HI Technical Bulletin Issue 2 February 2008
For registered members of the SAVA Certification Scheme
This month: • Interaction of tree roots on shrinkable clays • Rising damp • Feedback & contacts
Welcome to Issue 2 of SAVA’s Home Inspector Technical Bulletin The bulletin focuses on Home Condition Reports and associated non-energy issues. We trust that you will find the bulletin useful for your day-to-day work and we welcome any feedback you have about what you would like to see covered in future editions.
The interaction of tree roots on shrinkable clays Clay soils are defined as soils which have a large proportion of very small mineral particles with a diameter of less than 0.002mm. Characteristically they are plastic, smooth and greasy to the touch. The more clay in the soil, relative to any silt or other coarse grained material, the more pronounced are these characteristics. The ‘shrinkability’ or volume change of a clay is a function of its particle size and its mineral constituents, and is difficult to define and measure. In general terms the more ‘clayey’ the soil the greater its volume change potential; and the higher the proportion of non-shrinkable sand or silt, the less it will shrink or swell. A simple but crude test is to put a lump of clay in a bucket of water and leave it overnight. If it disintegrates the clay will contain significant amounts of sand or silt and the risk of shrinkage is diminished. However, if the clay remains as a solid lump, it will be pure clay and may have a potential for shrinkage or swelling. Clays in their natural condition contain a high percentage of moisture occupying the interstitial spaces. If this moisture is removed the clay particles will consolidate to take up the space occupied by the water and the shrinkage will correspond to the amount of water removed. If the clay then takes up moisture it will expand – resulting in surface heave. Changes in moisture contents can generally be brought about in one of two ways: a. through moisture movement near the ground surface where moisture evaporates during dry weather and is replenished by rainfall and by
upward migration from the water table; b. by transpiration of soil moisture through the action of the roots of vegetation, usually trees. Leakage from drains or water mains have also been known to have a similar effect. The demand for water from trees is directly related to the rate of transpiration from the foliage and in general, the larger the tree and the greater the leaf canopy, the greater will be its demand for water in dry weather. This demand will accelerate from spring time onwards as the new foliage grows. However, some tree species are more water demanding than others and the National House Building Council (NHBC) has classified trees by the water demand and their mature height (see Figure 1). Seasonal volume changes in shrinkable clays are generally confined to the top 600mm, but where tree roots are present the depth of clay affected may be 2.5m or more, although significant shrinkage is normally confined to the top 1.5m. The serious drying of clay soils to depths in excess of 1.0m is almost always associated with the removal of moisture by tree roots. In a dry summer the dry zones will extend and when the wet weather returns the slow rate of penetration of water into the more impermeable dry clay will be insufficient to replenish the soil before the next summer arrives. Thus the dry zone tends to become permanent.
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The spread of a rooting system is mainly dependent on the water demand of the tree, the available moisture in the soil, and, to some extent, on the species of tree. Oak, Poplar, Elm and Willow are considered to have the greatest spread of roots, whilst a line or group of trees can cause the roots to extend further due to competition for soil moisture. A commonly accepted rule of thumb for single trees is that the spread is equal to the height of the trees, or 1.5 times the height for a row or group of trees, extending to a depth of 1.8m for trees with a high demand, and 1.5m for others. This may be greater for trees in rows or groups.
The amount of shrinkage or swelling that a clay soil will undergo is not easy to determine. Laboratory tests have been made on direct measurement of the shrinkage of samples removed from the soil, but it is lengthy and the results have not been entirely satisfactory. A more practical approach is to determine the soil’s potential to shrinkage by examining the properties of the clay obtainable from simple standard classification tests described in BS1377 ‘Methods of Tests for Soil for Civil Engineering Purposes’. These are: a) the Liquid Limit, which defines the amount of water required to bring the
“The Liquid Limit defines the amount of water required to bring the clay to a very weak plastic consistency.”
Figure 1: Ref: NHBC Standards 2007
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clay to a very weak plastic consistency, expressed as a percentage moisture content; b) the Plastic Limit, which is the amount of water in a clay soil below which it is no longer plastic but breaks up when worked with the hands. Below the plastic limit there is insufficient water to fill the spaces between the soil particles and increasingly the voids become filled with air as the water content reduces and no further shrinkage occurs. These two extremes therefore represent the range over which a clay will behave plastically, within which the soil is saturated with water which lubricates and separates the
solid particles. Between these limits the clay shrinks as it dries and the volume change is in direct proportion to the amount of water removed. The difference, therefore, between the Liquid Limit and the Plastic Limit is a measure of the soil’s potential to shrinkage and is referred to as the Plasticity Index (see Figure 2 below). The NHBC has classified the shrinkability of soils as follows:
• • •
potential
High Shrinkage: plasticity index greater than 40; Medium Shrinkage: plasticity index between 20 and 39; Low Shrinkage: plasticity index below 20.
