A Classic Case of Roof Spread

Roof Spread Discovered During Condition Survey

Walls bulging due to roof spread

Walls bulging due to roof spread

I think most surveyors could tell you the theory behind roof spread and why it occurs but the reality is that we rarely see it in our day to day survey work; indeed this is the first case I’ve seen in 5 years so I thought it would be useful to outline the failure mode and the damage caused to the building concerned. The building is  large outbuilding on a property in Derbyshire, which was surveyed as part of a condition survey to the main building.

 

Roof spread failure mode

Roof spread failure mode

Cause of Roof Spread

Roof spread occurs when there is a failure to adequately support the main rafters of the roof so as to prevent them spreading further open in a scissor type action. Since the rafters are fixed to timber wall plates sitting on the head of the masonry wall, then roof spread pushed out those wall plates, in the process pushing out the masonry below. Often the  first signs are cracking, bulging or leaning to the head of the external masonry.

In this particular case the collar had failed at its connection with the rafter on one side and the rafters then scissored open causing  substantial damage to the external masonry as the wall plates were pushed out laterally.

There was also failure of the roof purlin to the same side and this had been shored up with a timber strut. In fact several additional support timbers had been installed to prevent further collapse of the roof structure, though none had been installed particularly well.

Tie bar installed through wall plates to prevent roof spread

Tie bar installed through wall plates to prevent roof spread

Temporary Structural Support

Interestingly we also noted that steel tie bars had been installed running through the building and through the wall plates to either side of the building. Again, the aim here was to provide lateral restraint and prevent the wall plates, and underlying masonry being pushed out even further due to roof spread. The building will be substantially refurbished, which will include complete renewal of the roof structure.

 

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Damp in Old & Historic Buildings

Managing damp problems in old buildings

CPD Session in Nottingham

There will be an opportunity to attend a talk I’ll be giving in dealing with damp in old and historic buildings on the 19th of May in Sandiacre, Nottingham. Full details can be found on the following link

The event is being organised by Jane Newton at the CIOB, whose contact details can also be found in the link above. The event is £5 to non-members of the CIOB and free to members and students. Whether you are a practising surveyor or just have an interest in old buildings then I’m sure you’ll find it fascinating.

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The talk should last about an hour with a Q&A afterwards. Topics that I’ll be covering are:

1. Tools of the Trade
2. The principles of moisture management
3. Back to basics: A focus on critical technical details.
4. Understanding moisture equilibrium
5. Cure or management solution
6. The holistic damp investigation process
7. A brief update on rising damp
8. Condensation damp & ventilation strategy.

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Collyweston Stone Slate

Possibly the best material you could have on your roof. 

Leicestershire property with Collyweston stone slate roof.

Leicestershire property with Collyweston stone slate roof.

We do a lot of conservation and heritage work and we surveyed another old grade 2 listed historic building this week, which was particularly fascinating for its roof covering of Collyweston stone slate. Indeed, the roofing material is probably at least partially responsible for the buildings grade 2 listing. Walls  to this building were  circa 500mm thick, with the original part of the building being around 400 years old and constructed of random rubble limestone. Collyweston stone slate gets its  name from the village in Northamptonshire, which is where these slates are made and it is a  material whose use is generally restricted to areas running along the limestone belt so can be found in Northamptonshire, South Lincs, Rutland and Cambridgeshire; the property in question is in Rutland. Collyweston stone slating has never been a large industry but it is now extremely rare and we believe that there are only two roofing businesses operating that specialise in this material.

About Collyweston Stone Slate

Collyweston slate is not actually slate, in fact it is limestone dating from the jurassic period that splits naturally along its bedding plane to form slates. Making these stone slates is incredibly labour intensive and skilful. In the 19th century the process was known as ‘foxing’ and involved a miner laying on his side and tapping away at the overhead seam with foxing picks. At some point the overhead seam would fall and miners would build up temporary supports for the seam using columns of waste stone. If the seam did not fall by the days end then a ‘lions tail’ would be used to lever the seam down; it would hopefully smash into manageable pieces when it hit the floor and hopefully not land on a miner. These pieces were known as ‘logs’ and it was important for the logs to remain damp because they were then left out in the open on a bed of shale so that freeze/thaw action could initiate splitting of the log into slates.

Diminishing courses on Collyweston stone slate roof

Diminishing courses on Collyweston stone slate roof

Even today, slaters rely on frost to split the log. Slates are dressed into various sizes and when you view a Colllyweston slate roof you’ll immediately notice that the slates are laid in diminishing courses towards the roof peak. To accomplish this, the  underlying timber laths, usually 0.75″ x 0.75″ sections are also laid in diminishing courses. They may be spaced at around 6″s near the eaves slate and decrease to a lath spacing of around 2.75″ near the roof peak. The slates are secured with oak pegs fixed through a hole in the head of the slate. These days the hole is drilled but traditionally they were made with a bill and elves.

