Is it an early Georgian flitch beam? Or a shallow truss?

Is it an early Georgian flitch beam? Or a shallow truss?

This week we will look at an interesting structural beam I found a little while ago. I was called to a mansion house in Kent, the building is Grade 1 Listed due to its very fine Georgian plasterwork to its ceilings on the ground floor. While the building is mostly Georgian, in the 1950’s it was extensively re-worked, the plasterwork was left untouched and remains preserved within the building.

The client called us in because the plain ceiling and cornice were cracking in several of the main rooms on the ground floor.

The two ceilings which were being looked at were the original ornately decorated 1735 ceilings and are the most important historical part of the building’s interior. Therefore great care was required in any proposed repair. Before any physical interventions were to take place we needed to understand the situation in much more detail, this understanding took place as follows:

• Historical research
• Environmental assessment
• Structural investigation
• Plaster investigation and assessment
• Consultation

This investigation work prior to any other work taking place allowed me to fully understand the situation and provide the client with a clear way forward which would be based on specialist advice and knowledge. Only after this process had been completed would a methodical and carefully prepared programme of repair work be planned and carried out.

The structural of the ceiling and floor above the second main room was found to be a double joisted floor, while it had an interesting ‘daub’ layer, possibly for sound insulation. There was nothing unusual in this floor type in a house of this age and grandeur. This floor structure had two principle timber beams, from which the whole floor was supported, it could be seen that these had deflected by 60-70mm at the centre.

The principal timber beams appeared to be in two halves bolted together with a central element which was raised at the mid point of the beam and was at low level at the ends with it ending before the beams entered the wall, leaving the two beams separated by an inch. This was observed in the floor above the main room, however the top of the principal beam of the hall was visible and this gap between the two beams could be seen.

 

The deflection in the principal beam had been identified as a reason for the large crack across the secondary rooms ceiling, however there was documentary evidence that this was the same in 2005, therefore the movement was not fast. It was agreed by all that this principal timber could be moving, however without long-term monitoring this would not be known. With the engineer’s agreement I investigated the construction of the principal beams, as these were similar to the principal beams in the main hall. What I found was an oak timber which may have been under some tension. It was held in a shaped recess between the outer timbers. The engineer had come across this type of beam only once before, in that case the central element was an iron shallow truss. It was thought to be a Georgian idea of increasing the strength of a timber beam, however it seems to have had a weakening effect as it required the removal of a lot of the timber from the outer beams.

 

 

The worst area of cracking in the hall was on the beam which had a larger amount of deflection,   horizontally along the plaster encasing the principal beam. This was first thought to be due to structural movement, however when the plaster conservator drilled several holes in this area no sign of structural failure could be found behind the plaster. Some areas of the beam were found to be laid to laths which had been fixed directly to the beams, resulting in the plaster being applied without the space for any nibs to form, this was resulting in the deflection of the beam to cause the ceiling to continue to crack.

 

Therefore once the plaster repairs were carried out it was agreed that we would place survey reflective markers throughout the ceilings at strategic points and a survey firm be employed to carry out periodical surveying, once a month for 18 months to ensure a full season is recorded. A visual inspection over this 18 month period would also be able to highlight any opening up of the repaired ceiling in the areas where the worst cracks opened up. This monitoring will provide confirmation of any movement during the course of the year. I will then monitor this and make further decisions with the conservation engineer once this data is gathered.

 

The investigation work undertaken on the principal beams provided information on the structural composition of the floor. The long term monitoring will be an on-going item over the next 18-24 months. It will be interesting to see what this monitoring shows, if there is movement then maybe some intervention will be required, however this will be only if the movement is beyond the tolerance deemed to be acceptable by the conservation engineer.