“The difference between the liquid Limit and the Plastic limit is a measure of the soil’s potential to shrinkage.”
Figure 2: Ref: G E Barnes
Figure 3: Examples of typical shrinkage cracking
HI Technical Bulletin The symptoms of clay shrinkage problems are stepped cracking in external brickwork with diagonal plaster cracks internally, usually associated with window openings, and seizing up of doors and windows (see Figure 3 on the previous page). The cracks usually open up after a long dry period and will close up to some extent, but
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not fully, during a subsequent wet period. The type of movement the building has undergone can often be deduced from the cracking pattern (see Figure 4 below). Where a tree was planted at the same time as a nearby house, its size and root spread would increase over 30-50 years as the tree reached maturity. Consequently, the clay
“The type of movement the building has undergone can often be deduced from the cracking pattern.�
Figure 4: Movement deduced from cracking pattern (ref BRE 251)
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shrinkage would develop gradually and any movement would be slight and would probably not be noticed. Indeed, some of the movement may well be taken up by strain within the fabric of the property whilst internal cracks would be filled when redecorated. When the tree is felled the moisture extracted by the tree’s roots suddenly ceases and the dried out clay re-absorbs the moisture it has gradually lost over the previous 30-40 years, causing expansion of the clay over a relatively short period of time, resulting in a reversal of the previous movement often causing very conspicuous cracking. However, once the clay has reached its equilibrium moisture content the soil and the property will have returned to its original status quo and no further movement need be anticipated. On this basis there should be no need for extensive (and expensive) underpinning operations, provided the foundations are adequate in other respects, although cosmetic work would naturally be required. The problem is knowing a) what the equilibrium moisture content is, and b) when it reaches this point. This may be done by, first of all, taking a range of clay samples from a point in the vicinity but well away from any trees, and measuring the moisture content which should give a good indication of the equilibrium moisture content; then comparing it with the moisture content near the building at risk. Alternatively a precise tell-tale system may be set up, readings taken at regular intervals and monitored. Once a consistent reading is obtained over, say, a period of 12 months, allowing for normal seasonal variations, one can assume that moisture equilibrium has been reached. The cure is not straightforward. Removing the tree, if it is a mature specimen, may only produce a worse problem with the subsequent heave, although eventually an equilibrium moisture content will be achieved. Pruning the tree to reduce its water demand will help in the short term but the subsequent more vigorous growth will, if anything, tend to increase the water demand in subsequent years and will only be effective if the pruning is undertaken on a regular basis. The same will apply to root pruning. If there are a number of trees/shrubs, it may be possible to carry out selective removal of the more moisture-demanding trees, leaving others to achieve a balance. If a tree is suspect, it is advisable to identify the tree species and check that it matches the roots found in excavations close to the house. Problems may arise if the offending tree is on the neighbour’s land or on the public
Issue 2 February 2008 highway. Once the identification is made it is best left to the Loss Adjusters to sort out appropriate action. If the problem has been particularly severe or the foundations are too shallow or suspect in other respects, the only effective long-term solution is to underpin the existing foundations to below the level of tree root activity - which is generally at least 1.5m below ground level - and undertake appropriate remedial work once this is done. Generally, cracks up to 1mm may be injected with a resin repair solutions, cracks between 1 and 5mm, if combined with sticking doors and windows, should be repaired by inserting stainless steel stitches such as Helibars (by Helifix Ltd), across the crack and bedded with a cementitious grout. If the crack is wider than 5mm, there is a serious problem and it would be advisable to commission a qualified structural engineer to investigate. Once appropriate action has been initiated, the offending tree (s) can be left, pruned or removed without any effect on the property.
