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.

 

Please follow and recommend our blog page:

Verging on poor build standards

A Quote for £3.5k on Potential Roof Repairs Triggers a Roof Inspection

Failed cement roof verges and undercloak

Failed roof verges are a common problem

We were recently called to a property in Stoke on Trent after a client was quoted £3.5k for substantial roof repairs relating to roof verge failure. This is quite a chunk of money considering that the roof was on a property built in the mid 90’s; surely we can expect a new roof to last longer than 20 years can’t we?

What we found on attending was fairly  representative of the issues we find when inspecting fairly modern roofing installations.

We found substantial failure of the cement verge fillets and resultant moisture damage to both the cement board undercloak and the tannelised roof batten ends.

Missing undercloak and wet rot to batten ends

Missing undercloak and wet rot to batten ends

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Unnecessary Recommendations.

We understand that the roofer had suggested works to replace the sarking membrane but since water ingress by wind driven rain or otherwise, had not been a problem then this was clearly an unnecessary recommendation.  Wind driven rain tends to only affect shallow pitch roofing and since  this roof pitch was circa 45 degrees then we saw no need to spend money addressing a problem that didn’t exist.

Defects

On inspecting the roof from a ladder we noted a number of key defects:

  1. Cracking, failure and close of cement verge fillets
  2. Wet rot to the roof batten ends
  3. Damaged and missing cement board undercloak
  4. Sarking membrane not draped into gutter and no alternative eaves flashing detail.

Failure of roof verges is incredibly common and often stems from two underlying problems:

  1. The mortar does not contain the minimum 30% required sharp sand.
  2. The verge tiles have not been bedded on the mortar in one operation.
Eaves flashing only visible in limited areas

Eaves flashing only visible in limited areas

It is a mistake to try and point verges after the tiles are already in place and given the depth of remaining cement verge fillets, we feel that this is precisely what happened. Failed verge fillets are top of the NHBC’s hit list because it is one of their most commonly reported defects on relatively new build properties. They also commonly report a failure to add sharp sand to the roof mortar mix.

Thinking the problem through logically, if the cement verges have failed then can we expect similar problems with a failure to ridge tile mortar beds? In almost all cases, the answer is yes… if they failed to add sharp and to the verge mortar then why should we expect to see it under the ridge tiles! A shot with a zoom camera confirmed evidence of failure to the mortar bed joints.

Failed cement mortar bed

Failed cement mortar bed

You may not know that the fixing requirement has changed for roof ridge tiles under BS5534 and it is no longer allowable for ridge tiles to be simply bedded on mortar; they must also be mechanically fixed now so the repair specification we produced covered this point. The repair specification included for stripping off the large format interlocking concrete tiles at the verges, renewing the fibre cement undercloak and cutting out and replacing tantalised roof batten ends affected by wet rot. Critically, the mortar mix was correctly specified and it was further detailed that verge tiles should be replaced on a fresh bed of mortar and nailed in one operation. Similarly it was specified that ridge tiles should be re-bedded on the correct mortar mix and mechanically fixed to the ridge. We would substantially extend the life of this critical roof detail if roofers understood the critical importance of adding sharp sand to their mortar mix.

Roof verge technical detail

Roof verge technical detail

 

 

Please follow and recommend our blog page:

Thatched Roofing

An Introduction to Thatched Roofing

 

1950's Partially Thatched Roof

1950’s image shows a mix of clay pantiles & Thatch

Grade II Listed Thatched Cottage in Nottinghamshire

Grade II Listed Thatched Cottage in Nottinghamshire

We carried out a full condition survey last week on a beautiful 600 year old cottage in Nottinghamshire. From my perspective the key point of interest was the thatched roofing and understandably we had a client who was particularly interested in its condition and the potential cost of any repairs of maintenance. When dealing with buildings of this age then obviously there is a strong focus on conservation so that often involves doing a great deal of background research into the history of the building and in this particular case there was an anomaly to clear up… Doing the background research we came across an image of the building taken in the 1950’s that clearly showed the roof to be only partially thatched. We were confident that the thatch predated the clay pantiles seen in the image but it was

important to prove it.

