Technical Information

Older buildings have solid walls with no damp proof courses. They often have an outer and inner skin of brick or stone with a masonry core. They were designed to absorb water from rising damp, rainwater and condensation then shed this moisture externally by evaporation in the wind and sun or internally by smaller air circulation.

External walls should be pointed with a porous lime and aggregate mortar and render should be with a suitable lime render. Modern paints can also provide a barrier to the walls shedding water and internal walls need similar treatment. Rubberised painted barriers should be removed, boarding and insulation fitted at low level act merely to disguise the problem, cement and plaster prevent water movement and modern paints seal the walls.

A suitable lime render and a breathable paint is often the answer, and much cheaper than the solutions many damp proof companies will suggest.

The drying process for very damp walls, which have not been able to breathe for many years does take time, perhaps 6-12 months in extreme cases, but given time and the right repairs the walls of old houses will dry out and a healthy equilibrium of moisture will return.

There are many other reasons why walls could be damp, which we will identify during our survey.

Living under thatch

From Anglo-Saxon times thatch was used universally to provide a waterproof roofing material for farmstead huts. While during the early and mid 20th century it was perceived as a poor man’s lot with many country dwellers using whatever material was cheap and available at the time to thatch their homes. Indeed, it was common for thatched roofs to be replaced with more durable materials such as pantiles and corrugated iron. Surprisingly, it is still not uncommon to find derelict thatched cottages with corrugated iron roofs and the old thatch beneath.

But things are changing. Thatch has become the chocolate-box icon of country living and increasingly builders are constructing new houses in thatch to take advantage of their increased popularity and premium prices when sold.

But despite this renaissance many people are still nervous about the prospect of living in a thatched home. From fire risk, increased insurance premiums and the costs of associated with its upkeep many mis-conceptions exist. The aim of this piece is to address these issues. Allowing you, the proud new or potential owner of a thatched property, to enjoy your home with the other estimated 60,000 thatched property owners in Britain.

Types of Thatch

Throughout the centuries practically all types of grass and shrubs have been used to roof dwellings with the occupants ignoring aesthetics in favour of what was most abundant and to hand. But today, three thatching materials are predominantly used in Britain.

Water Reed

Water reed is the most durable of all thatching materials. It is often called ‘Norfolk reed’ but the majority of reed used in the UK now comes from a host of different countries. Water Reeds thatch has a compact, even texture with clean cut edges and are normally topped with a block ridge using sedge grass. As reed is not susceptible to bird or vermin damage, roofs are seldom wired except for the ridge. In deed, the wiring of a reed thatch causes debris to collect, which in turn prevents water from being freely dispersed, resulting in dampness and the build up of moss.

Wheat Reed

Commonly called Devon wheat reed, it is, not reed at all, but winter wheat straw. The confusion is caused by its appearance and application being similar to water reed. The straw is cut with a binder and comb-threshed which removes all ears and leaves. Because the thatching bundles are prepared with all butt ends facing downwards it has a ‘clipped’ appearance. Combed wheat can be topped with either a block or flush ridge of the same material and is often wired overall.

Long Straw

Long straw is the favoured material of conservation officers in much of Southern England and East Anglia and has been the cause of some heated debate between local councils and thatchers. In some cases the battle has reached the national press as many property owners choose the durability of imported reed rather than sticking to regional aesthetics. Many thatchers argue it is very difficult to pin down the most appropriate material for any region due to the variety of materials used to thatch a house during its lifetime.

In appearance a long straw roof has a much softer, rounder shape and is generally deeper than reed or wheat straw. It is also decorated on the surface with liggers of hazel or willow in a criss-cross pattern. The key difference between long straw and wheat reed is while long straw is being threshed both the ears and butts of the straw are being jumbled together which is how they are applied to the roof. Wheat reed has all the ears and leaves removed.

