Tag Archives: Phase 1 problem

Problems with a corbel

Post 23 (a) North East Ceiling Corbel 14 March 2010
Intact boss and corbel before any damage
Post 23 (b) Corbel & Clean Plaster Break from Below Left.
The base of the corbel after removing the boss

In an earlier post (on 1st June 2104), we showed what had happened to one of the decorative bosses in the gallery of the former East Kirk when wet rot had destroyed the strength of its internal wooden structure. No-one was hurt when it crashed through the floor overnight, but there was concern about the safety of all the other decorative bosses, so they were carefully checked. One was found to be loose, so the decision was taken to carefully remove it. The first two pictures show an intact boss and a close-up of what remained after it had been removed. This was in an area which had already been treated for dry rot. On looking inside the small opening of the corbel, there was some concern that there could be some dry rot inside the plasterwork – but it was not possible to see enough to be sure. Complicating the matter was that nothing was known about the internal structure of the corbel. Obviously there would be structural wooden beams, which were supporting one of the main roof beams. If there were dry rot it could rapidly destroy the strength of the wood and so potentially cause a partial or total roof collapse.

Post 23 (c) Plaster removed from corbel showing structure of beams P1020025 Corbel 01.06.2011
The interior structure of the corbel

The decision had to be taken to remove all the decorative plaster from this corbel so that any necessary treatment or replacement could be carried out. However, before this could be done, it was necessary to install some scaffolding to support the roof beam, just in case the wood had already lost its strength and it was only the plaster which was supporting the roof beam. Fortunately it became obvious that there was no dry rot, which was a great relief! Removing the plaster also revealed the internal structure of the corbel. This showed only two support timbers, resting on a stone protruding from the wall. One timber was vertical (the king post), whilst the other went up at a steep angle, far more steeply than the decorative plaster work would have suggested. The final photograph shows the exposed wood inside the corbel (with the support scaffold still partially in place). All the plaster was removed carefully and kept, so that it will be possible to reinstate the corbel when the interior build takes place, either using plaster or fibreglass.

Dry rot

In a previous post, we showed pictures of the dramatic effect that wet rot had on the structure of one of the bosses in the former East Kirk. This was caused by water getting in through the leaking roof. We have also had problems with dry rot. The name is slightly misleading since it too has a need for an increased level of moisture, but this is lower than with wet rot. Dry rot normally thrives with 20-40% moisture.

 

The fungus Serpula lacrymans is the main cause of dry rot. An outbreak starts with a spore germinating when conditions are right (often about 30% moisture). This develops thread-like hyphae which are capable of ¬†finding ways through stone and mortar as well as wood. A mass of hyphae forms a mycellium, which can look like grey fur (as shown in the first photograph). Post 18 (a) 01.06.2011Growth of the fungus in wood rapidly destroys its strength so that it may no longer be capable of load bearing in a building. Eventually the fungus will produce a ‘fruiting body’ to release more spores. The hyphae can travel considerable distance from the original spore. One of the outbreaks we have had started inside the gallery structure and was first seen two storeys lower and about 20 feet away.

 

Growth of dry rot on arch head of South Window 2nd from West 7sept09Often dry rot will start when a building is starting to dry out. This has been the case with the former East Kirk building, where there have been a number of outbreaks. The fruiting bodies can appear on wood or stone, as shown in the photographs. The first one shows a fruiting body on stone (above a window in the gallery).

 

 

Fruiting Dry Rot NE Corner of St Mary's Chapel 1 June 2011The second shows it on some of the 17th century carved wood in St Mary’s Chapel. The orange colour is caused by the spores ready to be released.

 

 

Treatment is possible, but the sooner the better – so constant vigilance is required where there is a known risk.

 

 

 

Problems with wet rot

For many years there were problems with rain and snow getting under the slates on the roof of the former East Kirk of St Nicholas. Historically, the roof had a very low pitch – 18 – and when it was given a slate roof after the fire of 1874, it would have required large overlap of slates to ensure no ingress in driving rain or snow. Over the years this led to a number of problems which had to be sorted out during Phase 1 of the restoration work.

North East Ceiling Corbel 14 March 2010In this post the focus is on one particular area on the north side of the building. The ceiling in the former Kirk has decorative plaster works, with corbels and bosses along the walls. The first photograph shows these structures. The wood inside the boss nearest the camera was, unknown to us, being damaged by wet rot caused by water running down the inside of the wall behind the lath and plaster.

 

Rotten & Damaged East End of Corbel BossDuring the Phase 1 works, the wood gave way and one morning it was found through the gallery floor as shown in the second photograph. It is fortunate that this happened overnight and that no-one was underneath. We will post about the actions we had to take later in the year.

With the new roof in place the walls are now perfectly dry, but are being monitored on a weekly basis.