This is a grade II listed building and the provenance of the roof may well influence any listed building consents that may be required. More important, since it was immediately obvious that the building needed a re-thatch.

 

What we managed to find was two comparable street scenes taken over a century apart that proved that the thatch predated the partial clay pantiling.

Thatched Ridges

We also noted that in the early streetscene that roof was seen to have a flush ridge, as opposed to the  current decorative patterned ridge. The flush ridge, gives rise to the possibility that the thatching material has been changed over the decades.  Long straw is generally installed with a flush ridge. However, the patterned ridge, as currently installed, was a post world war 2 fashion and it is more likely that the change in ridgeline simply followed the fashion for patterned ridges. It should be noted that patterned ridges are more problematic and have a reduced lifespan. This is because the hazel liggers, used to hold the ridge in place, impede rainwater runoff from the ridge, creating depressions and moisture pathways into the straw, which promotes rotting of the straw. There is a strong argument for returning the roof to a flush ridge should the cottage be re-thatched. If the current patterned ridge is retained then the client should be prepared for more frequent ridge changes.

Streetscene showing existing Thatch Roof

Streetscene showing existing Thatch Roof

Streetscene proving thatch predated the partial clay tiling

Streetscene proving thatch predated the partial clay tiling

 

 

 

 

 

 

 

Types of Thatch

 

Exposed horizontal sways & spars due to loss of thatch material

Exposed horizontal sways & spars due to loss of thatch material

 

Traditionally there are three types of thatching material, Water Reed (Traditionally from Norfolk though most reed is now imported due to high demand), combed wheat reed (straw), or long straw. Water reed requires a complete roof strip back to timber when it is renewed, whereas combed wheat straw (reed) and long straw can be partially stripped and re-thatched. The material on the roof of this particular cottage is combed wheat reed.

Thatch Defects

we noted a number of issues, which included:

• Heavy build up of moss to some areas of the roof, particularly near the chimneystack to the northeast corner of the building.

• Substantial stripping of straw material and exposure of both the hazel fixing sways and plastic fixing spars, particularly to the south elevation of the roof.

• Complete failure of the mortar fillet flashings around the base of the chimneystacks.

Exposed plastic spars (Staples)

Exposed plastic spars (Staples)

It was particularly interesting to note that the ‘U’ shaped fixing spars are manufactured plastic fixings, as opposed to the more traditional hazel fixing spars but this is unsurprising, since handmaking hazel fixing spars is incredibly labour intensive and something of a lost art.  It’s also  worth noting that even the wire netting, installed to prevent birds from stripping material from the roof, can help aid identification since only the ridges are generally netted for water reed, whereas straw is completely netted.

Cost of New Thatch

It is clear the roof needs re-thatching in the short term, and this would also include installation of a new ridge. You should generally budget for circa £120.00 per square meter, though thatcher’s often quote by the ‘square’, which is equal to 100 ft2 or 9.3 m2. The total cost for a square would be in the region of £1116.00 net.  In this particular case we calculated circa 130 square meters of thatch so replacement may cost in the region of £18,720.00 inclusive of vat at 2016 prices. For context and an historical perspective this is circa two thirds of the National average wage.

New ridge to thatched roof

Decorative ridges need more frequent attention than the main roof

Moss build up causes underlying thatch to rot. You have the roof 'demossed' as required.

Moss build up causes underlying thatch to rot. You should have the roof ‘demossed’ as required.

When inspecting the roof structure we would always check the timber moisture content of the roof to ensure that the roof  timbers are not at risk of timber decay. The ideal moisture content for roof timbers is between 8-12%. This is slightly elevated but well below the 20% danger threshold and what roof timbers could be visually inspected,  appeared to be sound.