How Long Will it Last

The durability and life expectancy of your thatched roof is affected by many factors. For example: ·

The ability of the roof to shed water is dictated by the pitch of the roof. Roofs with a more shallow pitch will retain moisture more readily and decay will be accelerated.
Climatic conditions from high humidity and the wind also have impact. In some parts of Devon high humidity in weather patterns shortens the life expectancy of thatch. While parts of East Anglia benefit from North Sea winds which aid drying.

The above factors’ impact is reduced or increased by the quality of the material used, the skill of the thatcher, proximity of trees and correct and regular maintenance. Generally speaking if the thatch is not exposed to extreme conditions the following figures can be used for guidance until the advice of a respected local thatcher is sought.

Water reed 55-65 years

Combed wheat 20-40 years

Long Straw 15-25 years

But of course, there are exceptions. There are examples of long straw lasting over 40 years and reed over 100 years.

Type of repair

Before describing the basic repairs for thatched roofs it is important to stress that if a property requires total re-thatching listed building consent is required. The owners should also seek advice on how to record evidence of old work or techniques such as woven reed matting before they are removed.

Dressing up

Thatch deteriorates gradually causing the thatch to loosen slightly from its fixings. In dressing up, the thatch is knocked evenly and securely back up in to their fixings.

Brushing down

The surface of the thatch may at the same time be brushed down to remove loose material and moss, which can retain moisture and gradually damage the body of the thatch.

Repairing holes

Thatch which is well-laid and maintained should not normally be damaged by wind or develop holes. If isolated holes do appear they can be repaired provided they are not associated with major underlying problems such as major general slippage.

Checking New Work & Repairs It is difficult to check the quality of thatching due to regional differences in the craft and personal working styles. This is made even more difficult by the quality of the work being underpinned by not only the visual appearance of the thatch but the standard of work hidden from view. If you are in any doubt over the quality of workmanship consult your local Master Thatchers’ Association for an opinion.

Cost of thatching

The cost of thatching depends on many factors, these include:

Size of roof
Shape and design of roof
Height to ridge
Height to eaves
Types of features
Access to the site & roof
Types of thatching material to be used
Existing thatch & timber condition
Coat thickness required – depending on thickness of existing thatch

Hidden costs exist because a thatcher, when estimating, may not be able to determine the condition of the roof timbers or the existing thatch. Therefore it is important to seek at least three estimates of the work to be undertaken with additional notes outlining the costs associated with any timber repairs. Be wary of accepting ‘off the cuff’ estimates over the telephone or thatchers who are not prepared to provide additional costings for any additional work.

On the cost front thatchers work in ‘squares’ (around 10ft by 10ft) costing between £600 and £800 pounds per square.

Building For Thatch & Extensions To Thatched Properties

Before designing a new thatched home or extension to a thatched property its is important the following points are considered.

Consult with a good, experienced thatcher at the early design stages on roof construction, pitch, shape and overall design
Decisions of an informed nature will need to be taken on where any intended joins to existing thatch can be made.
Present thatch levels, will be due to the age of the thatch and the existing number of coats of thatch. Because of this it may be impossible to match the existing thatch level with the application of a new coat.
Make sure any person has true expertise before accepting any comments related to the use and performance of thatch.

Thatch & Fire

Fortunately for our safety and the preservation of our heritage thatch fires are not as prevalent as many ordinary people think. The illusion that thatched houses are continually burning down results from the media giving such high profiles to fires when they occur. In fact, only one death has been recorded during the 20C in a thatched house fire. UK fire brigade and insurance statistics suggest only 1.5% of the total thatched property stock in the UK are likely to suffer a thatch related fire in one year.

Until recently it was thought that the majority of thatch fires were caused by sparks. Research by the fire brigade and RHM Technology indicates the problem may lay further down the chimney stack. Even chimneys which appear in good order may have defective pointing or cracks which allow hot gases to escape from the flue into the thatch. The leaking of hot gases creates hot zones which in time may combust and cause a thatch fire.

