurt” nearby. For the inside walls, curved corrugated steel sheets (from outdoor pig arks) with reed matting proved ideal.

A circle with a 10ft internal diameter, with a small extension to mimic the nave, was orientated east:west. Incidentally, it was possible to build square corners in flint.

Then 3ft deep trenches were dug, filled and rammed with alternate layers of flint and ground chalk. These walls were 2ft 6 ins thick but in practice some bulged to 3ft. A double splay round window was included, using a basketwork form once displayed at the Royal Norfolk Show to display medieval building techniques.

The flints were gravel pit rejects, roughly the size of a half brick, and laid one by one in sand-lime mortar. This was made by slaking quicklime with sharp sand in 8ft square bunkers. Layers of damp sand, then six bags of quicklime, were spread, and watered to start the chemical reaction. More sand was spread, the bunker was covered and left for a week. In hindsight, it should have been better left much longer. It was removed, slice by slice, and put into a conventional “concrete” mixer.

Laying flints was easier than first thought. To protect the walls in summer straw bales were used to keep the heat off. If mortar dries too quickly during hot spells, it will not be as strong or as durable.

Finally, a roof was laid to keep the tower waterproof. It was quickly taken over by the gardeners, who found it perfect for overwintering sensitive plants and for propagating seedlings!

The final piece of the project was equally instructive. As it had to be completed by autumn 2001, the shuttering was removed earlier than ideal, partly to show the finished tower. It was very white.

It illustrated how churches may have appeared when first constructed. They were commonly rendered with lime plaster over the mortar that exuded between the wattles.

Unfortunately that Christmas, a hard frost caused damage and part of the north wall collapsed as the outer layer of flints fell out.

This was surprising because the wall had appeared very hard when the shuttering was removed several months earlier. Yet, now a pen knife could now be pushed easily into the mortar. It was as soft as when laid earlier that spring. Why?

Yet a few days later, the mortar was almost rock hard – even for a knife blade. What had happened?

Lime mortar cures very slowly from the outside of a wall, inwards inch by inch. It reverts to calcium carbonate (CaCo3) as water evaporates and carbon dioxide from the atmosphere is absorbed.

This starts as soon as the mortar is laid. But if the wall is two or three feet thick, it takes a very long time, possibly just an inch per year in a thick wall; some may never cure

Maybe this explains why such buildings remain flexible unlike those built using hard Portland cement render. At Swaffham, these fallen flints exposed the soft internal lime mortar, which gradually hardened.

* The tower can be seen as part of a visit to the Eco Tech centre.

Lime mortar: a ‘green’ building material?

The lime process starts with CaO or calcium oxide (quicklime) which has been produced by heating chalk or limestone in a kiln to drive off carbon dioxide or CO2.

Limeburning is one of the oldest chemical processes dating to prehistoric times.

The quicklime is added to sand, where it is “slaked” by adding water. It then becomes what is called slaked lime calcium hydroxide.