Fire Risk

Chimneys and flues are a particular concern when dealing with thatched roofing since a high percentage of fires started in thatched roofing are caused by chimney and/or flue defects. However, that should be contextualised against the view that fires in thatched roofing occur no more frequently than they do in more traditional roof coverings and the frequency of fires has been greatly exaggerated. Statistics prove this point and insurers are well aware of this fact but the comparable incidence of roof fires is probably down to the fact that owners of thatched roofing understand the risks and take the necessary sensible precautions to mitigate the risk. Some of the general fire safety considerations required for ownership of thatched roofing include:

• Have the chimneys swept twice a year

• If solid fuel fires are being used then burn only seasoned wood or coal

• Flues should be lined with a suitable liner if using solid fuels or a stove

• Chimneystacks should be kept in good repair

• Always have hard wired smoke detectors installed to the property

• Ideally, rely entirely on gas central heating and decommission chimney flues.

There has been a trend towards fire proofing the underside of thatched roofing by installing Rockwool slabs or spraying fire retardant onto the thatch but there are some concerns that the Rockwool could impede airflow through the thatch, thereby promoting rotting of the thatch, and similarly there is a risk that fire retardant chemicals may also promote rotting of the thatch.

Substantial failure of cement mortar fillets

Substantial failure of cement mortar fillets. Was ‘P L’ the last thatcher?

We noted a number of obvious visual defects to the chimneystacks

• Repointing was required to the two high level stacks on the main roof ridgeline. Previous attempts at repointing have been poor and the mortar has already failed.

• There is substantial failure of the cement flashing fillets to the base of both chimneys to the main roof ridgeline.

• We also noted that the cement flaunchings, which secure the clay chimneypot, appear to be failing on the central chimneystack.

We do not have a problem with the use of cement mortar fillets though lead can also be used; however, they have probably failed prematurely because hard Portland cement rather than more flexible NHL 5.0 lime mortar was used.

Thatched roof new ridge in progress

Thatched roof new ridge in progress (Summer 2016) but chimney flashings not yet complete.

There is no doubt that thatched roof ownership comes at a high comparative cost when you consider that straw thatch may require a re-thatch every 20-30 years and ridges need replacing every 10-15 years but these are often, as in this case, very special buildings, and they are worth the high cost of ownership.

 

Please follow and recommend our blog page:

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

Please follow and recommend our blog page:

Cutting Corners – Builder Short Cuts

A Number of Builder Short Cuts Adopted by Builders

Builder Short Cuts – We come across so many short cuts adopted by builders that I thought I’d start a regular post highlighting some of the strange decisions made by site trades people to save on time or money. As you’ll see, some of the decisions taken make no sense whatsoever. See for yourself.

Short Cuts 1

New consumer unit

New consumer unit

Why not use the circuit labels supplied?

New Ground Spike

New Ground Spike

Laid directly over the patio as a trip hazard.

Need a matching slate?

Need a matching slate?

This will do fine!

Need to remove downpipe?

Need to remove downpipe?

No need, just render around it.

Use of plumbers mate to stop leak

Use of plumbers mate to stop leak

Quicker to tighten unions on valve?

Need a roof vent?

Need a roof vent?

Just hide duct behind rafter.

Need another roof vent?

Need another roof vent?

Just hide duct behind rafter.

Double glazing in timber frame

Double glazing in timber frame

Needs installing on glazing bars.

Installed drains in wrong place?

Installed drains in wrong place?

Just grind a hole in your Aco channel.

Not maximising access costs

Not maximising access costs

Ignoring other defects is not smart.

No frost protection on condensate

No frost protection on condensate

A critical omission

Need a newel post fixing kit?

Need a newel post fixing kit?

Just nail them on!

 

Please follow and recommend our blog page:

Trouble at the Top – Roofing Faults

Trouble at the Top (Roofing Faults) – This article was recently published in the ICWCI’s ‘Site Recorder’ magazine and is being reprinted here for readers of this site. Thanks to Anne-Marie Elmer at the ICWCI for doing such a great job of formatting the article for print.