“The symptoms of clay shrinkage problems are
Generally a property damaged by tree root activity will be covered under the ‘subsidence’ clause of the insurance policy, subject to the excess, normally £1,000.
stepped cracking
R M Higgins,BSc, CEng, FIStructE; © RMH, 2007
diagonal plaster
References: BRE Digest Nos. 240 & 241. ‘Low rise buildings on shrinkable clay soils’ BRE Digest No. 251. ‘Assessment of damage in low rise buildings’ BRE Digest No. 298. ‘The influence of trees on house foundations’ BRE Digest No. 343. ‘Simple measuring and monitoring of movements in low rise buildings, Part 1’ 1989 BRE Digest No. 386. ‘Monitoring buildings and ground movement by precise levelling’ 1994 BRE Digest No. 412. ‘Desiccation in clay soils’ 1996 BRE Good Repair Guide 2. Damage to buildings caused by trees. BS1377 : 1975 – ‘Methods of Tests for Soil for Civil Engineering Purposes’ BS5837 : 1991 – ‘Trees in Relation to Construction’ BS 8004 : 1986 – ‘Foundations’ NHBC Standards 2007 Tree Root Damage to Buildings – P G Biddle Physiological Characteristics to Trees and Damage to Buildings by Root Activity – M P Coutts Helifix Ltd. 21 Warple Way, London, W3 0RX (Tel: 0208 735 5223) Soil Mechanics Principles and Practice – G E Barnes Tree Root Investigations Ltd. 3 Langley Drive, Kinnoull Hill, Perth, Scotland, PH2 7XE (they will also identify tree species from twig or leaf samples). Webber. Dickens House, Maulden Rod, Flitwick, Bedford, MK45 5BY (Tel: 01525 718877) for crack repair systems.
internally.”
in external brickwork with cracks
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Rising damp Dampness is a very common problem in properties and the English House Condition Survey of 1991 estimated that there were 1.5m dwellings in England unfit for human habitation with 22% of these houses suffering from dampness. These statistics do not take account of non-domestic properties such as shops, offices, factories etc. which also suffer from dampness. One of the more common forms of dampness is rising damp most prevalent in properties built pre 1919. Rising damp is the migration of ground water into porous building materials of the walls by capillary attraction. The severity of a rising damp problem depends upon the degree of moisture in the ground, drainage of the land, the porosity of the building materials and the effectiveness of any damp-proof course (DPC).
“Rising damp is the migration of ground water into porous building materials of the walls by capillary attraction”
Rising damp usually only affects the base of a wall, up to 1m above ground level (see Figure 5). Normally there is an abrupt change from high to low readings at the top limit of the rising damp. The rising damp contains ground salts in solution and as the moisture evaporates the salts are left behind, leaving a ‘tide mark’ at the top limit. This can be detected by use of a moisture meter. The salts are hygroscopic, that is they attract moisture from the atmosphere and exacerbate the rising damp problem. The salts are general unique to rising damp as they are carried into the walls from the ground. However, similar salts may be present in walls suffering from penetrating damp in coastal areas or where the escape of flue gasses has affected walls. Rising damp has been a problem throughout the years and prior to the installation of DPCs a dado rail was fixed to the internal face of wall 1m above floor level. The dado rail would allow for redecoration of the damp wall covering without affecting the wall above the 1m level that was not affected by rising damp.
Figure 5: Example of rising damp
Rising damp can be prevented by incorporating a physical barrier of impervious material or DPC. Early DPCs were made of natural slate, engineering bricks or lead. Modern damp-proof courses are often bitumen treated hessian or PVC sheets and should be at a minimum height of 150mm above ground level. The Building Regulations since 1965 have insisted that all properties have a dampproof course. Prior to 1965 local Building Bylaws, which differed throughout the country, enforced their use. In the NorthEast DPCs were regularly used during the 1920s and were enforced by the Public Health Act of 1930. How is rising damp caused? Rising damp is always associated with defective or missing damp-proof courses in walls or damp- proof membranes (DPMs) in concrete ground-floors. Properties built with a DPC may still suffer from rising damp, if the damp-proof course or dampproof membrane is non-continuous, defective or is bridged (see Figure 6).
Figure 6: Rising damp or mould
Isolated areas of rising damp are common around the chimney breast since the DPC is carried around the wall but is not continued into the chimney breast. This did not present much of a problem whilst solid fuel was burnt and the chimney remained dry. However, with the advent of central heating solid fuel fires became redundant and the dampness can be quite pronounced. Another area of missing damp-proof courses and membranes is just above concrete floors in houses built in the 1920s and 1930s. The properties often have been DPMs and DPCs. However the damp-proof membrane of the floor does not always extend into the wall and the DPC in the wall can be some 75mm to 150mm above floor level. A band of rising damp occurs within this gap and is usually hidden by deep skirting boards which often show evidence of rot.