Therefore faced with the difficulty of repairing a chimney where cracks and holes are incredibly difficult to discover it is advisable to line the chimney. Avoid using metal liners with insulating wool and stick to ceramic type liners with the open area surrounding the liner backfilled with clay fireproof granules. If you choose to install a wood burning stove the installation of a chimney liner is imperative due to the high temperature of the gases generated by the stove.

Insurance

As with thatch fires, thatched homes have a reputation for costing the earth to insure. But with the rise of specialists insurers and an increasing understanding of the physical characteristics of thatched homes this is not the case. You can gain specialist adivse on how to insure period properties through the insurance section of the Period Property UK website (click here), or through one of the three specialist insurers listed below.

Thatching Advisory Service – 01256 880 828  Thatch Owners Insurance Agency – 01403 321245  Country Insurance Services – 0345 660 063

All three companies specialise in insuring thatched homes – as their names depict – and because of this they are able to help you protect your home and make life easier due to their contacts with specialist contractors who undertake work if you have to make a claim.

The Vocabulary of Thatching

Spars: Split Hazel or willow sticks sharpened at each end and twisted in the middle to form a staple. Spars are used to secure new thatch to an existing roof.

Liggers: Split Hazel or willow rods used to form a decorative pattern on ridges and around the edge of long straw roofs.

Sways: Rods of wood or metal between 1 to 3ft in length used to secure each course of thatch laid horizontally across the thatch and fixed using crooks.

Crooks: Metal rods some 20 to 30m in length which are driven into the rafters to secure the sways.

Flush Ridge: Literally a ridge applied to the same level as the roofing thatch. Found on long straw and combed wheat thatched roofs.

Block Ridge: A ridge applied to stand proud of the roofing thatch. Normally thatched in sedge on a reed roof.

Leggett: A wooden bat shaped tool with a flat face. The face is ridged or has nails driven in it and is used to dress water reed and combed wheat into place.

Finally As with all things in life, real quality shines through. If you want your thatch to look its best and last you need to use a well established and respected thatcher. When making enquiries for thatching work be careful. If they say they can start work straight away seek further information and view their work both new and old. From my own experience, the best thatchers have waiting lists stretching to years. Therefore planning ahead is essential.

Rising Damp

Rising Damp

Rising damp is widely misdiagnosed in existing buildings, based on the incorrect interpretation of visual evidence and the readings of moisture meters. Because of a highly successful sales campaign by specialist remedial contractors installing injected ‘chemical damp-proof courses’, this misdiagnosis of rising damp has also become synonymous with a diagnosis of a lack of an ‘injected chemical damp-proof course’.

Although this has been very good for their business, it has often resulted in a waste of clients’ money and resources; original plasters and finishes have been destroyed in the process of installation, and unnecessary damage has been caused to original structures by the drilling of irrigation holes. In addition, money that might have been spent on more cost-effective maintenance or repair works has been wasted.

Whilst injected chemical damp-proof courses may provide some protection for certain types of structure, if properly specified, their general application is rarely the most cost-effective way of controlling damp problems in buildings.

Cause and effect

The most common source of moisture in the base of the walls of buildings is from defective ground and surface drainage. This is present to some degree in almost every building in the country, due to a combination of such factors as rising ground levels, the failure of ground drainage systems, and the increased use of concrete or finishes around buildings without consideration of drainage slopes.

Damp conditions at the foot of walls may be greatly increased by condensation. This occurs when warm moisture-laden air cools to dew point (the temperature at which moisture condenses) against a cold surface. Such cold surfaces commonly occur when the insulation value of the external wall is reduced by water penetration, as described above. Intermittent occupancy with intermittent heating provides the conditions for condensation of further water on these cold damp surfaces, particularly in ground floor bedrooms. These phenomena are the main causes of damp in the base of walls rather than ‘rising damp’ alone.