Screen Shot 2015-09-02 at 11.14.09 Screen Shot 2015-09-02 at 11.14.28

Please follow and recommend our blog page:

Importance of High Quality Cameras

The Importance of a High Quality Camera and Interchangeable Lenses to The Survey Process

Screen Shot 2015-08-26 at 14.21.50

Windmill during demolition phase of conversion in 1981

I was recently commissioned to carry out a full structural survey on a converted windmill in Nottinghamshire. I’ve surveyed a water mill before but this is my first windmill. Having said that the building was reduced in height as part of the residential conversion that took place in 1981 and no working parts were retained. Once the height was reduced a cut timber conical roof was installed that was fully covered with butt jointed timber planks prior to being covered with a bitumen and felt roof coating. This would not have been my choice of roof covering and perhaps it was a commercial decision taken by the developer or perhaps it was a more reasoned decision based on the buildings height and local  exposure to high winds? After all, windmills are generally sited in windy locations and we have to consider the potential for roof chatter or even tiles being ripped from the roof in high winds. Roof chatter derives its name from wind uplifting the tiles so they make a chattering sound in high winds; usually caused by insufficient nail fixings.

Screen Shot 2015-08-26 at 14.18.45

Conical roof nearing completion in 1981

It is difficult to inspect a roof at this height from the ground and this is where the zoom lens and a decent camera really pays dividends. By walking a good way from the property I was able to zoom in and take shots of the roof from all angles and the resultant images when blown up on a computer screen provide valuable information with regards to final advice that is included in the full survey report. In fact the roof looks to be in reasonably serviceable condition but it has already overstretched its expected life cycle by quite some years. Without the images obtained I’d have been extremely uncomfortable in providing accurate advice relating to the roof condition and as a general principle I find the high quality zoom lens and camera to be an essential part of my surveying kit. For an unusual building of this height, it was absolutely critical.

OLYMPUS DIGITAL CAMERA

Zoom Lens Reveals Enough Detail to make Informed Decisions about Condition. Note lightning protection system to left of RWDP.

Please follow and recommend our blog page:

Head Above The Parapet

A regular client with a portfolio of properties recently sent me some images of a particular damp problem to the rear kitchen extension of the property they had recently let. They knew there was some minor dampness to the head of the kitchen wall but expected that this was a result of the property standing vacant for a long time and they thought it would dry once occupied.  To my mind there is no good reason for vacant properties to be damp but in any event on seeing the images it was fairly obvious that the property had a severe penetrating damp problem to the head of the rear kitchen wall.

Penetrating damp above kitchen window

Penetrating damp above kitchen window

 

They appointed me to investigate the problem and when I arrived I noted that it was a single storey extension with a parapet wall directly above the damp kitchen wall. It has got to the point that if I see a parapet wall I know it has defects and has been detailed incorrectly so I always open up to confirm.Out came  the telescopic ladders and I accessed the flat roof deck to be met with the following site.

OLYMPUS DIGITAL CAMERA

Narrow coping stones to parapet head

It was immediately obvious that the coping  stones were too narrow for the application but I also noted that the mortar joints were heavily eroded and seemed to be comprised almost entirely of red sand. I scraped the joint and it was clear that the sand had only been mildly threatened with the possibility of any cement.

OLYMPUS DIGITAL CAMERA

What, no cement?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I lifted off a coping stone and as expected saw that critical technical details were absent. The was no  hard support across the cavity and no physical damp proof course installed.  The parapets would continue to take in rainwater until these defects were corrected.

OLYMPUS DIGITAL CAMERA

No physical DPC or hard support installed

 

 

 

 

 

 

 

There are a number of proprietary systems on the market now for securing copings, such as cavity lock systems, and there are even proprietary hard supports available to bridge the head of the cavity but in the absence of any of those systems I have detailed the method that can easily be achieved by any reasonably skilled builder with readily available tools and materials.