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Damp-proof courses may have been damaged due to repairs and alterations or due to subsidence or settlement of the property. A break in the damp-proof course or membrane allows water penetration. Bridging of the damp-proof course is another problem. Soil or paving against the external wall may be above the damp-proof course allowing moisture to migrate into the wall above the DPC. The cavity may be blocked with fallen debris or construction material which results in bridging over the DCP and allows damp to penetrate through the wall. External render coats and internal plaster finishes may extend below the DPC level allowing moisture to pass between the wall and the finish due to capillary attraction. A concrete ground-floor slab may be higher than the damp-proof course and’ if it has no damp-proof membrane built into the wall, it can bridge the DPC.
Figure 8: Plugged holes showing presence of injected DPC
There have been several treatments tried and tested throughout the years as follows: Chemical injection damp-proof course The most common method of installing a retro fix DPC is by chemical injection to BS6576: 1985. Holes are drilled into the walls of the affected property at a minimum of 150mm above ground level at 100mm to 200mm centres to BS CP 102:1973. Chemicals, such as polyoxo aluminium streate, silicone resin or potassium methyl siliconate, diluted with white spirits or water, are injected into the holes and gravity fed into the pores of the building fabric. The chemicals line and bond with the pores and change the angle of contact between rising moisture and the pore resulting in the reverse of capillary attraction, a downwards movement. Alternatively, chemical mortars and gels can be inserted into masonry rubble walls and work on the same principles. See Figures 7 to 10 for examples of these processes.
“The most common Figure 9: The problem
retro fix DPC is by chemical injection.”
Figure 10: The solution
Damp-proof membranes If a solid ground-floor suffers from rising damp a DPM should be installed. This may involve removing the existing floor and relaying the concrete over a 1200 gauge Visqueen DPM incorporating sub-floor insulation. Alternatively, if levels allow, a minimum 65mm screed could be laid on top of a 1200 gauge DPM over the floor slab. There are thin resin based or bitumen screeds less than 5mm thick which can be laid over sound concrete floors. This will also provide a tolerance against rising damp. If laying a new DPM, it is essential to build the DPM into the wall at DPC level. Figure 7: Installation of Injected DPC
method of installing a
Capillary tubes These were developed in the 1930s and manufactured by Royal Doulton. Capillary tubes, known better as Doulton tubes, are
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inserted into the walls 150mm above ground level. The tubes are made of a porous ceramic material and are bedded into the external walls in a porous mortar as retro fix dampproofing system. The tubes are inserted into the walls at close intervals with a slight fall outwards. The tubes porosity attracts moisture from the surrounding damp wall which drains and then evaporates into the outside air. The tubes do appear to work; however, they are bulky, difficult to install, expensive and they can attract hygroscopic salts which prevent effective evaporation. Ideally the tubes should be replaced every five years. Physical damp-proof course This can be achieved by removing 1m sections of the wall at a time and rebuilding with a physical DPC, often in slate. Tanking As previously discussed, hygroscopic chlorides, sulphates and nitrates are transferred into the wall and will attract moisture unless they are removed. It is essential that the plaster is removed to 300mm above the top level of the rising damp on all walls affected by rising damp. The walls should be replastered using a sand cement render with a render guard waterproof additive and finished with a skim of gypsum plaster.
Ensure that all ground levels are a minimum 150mm below DPC level by lowering garden levels and paving. Ensure that render coats and plaster finishes do not extend below DPC level. Carefully cut out brickwork at 1.m intervals and remove any debris and obstructions from the cavity. Drainage Laying a field drain or ‘French’ drain around the perimeter of the property to try and reduce water table levels has met with some success but is only useful if the ground suffers from a high water table. Electro-osmosis In 1807 Professor Reuss demonstrated that a direct current from electrodes would drive water from wet quartz. In Switzerland the Ernst brothers developed the system in the 1930s as a method of damp-proofing properties. The system at best proved to be unreliable. ALAN HOLMES, MBIAT, ACIOB References: Sovereign Chemicals BS CP 102 BS 6576: 1985 Defects and Deterioration in Buildings, B A Richardson BRE Digests Nos. 245, 364 & 380.
Clearing obstructions If the DPC is bridged, it may be necessary to remove the material bridging the DPC.
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