<%@LANGUAGE="VBSCRIPT" CODEPAGE="CP_ACP"%> DPC Training Manual
You are here - Homepage > DPC & Timber Treatment > DPC Training Manual

 

 
o o o o o
 
 
DAMP & TIMBER TREATMENT COURSE (top)  
     
  This document has been produced solely for delegates attending the PAM TIES LTD. DAMP AND TIMBER TREATMENT COURSE, and is intended only for private study use. The information contained within this document is confidential, and no warranty or guarantee may be assumed. Any persons making such assumptions do so at their own risk. Reproduction in whole or part is strictly forbidden without the prior consent in writing from PAM TIES LIMITED.
  
     
 
RAIN & DRIVING RAIN (top)  
     
 

Rain falls and wets a roof, which is there to cover a building and protect it from getting wet from rain, and it usually does this admirably. Driving rain, however, i.e. rain not falling vertically, but driven sideways by strong wind, falls both on the roof and on the windward walls. The roof does not protect the walls from the driving rain, except where there is a very large overhang, a feature that is not popular in the U.K., but it is more popular in Switzerland where it protects the house from deep snow against the walls.

  
  Walls therefore have many functions: they have to hold up the roof; protect and enclose the building against wind, rain, snow, sleet and hail; insulate; keep out solar heat; create privacy; and partly insulate against sound.  
  Many walls – solid ones – built years ago did not keep out driving rain sufficiently. It may have been supposed that a one-brick wall built in a good frost resistant facing brick, such as London Yellow stock and proved to be fairly good for London suburbs for weather tightness and frost resistance would therefore generally be suitable for almost anywhere. This is not the case. There is far more driving rain in some parts of the British Isles than in others: one can reckon three times as much on average in the west of Ireland as in London, and still much more for Snowdonia, Cumbria or Argyll (7 times that in London).  
  Atlas’s contain annual rainfall maps: rainfall, not driving rain. The rainfall map of the British Isles is interesting, showing for the mountains of Western Scotland, Cumbria and Snowdonia over 100 inches of rain (over 2.5m), but for much of East Anglia and the East Midlands only 20-25 inches (0.5 – 0.63m).  
  Driving rains is a different matter. This is borne out by the Driving Rain Contour Map of the British Isles, the highest winds occur in general from the prevailing directions of SW and W as for rain generally, but in addition some strong winds come from S and NW in some areas. The hills and mountains in the west of Britain therefore receive the highest winds and greatest rainfall, and therefore the most driving rain.  
  Rosettes indicate by the length of their arms the amounts of driving rain from twelve directions equally spaces round the compass, the rosettes applying to certain chosen locations.  
  Fig. 1. Shows rosettes for Plymouth and Edinburgh. Note the strong South driving rain at Plymouth ant the strong north-easterly at Edinburgh from across the Firth of Forth. These rosettes do not quite correspond with wind “roses” shown on some atlases, as the wind rose show the frequency of the wind in different directions, not the mount of wetting of walls.  
     
 
DRIVING RAIN INDEX (top)  
     
  The index numbers on the driving rain map, ranging from less than 2.5 to 15, are obtained as follows. The annual rainfall in metres is multiplied by the average wind speed in m/s, giving m²/s. The rainfall figure taken is that falling on the ground. This suffice because when tests have been made of rain driven onto walls it is found that this amount is proportional to the product: rainfall on ground x wind speed during rain. The latter, wind speed during rain, is not normal meteorological recording like average wind speeds, which are used instead for the index.  
  Examining the differences in index in the U.K., one can see that a large area of London, Essex and the East Midlands up to central Yorkshire has an index below 3. Dartmoor has 10, the Lake District 10 and considerable areas of Argyll and Inverness-shire 10-15.  
  Records of maximum hourly driving rain at 23 stations in the British Isles, taken over a period of ten years, are given in a table in Digest 127. The range is from 0.08 (Kew) to 0.21 (Glasgow), but there is a freak exception at Plymouth where 0.4m/s was recorded for an east wind in July.  
  A rate of 0.2m²/s would correspond to about 50 L/m² per hour on a vertical face. Since the wind does not blow square on to a vertical face but blows angle wise and is deflected round corners, one may expect more rain to be driven onto corners and local areas than on the whole façade. The whole façade would be more likely to receive half the quoted figures. Though the rosettes show driving rain from the different directions over the year they do not show the directions of what might be the most intense short period driving rain – a direction not to be suspected from the rosettes. For instance, London has its most intense driving rain from the east, but the rosette shows between S and W as the directions of greatest annual driving rain.  
     