Life Cycle Costs

Incredibly, Collyweston stone slates are capable of almost continuous reuse, which makes them possibly the cheapest roof covering you can buy if you calculate life cycle, rather than upfront costs. It is the oak pegs or underlying timber laths that are likely to be the weak link so for very old roofing it is not unusual to have to strip and relay the roof to renew oak pegs or laths. This roof was stripped and relaid around 1989 but all the slate was reused.

Erosion to limestone parapet copings

Erosion to limestone parapet copings

Substantial oak frame takes the load

Substantial oak frame takes the load

As you can imagine, there is substantial weight in a Collyweston stone slate roof so the underlying timber frame has to be substantial and you will generally find an impressive and substantial oak frame taking the load, as can be seen in this case.

OPC mortar fillets in remarkably sound condition.

OPC mortar fillets in remarkably sound condition.

There was very little wrong with this roof, bar the heavy erosion seen to the edges of some limestone parapet wall copings and the fact that the base of the chimneystacks and parapet walls flashings had been filleted with OPC mortar. We’d have preferred that NHL 5 lime mortar was used but since both the slates and the parapet walls are limestone then there is little cause to worry about differential expansion and subsequent cracking to the mortar fillets.  The fillets were in remarkably sound condition and generally where we encounter OPC mortar fillet roof flashings, they have generally cracked or failed altogether.  After some relatively minor repair work to the parapet copings and occasional ongoing maintenance work, we feel pretty sure that the roof will be good for another 400 years.

As a postscript to this piece, I noticed that it was found and retweeted by the last company still mining Colllyweston stone slate, Claude N Smith & Co. That in turn led me to one of their Youtube videos, which is absolutely fascinating if you can spare a few minutes to watch. Mining Collyweston Slate

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Do physical damp proof courses fail?

Why DPC Injection Work is Rarely Required. 

The damp proofing industry in the UK commonly promote two statements that are fundamental to this industry. Firstly, they promote rising damp as a common occurrence and we can comfortably state that this is simply untrue. It is an academically proven fact that  rising damp is incredibly rare.

The second claim, which is also fundamental to an industry that sells retrofit chemical injection and re-plastering is that physical damp proof courses commonly fail.  We have reviewed many many reports from these ‘specialist’ companies and the absence or failure of an existing physical DPC is commonly cited as justification for installing a retrofit chemical injection system. Moreover, you have all commonly seen retrofit chemical injection work installed where physical DPC’s already exist.

Do Damp Proof Courses Fail?

Bitumen felt DPC

Bitumen felt DPC

Originally pointed over but extrusion has blown the mortar. This DPC has not failed.

Very old slate DPC

Very old slate DPC

Fully functional slate DPC but bridged by rainsplash due to high ground levels

Hidden slate DPC

Hidden slate DPC

Physical DPC simply bridged by soil banked against the wall. Guess what the solution is?

Functional Slate DPC

Functional Slate DPC

DPC still fully functional despite being bridged by OPC mortar at the bed joint.

Visibly Functional Slate DPC

Visibly Functional Slate DPC

Despite localised flooding due to a blocked gulley.

New Polyethylene DPC

New Polyethylene DPC

New plastic DPC's are commonly bridged by poorly informed builders

Correctly installed Polyethylene DPC

Correctly installed Polyethylene DPC

DPC installed with the required overlap to prevent bridging at the bed joint

Damp proofers

Damp proofers

Why let the presence of a functional DPC get in the way of selling you another one.

More pointless injection work

More pointless injection work

Injection work of this sort is inappropriate for old properties and nothing short of vandalism.

No physical damp proof course present

No physical damp proof course present

Not a problem in this windmill provided the wall base had been allowed to breathe.

Cracked rainwater gulley

Cracked rainwater gulley

The lack of a physical DPC need not be a problem if local ground moisture is managed.

High ground levels and blocked gulley

High ground levels and blocked gulley

The solution to dealing with wall base damp very rarely needs 'specialist' treatments.

No DPC present & high external ground levels

No DPC present & high external ground levels

A problem that was cured by reinstating critical technical details.