Damp masonry at the base of walls may lead to a number of problems:

The moisture content of the structure may rise to a level at which decay organisms may grow, or the materials themselves may be adversely affected. For example, timber skirting boards or built-in bonding timbers along the base of walls may become infected and decayed by dry rot, wet rot, weevils or woodworm.
In very damp conditions, the inorganic materials themselves may lose their structural strength. This occurs most spectacularly with walls made of cob (earth) soaked with water.
Damp conditions on the surface of walls, particularly in conjunction with condensation, allow the growth of moulds both on the surface and within porous or fibrous materials, such as wallpapers or carpets fitted against the base of the wall. Not only is this aesthetically unacceptable and damaging to finishes, but it can be a significant health hazard to occupants.

Where evaporation takes place, the deposition of soluble salts on the surface or within the pores of materials can cause aesthetic and structural damage.

Treatment options

As described above, ‘rising damp’ is only one of many mechanisms resulting in high moisture levels in the base of walls, and even when it is a significant factor, it is rarely the primary source of moisture. The management of problems due to high moisture levels requires the proper identification of the moisture source and the defect responsible, before the most cost-effective solution to the problem can be determined.

Damp and its effects may then be controlled by adopting one or more of the following measures:

The provision of suitable moisture sinks to dissipate the moisture at its source without causing problems to the structure or occupants, and the repair of any contributing defects acting as moisture sources, such as broken pipes.
The introduction of either physical barriers using damp-proof membranes or materials to form a ‘damp-proof course’ or hydrophobic (water-repellent) materials as in ‘chemical damp proof courses’.
The isolation of vulnerable materials such as timber and interior finishes from damp fabric.

Moisture barriers

The control of moisture movement using either damp-proof or hydrophobic materials to create a relatively less permeable ‘moisture barrier’ is not necessarily a cost-effective option in controlling damp problems and may even be counter-productive. This is because use of relatively impermeable materials will restrict moisture movement and hence drying. As a result, moisture may be ‘locked’ into damp materials for many years causing chronic problems. Moisture may also be prevented from dissipating from permeable materials, resulting in the build-up of moisture or even damper conditions in localised areas. This may result in moisture moving into previously dry structures or evaporating from previously unaffected surfaces, causing further salt efflorescence. One reason why those injecting ‘chemical damp-proof courses’ generally insist on re-plastering treated masonry with a salt-proof and waterproof mixture, is to cover up these potential problems.

‘Chemical damp-proofing’ may provide a useful barrier to damp in the short to medium term, or at least a ‘nominal damp proof course’, where the walls are of uniform construction such as sound brickwork laid with strong cement mortar; especially if they are combined with a ventilated dry lining system or other building detail which allows moisture to dissipate. However, any gaps which are left, or which appear over time as the material deteriorates, may lead to an accelerated rate of decay.

Surface water drainage

The most cost-effective way of preventing damp problems in buildings, including those resulting in damp masonry at the foot of walls, is to minimise moisture sources and provide adequate passive moisture sinks to dissipate any penetrative moisture so as to make the system fail-safe. This should start with the provision of adequate ground drainage around the building to minimise water penetration to the foundations, and the re-detailing of surface drainage so as to ensure surface water is drained clear of the foot of the walls.

It has become fashionable to specify ‘French drains’ to help with this process. However, these are often poorly specified and soon become ‘French ponds’ in UK conditions. This may be because the base of the drain has been inadequately levelled or drained to keep water out of the foundations and the gravel infill has become contaminated with soil and debris, preventing proper moisture drainage and evaporation from the foot of the wall.

In the UK, the more traditional and more effective detail is to use a ventilated and drained ‘dry area’ around the foot of the wall. These are commonly covered with York stone slabs in order to prevent debris accumulating in the drained dry area and to minimise maintenance.

Alternatively, a perforated plastic land drain can be laid to falls in a trench lined with geo-textile and back filled with ‘beach cobles’ or large diameter hard core. Proprietary external ‘drained cavity systems are also available.