 
USE OF THE DRIVING RAIN INDEX (top)  
     
  To what use can we put the index and knowledge of rosettes? It is difficult to be precise in applying this knowledge, but applications do have some value. Consider the one-brick solid wall built in good facing bricks: the construction might be sufficient to keep out rain (but not good enough for thermal insulation) over the more sheltered areas of England mentioned – from London and Essex up to the Wash, up the East Midlands to central Yorkshire, but excluding any area close to the coast. This area has a driving rain index under 3.  
  “Moderate” conditions obtain over the south of England, parts of the South West, Norfolk, West Midlands, the North East of England and the eastern half of Scotland. Here a one-brick wall, which might be used for a garage, would be of doubtful
water-tightness, with an index of 3-7m²/s. for all the western areas of Wales, Cumbria and Scotland, where “severe” exposure occurs (over 7m²/s), design for waterproofness should be considerably better. Cavity walls would be essential and probably a greater “lap” of tiles and slates.		  
  It should be remembered that porous bricks may absorb water quickly in driving rains, but will also dry out quickly by evaporation aided by wind and sun when the rain is over. Short sharp showers therefore are likely to penetrate so much as prolonged driving rain. Bricks for this exposure require to be comparatively non-absorbent like engineering quality bricks, or to be very adequately rendered or pebbledashed. Pointing to brickwork should have weather struck joints.  
  Driving rain can sometimes cause a stream to run down walls as it does down windows. Design should be such as to throw running and dripping water clear of the building, from cills, parapets, and projecting string courses. Such dripping water falls on people below or on to plinths. Projecting plinths in general are not a good design feature. Good detailing to throw water clear where it can cause no harm is part of an Architects duty.  
  A sequence to severe wetting in winter is the effect of frost – the worst form of attack, which disintegrates bricks in general, and also, attacks stones and any porous concrete or cracked concrete. In brickwork it applies more to certain locations such as parapets, self standing walls and retaining walls, that to plain walls of buildings.  
  One must again emphasise that parts of a building, other that the roof, will collect more driven rain than others, e.g., chimneys, parapets, cornices, quoins and corners of buildings, plinths, projecting string courses, cills, balconies and canopies.  
  One of the effects of wetting a wall is to cause moisture expansion. The effect is general to all porous materials, but is quite small in clay bricks and larger in sand-lime bricks.  
  It is usual in long runs of walls, especially self-standing walls, to allow for moisture expansion and contraction together with thermal expansion and contraction. The order of these two movements is comparable for some materials. In general, of course, when there is heavy rain causing moisture expansion, it is unusual to have high thermal expansion since the conditions are not normally coincident. Thermal contraction in winter tends to be partly compensated for by moisture expansion.  
     
 
TEMPERATURE CHANGES (top)  
     
  Even in a temperate climate a dark surface can rise to a high temperature as 70°C as it absorbs solar energy, though the air temperature will be considerably less. A surface may drop in winter in England to - 10°C, a change of 80°C. The effect on building materials is worth examining. Coefficients of expansion vary for building materials (which include metals), but in general lie between the extremes shown in the following table.  
     
