The monument had suffered the harsh salt air weather conditions . The erosion was also further exasperated by the re-pointing work carried out during the early 1920s where hard cementatious pointing had replaced the traditional lime mortars. The project aimed to make it structurally sound and functional, while at the same time considering authenticity and aesthetic appearance.

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Lindisfarne – also known as Holy Island – was the site of the earliest Christian monastery in Anglo-Saxon Northumbria. Irish monks settled here in AD 635 at the invitation of the Northumbrian king, and the monastery became the centre of a major saint’s cult celebrating its bishop, Cuthbert. The community moved inland after Vikings raided the island in 793.

Source: English Heritage

Removal of Cememt Mortars

Where stone decay is evident on Lindisfarne Priory it is has often been caused by harmful salts within the stone units, this has in many cases been due to the fact that the joints within the wall have been over pointed with cement based mortar. Sand stone is porous and salt is soluble the salt gets into the paws of the stone and when the water evaporates the salt in the stone crystallizes, cement over pointing will limit the water evaporation from the stone unit and when the salt crystals expand in the stone this will lead to a breakdown in the stone, this is often found in the form of a friable powdery surface, it is best if possible to stop the salt from contaminating the sand stone in the first instance.

Moisture needs to drain from the stones the mortar should be weaker than the stone to ensure water evaporation. Cement mortars are very hard in general and therefore should not be used on historic structures Always use a lime based mortar that is more pours than the stone to allow for adequate drainage of moisture from the stone unit.

English Heritage had previously identified the problems associated with the use of cement in mortars on English Heritage properties including Lindisfarne Priory. Various methods of removing this cement pointing had been tried with some success.

It has always traditionally been unacceptable to use power tools on a historic structure however with a drill and a skilled operative it is possible to remove cement based mortars from these structures with care taken not to damage the stone units within the wall. The following pictures show the drilling out of the cement mortar and the cement and grout removed; please note the depth in which the cement material has been removed.

In the case of Lindisfarne Priory which had previously been consolidated with a concrete mix probably a 1:2 cement/ aggregate mix including some large aggregate within this recipe, the mix is known as {Scotch Pointing} the name scotch pointing originated from when the English Heritage properties within the north of England were manage by their Edinbrough office, they had a standard mix with exposed aggregate finish which is very common and is now referred to as scotch pointing. In Scotland it is referred to as Stirling grit finish.

Stone Replacement

The difficulty in deciding where to start replacing eroded stone is matched by an equal difficulty in where to stop; these four following considerations need to be noted before deciding to replace masonry.

  1. Authenticity, would the retaining of original stone preserve the buildings integrity and character?
  2. Aesthetic, the appearance of the building does it depend on retaining the marks of time?
  3. Structural soundness, will the building collapse or will there be serious failure if stone replacement has not been decided?
  4. Functional, is the building still performing in the way it was meant to be?

In most situations stone is simply replaced because ascetically is does not look right and some times this decision is fine, however, we should always remember the rule of conservation is minimum intervention. At first going into a situation where a survey is required for a stone façade with minimum intervention in mind the decision to replace stone should always be the furthest thing from your mind unless the stone is of a structural nature then there is no option but to replace.

Team Force removed and replaced the string course on the front facade. The decision to replace these stones was based on protection of the arched window below them. The existing weathered string course had badly corroded and was not functioning as it should and the vousiors within the arched window were being affected by this.

Over the arch window Team Force inserted supporting steel dowels into the stone joints for extra support.

When a decision is made to replace a stone on a historic monument the normal criteria accepted by English Heritage would be to bed the new stone to the original building line.

his method involves plumbing in the new stone to the lesser worn stones within the same location, pick three or four stones that are not showing much corrosion and plumb the new stone in place allowing for a further 5mm or so and this should give more or less the same projection as the existing stones. Unfortunately the big lesson to learn is that this is only the opinion of some, by all means build the new stone out to what is thought the original building line, however, sometimes the stone in question looks very different than you first imagined. It is important for the new stone to project slightly, however, stones on a historic structure should not hide the fact that it is a new stone and at the same time the new stone does not want to stand out like a beacon. The replacement stone needs to be subtle and sympathetic to the structure, correct tooling is therefore essential.

Lime as a Binder

The use of lime and well graded sand is very important on a historic structure. The lime binder needs to be weaker or more porous than the surrounding stone units. The reason for this is that when a wall becomes wet the moisture within the stones will need to disperse, lime mortar providing the lime binder is more permeable than the stone will draw moisture away from the surrounding stones, the moisture can then disperse out of the mortar joints.

The main function of a stone wall bedded and pointed with lime mortar is that when the wall is significantly wet due to driving rain the wall will get wet then dry out quickly, this will mean the stone units are less likely to corrode.

Lime mortar was used in the conservation of Lindisfarne Priory, the lime and sand mix a 1:3 lime, fine and sharp sands, as per the specification was said to be the correct recipe. However before the mortar work was carried out we decided to conduct a test on the sand and the binder taking into consideration their bulk densities. The well graded sand was tested and found to have a 37% void ratio and the fine sand had a 45% void ratio this was thought to be wrong. Because of the high void ratio of the fine sand there would not be enough lime within the mix to wrap around the sand particles.

We decided to use the sharp sand only in the final mix as the sharp sand was in fact well graded type sand and best suited. We then weighed the correct amount of lime binder to suit the sand type and taking into consideration the lime binders bulk density we determined that the mix was in fact a one binder to two and a half well graded sand. This when measured into containers made a 1:3 mix by volume. Care needs to be taken as some limes and sands have different bulking densities and this alters the mixing ratio by volume. Understanding how it all works is the key to success.

Construction Excellence North East Award 2011

The first time Team Force won an award at the CENE ceremony.

View Application (PDF)

Project Details

  • Client: English Heritage
  • Cost: £118,000
  • Duration: August 2010 - March 2011
  • Awards:

      Construction Excellence North East Award 2011

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