Wall construction

The use of impermeable finishes, such as sand/ cement renders, around the base of external walls is a common cause of damp problems. These prevent moisture evaporating from the foot of the wall, forcing it into the interiors. As with all impermeable materials, they eventually fail, generally due to cracking. This allows water to penetrate into the foot of the wall, but prevents drying. The use of more traditional breathable lime mortar renders, and the correct detailing of renders to shed water clear of the base of the wall and to prevent ‘bridging’ of any existing damp-proof course, would be the preferred solutions.

Cavity wall construction may provide a way of dissipating moisture and preventing it penetrating into the building, provided the cavity is through ventilated. This may be compromised by debris or the ill-advised injection of proprietary insulation foams. These defects may also bridge existing damp-proof courses, allowing water to penetrate to interior finishes. In some cases, the most cost effective solution is to reinstate a through-ventilated cavity.

Generally, failures in existing damp-proof courses are the result of bridging by inappropriate repairs and alterations, by raised ground levels or by localised damage due to structural movement or poor building work. If a damp-proof course is an original design detail to control moisture movement in the structure, it may be necessary to carry out local repairs. This is best done by ‘cutting in’ a new layer of damp-proof material locally rather than by the general injection of hydrophobic solutions into the masonry to create a ‘moisture movement restricting barrier’.

Ventilation

Traditional buildings built in damp or potentially damp sites commonly included through-ventilated sub-floor cavities, cellars or basements. These act as sumps to allow the evaporation and dissipation of moisture from the structure before it reaches occupied areas or vulnerable finishes. Indeed, in some parts of the country it is not uncommon to find streams running through the cellars or basements in old farmhouses. These were presumably retained as a source of water for domestic use. However, if the ventilation of a basement, cellar or sub-floor cavity has been restricted, moisture can build up and penetrate vulnerable structures. This can occur, for example, by earth and plants clogging air bricks or by the ill-advised application of relatively impermeable materials. The solution to these problems if they develop, is to re-establish ventilation, not to start applying further damp-proof materials.

As described earlier, the reinstatement of a through-ventilated suspended floor is generally preferable to its replacement with a concrete slab. The requirement for the continued dissipation of moisture does not preclude the use of basements and cellars as occupied areas, but means that walls should be kept ventilated and not sealed. This can be achieved by using through-ventilated dry lining systems rather than impermeable finishes or tanking materials, which would only force moisture into adjacent structures above or to the side. Traditionally, dry lining has been produced by the use of timber panelling spaced from the masonry with battens or the use of lath and plaster. In all cases, the cavity behind should be ventilated at the top and at the bottom to allow through-ventilation to dissipate moisture, as otherwise moisture will accumulate to cause damp and decay problems. This commonly happens when insulation material or debris is allowed to block the cavity behind lath and plaster or when impermeable paint layers accumulate over timber panelling. These defects are easily solved and the traditional ‘farmhouse’ technique of timber panelling to dado level can be an attractive and cost-effective solution to problems of damp penetration or condensation affecting the foot of masonry walls. Modern materials and techniques may be used to achieve the same end, and many products are available on the market to allow the cost-effective provision of through-ventilated dry lining systems, including specialist plasterboard systems and studded plastic membranes which can be used to form vertical damp proof course details behind the dry lining.

Conclusion

Even with the loss of traditional skills and the complexities introduced into building by new materials and new styles of occupancy, the conditions resulting in damp to the base of walls can easily be avoided with a little thought and scientific understanding. Indeed, new materials and techniques can often be used to advantage if their properties are analysed as potential environmental controls. In contrast, the misdiagnosis of rising damp and the general application of particular products and techniques without considering the consequences lead to the unnecessary waste of the increasingly limited budgets available for maintenance and refurbishment.

A more rational approach to the diagnosis and treatment of damp problems in buildings is only good building practice, which Hughes Surveyors and their scientific consultants promote in the interest of sound building and public health.