I always include a drawing of the correct technical detail in my reports so that this can be given to the contractor because it is rare to find builders who understand the required technical detailing, which is odd because parapet walls are becoming a very common feature  on some new build sites. If you’re interested in that detail then here it is…

Screen Shot 2015-07-25 at 07.43.45

Parapet wall technical detailing

 

 

Please follow and recommend our blog page:

How to not strengthen a traditional cut timber roof

I am currently engaged on a construction dispute in which the cost of building an extension somehow escalated to around £60k over and above the original £70k quote from the builder. I was initially called in to inspect and assess building defects relating to the works and the client was quite staggered at the range and number of defects found and I simply could not see where the additional £60k had been spent.

The builder had claimed an additional sum of money for ‘roof strengthening works’ and anecdotally I am informed that the client received a call from the builder when discovering that the roof rafters were structurally unsound and indeed, they were. At some point in the buildings history, changes had been made to the roof pitch and/or height and new rafters of the correct length and cross section should really have been installed to stretch from ridge board to wall plate. However, the builder at that time simply decided to nail extra lengths of timber onto the rafters and the roof was completed in the manner you see in this image.

Historic alterations to the buildings roof left the roof structure in this condition.

So, our builder, bless his cotton socks, was absolutely right; the roof structure was unsound and required strengthening, something that he could deal with at additional ‘reasonable’ cost.

Sadly however, what the builder actually did was make the same mistake again and simply nailed on even more bits of wood so we ended up with a three leaf rafter sandwich. Rafters were not properly coach bolted together and neither were the repair sections long enough to reach the wall plate. In fact, even the sections were undersized and the builder dealt with this by packing between the purlin and the repair sections with bits of cut plywood.

Three ply rafter sandwich but not much meat.

These were fairly major roofing work to a portion of the building and what is worse is that the builder only saw fit to report on the sections of the roof affected by his work to the new extension.

The main roof of the building was also structurally  unsound and whilst this had nothing to do with the current builder, surely a responsible builder would have reported this to the client and given them an opportunity to deal with the whole roof structure whilst works were ongoing.

New ‘strengthening’ rafters cut short and packed with plywood

I do know that the client would have taken the opportunity to deal with these structural defects in the whole roof but as it happens they also have to deal with the part of the roof that was allegedly strengthened . Needless to say, the cost of ‘roof strengthening’ works forms part of a rather large defect and cost schedule that is currently in the hands of my clients construction lawyer.

 

 

Please follow and recommend our blog page:

An Interesting Case of Roof Spread

Roof SpreadI was carrying out a dilapidation survey in the West Midlands quite recently and whilst checking the roof space came across an unusual case of roof spread. I’m sure you’re familiar with the basic cause of roof spread but if you aren’t then I’ve included a basic diagram that shows how unrestrained rafters can apply horizontal thrust to the walls and make them unstable. Generally this is due to inadequate collar ties or ceiling joists or indeed failure of those timbers.  In this particular case there were no external signs of cracking or structural problems and neither was I particularly looking for, or asked to investigate structural cracking in the building. However, on inspecting the first floor store room I noticed substantial cracking to the dividing party wall that warranted more detailed investigation.

There was a long crack to the internal corner of the ceiling to wall junction on the front elevation of the property and a further diagonal crack cutting down through the dividing party wall.

SRCrackThe damage was indicative of roof spread but was not quite serious enough to manifest any obvious visual problems to the front elevation of the property.

The roof space to this particular property was not fully accessible or safe to access so I had to inspect as best as I could from the loft hatch and after a couple of minutes I looked directly above my head and immediately noticed the problem. The ridge board was completely split which had resulted in a section of the rafters dropping just enough to apply horizontal thrust to the wall plate on the front elevation. If looking for the obvious and common cause of roof spread then this particular issue could so easily have been missed, especially  when limited to a head and shoulders inspection from the loft hatch.

 

Split Ridge

Split ridge board causing rafter instability and subsequent applied horizontal thrust to the wall plates.

 

 

Please follow and recommend our blog page:
1 2