Bitumen DPC

Bitumen DPC

Extruded from wall but fully functional

There are of course legislative requirements for the insertion of a physical dpc in new buildings. Approved document C, Section 5.2, states that walls should: resist the passage of moisture from the ground to the inside of the building; and not be damaged by moisture from the ground to any part which would be damaged by it. This requirement is met if a damp proof course is provided of; bituminous material, polyethylene, engineering bricks or slates in cement mortar or any other material that will prevent the passage of moisture.  However, relatively speaking this is modern requirement and we have many thousands of properties in the UK that do not have have a physical damp proof course installed and yet they manage moisture perfectly well despite non-compliance with the modern requirement for a physical DPC.

I personally carried out a comprehensive review of this very question and  what became clear is that the majority of academic commentary cited bridging rather than failure as the key issue, in fact it is fair to say that there was general agreement on this point. We  found only two cases where commentators cited their view that DPC’s fail, in both cases these were unproven opinion rather than proven fact. Here is an opinion given by Trotman P, Sanders C, Harrison H (2004)…  Physical dpc’s can fail occasionally, particularly those formed by engineering bricks or overlapping slates, following breakdown of the mortar; bitumen felt dpc’s can become brittle with age.  The ‘breakdown of mortar’ is the most interesting point in this statement but the idea that an engineering brick can fail is simply wrong. The authors do not go on to explain their point but we can only assume that this idea is linked to occasional building movement that results in cracked engineering bricks at DPC level. A crack in a brick or a slate DPC will not result in capillary rise in those units and we are firmly of the opinion that engineering brick DPC’s do not fail. Moreover they are the simplest physical DPC to visually inspect. The key controversy must focus on hidden DPC’s installed to the mortar bed joint. These can be formed from a wide range of materials including poured bitumen, bitumen felt, lead, copper, overlapping slates and probably one or two more that currently escape my mind. They are  often not even visible at the bed joint and this may be due to being hidden by high external ground levels, or more commonly, they have been pointed over. Both issues are clearly bridging issues rather than DPC failure and if you have a bridge then the simple solution to that problem is to remove the bridge.

To my knowledge no one has carried out a piece of research into alleged DPC failures and published their findings. It can’t be done by the damp proofing industry because they have a vested interest in promoting the idea of DPC failure. It would need to be an independent piece of work  that to my mind would be a valuable piece of research. I have considered co-ordinating this with a demolition company so that every time a building is taken down we can thoroughly inspect the DPC in the process. We have removed bricks from walls on many many occasions to inspect cavities and where we do this we have consistently found the old physical DPC to be intact and fully functional.

We have previously written that Portland cement degrades over time, initially it is resistant to rising damp until after many years of degradation it then becomes the major moisture pathway for rising damp. Where a continuous physical barrier is installed then clearly this is not a problem but this fact may well form at least a partially valid argument towards a claim that an engineering brick DPC has failed. Technically there would be nothing wrong with bricks but the mortar perps may allow rising damp via diffusion. Interestingly we have seen where perp joints have been left open on engineering brick DPCs and this would completely mitigate for this potential issue.  However, in all alleged cases of DPC failure,  what we commonly recommend is that so long as there is a provision for adequate wall base ventilation then this does not become an issue. It is all about maintaining moisture equilibrium, which is ensuring that moisture is evaporating off the wall as fast as it is rising.  Similarly, where we find that physical DPC’s are hidden we simply treat the building as though a physical DPC is not installed so that if external finished floor levels are a minimum of 200mm below internal finished floor level then this need not be a problem. There are thousands of properties in this country that perform perfectly well without a physical DPC and they generally do so because moisture equilibrium is maintained in their walls due to the fact that they are left bare, they are correctly  repointed with lime mortar, there is adequate subfloor ventilation, external finished floor levels are not too high and local ground moisture is managed.  You can of course apply all or most of these principles to a building that has a physical DPC installed, even one that has allegedly failed and you would mitigate for the alleged failure.

We are lucky enough to carry out a great deal of survey work on the Crown Estate. We deal with some very old historic buildings that were originally built to a very high standard. We are seeing properties over 150 years old where ordinarily we would not expect to see a physical DPC installed but on this Estate they do,  and this gives us a rare insight into some quite unique properties. Many of the images contained within this blog are from the Crown Estate and we are consistently finding perfectly functional DPC’s in some of the oldest properties to have physical DPC’s installed.  I may not have proven through this blog that physical DPC’s don’t fail but I can state with certainty that no one has proved that they do. We do not believe that physical DPC’s fail so if one is installed then you should give careful thought as to why you would even consider installing another unproven retrofit chemical injection system in the absence of any proof that the existing physical system has failed. We have always taken a balanced view on retrofit DPC injection because pragmatically there are times when lowering external ground levels may not be an option but the fact remains that we very rarely have a need to specify these management solutions because our focus is always on curing rather than managing or hiding the problem.

 

 

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