 
MATERIAL
Aluminium
Brick
Copper
Duralumin
Glass/Plate
Iron

Lead
Marble
Masonry
PVC
Sandstone
Steel
Timber (hard, along grain)
Timber (pine, along grain)
Timber (hard, across grain)
Timber (pine, across grain)
THERMAL EXPANSION (Coefficient x 10)
25
9.0
17
24
9
12
29
11.7
4-7
60
7-12
11-13
3-9
5.5
32-61
34
 
     
  Using table find the maximum expansion of a 10m length (a) of metal for an 80°C rise in temperature and (b) brickwork for a 70°C rise in temperature.  
     
a.
 Aluminium (lead is laid in separate sheets):  
 
Expansion = 10 x 25 x 10 x 80m
  = 20mm
  		 
b.
 Brickwork:  
 
Expansion = 10 x 9.5 x 10 x 70m
  = 6.65mm
 
     
  Brickwork may accommodate expansion in its joints if the mortar is not too dense, but with long lengths, and to allow for moisture expansion, design should incorporate the possibility of movement. Mastic-filled joints, right-angled returns or chevron-plan shapes may accommodate this.
  
  Roof sheets in general are jointed with loose connection and will permit expansion. Rainwater gutters of steel or cast iron may expand 15mm for a 20m length. This is seldom allowed for but, being fixed to softwood eaves, the differential expansion will be less owing to the slight timber expansion. PVC gutters present more of a problem since the expansion is about 5 times that of steel. As this is so considerable, the joints must be flexible to accommodate the expansion, but even so they tend to fall.  
  In timber, moisture changes affect the width far more than thermal changes, so the thermal changes may be neglected. Thermal expansion in length of timber is small and likely to be partly counteracted by drying shrinkage as the timber warms. However, the time scale differs. Timber may warm up quickly but dry slowly, taking longer to shrink.  
  Another effect due to temperature rise is the softening of a material. This particularly applies to asphalt and bitumen on roofs and asphalt on plinths, skirtings, etc. as a surface temperature rise of 70° is possible, it is better to cover flat asphalt roofs with marble chip or something similar to reflect solar radiation. Plinths cannot be covered, and are sometimes seen to begin to run into “curtains”.  
     
 
HUMIDITY & LOW TEMPERATURE (top)  
     
  Apart from actual rain falling on to buildings, the air surrounding them may vary over a wide range of relative humidity – from say, 40% to 100%. It is about 100% in cold and foggy or misty weather, and the air close to the ground rises to 100% humidity on most nights during the year in the U.K. Water or water vapour may thus readily be absorbed by materials externally and in this may affect their behaviour. Some effects are: -
  
1
 Swelling of timber across grain in particular, e.g., in gates and doors, which therefore have a habit of sticking or binding in wet seasons.  
2
 Condensation on to metal – metals like iron corrode not only when there is condensation or rain on them, but also when the humidity rises (at 90% relative humidity, rusting accelerates considerably over that at low humidities, say 40-50%, so tools, etc., in unheated sheds and garages often rust quite fast in winter although no condensation may have actually deposited upon them).  
3
 Condensation onto un-insulated concrete or asbestos cement, which often occurs in garages and outhouses – it occurs under nearly flat asbestos cement roofs and drops onto cars or whatever stands below.  
4
 Frost, sulphate attack, exfoliation etc., which have one thing in common – water is needed. Further study of these will follow in the next section.  
     
 
ATMOSPHERIC POLLUTANTS AND THEIR EFFECTS (top)  
     
  The normal atmosphere contains about 20% oxygen and about 80% nitrogen and the rare inert gases, plus water vapour and carbon dioxide, CO², which normally forms rather less than 0.04% of the air. Increase of CO² to ten times this amount cannot be called a pollutant as ground air (air in the top soil) contains 250 times the amount of CO² of the open air. If we look at the pollutants of the open air there are few that concern us. (We will exclude lead from petrol fumes, as this does not affect buildings). The principal ones are sulphur gases given out in combustion, which turn into sulphur acids in atmospheric water. Apart from these gases, there are visible particles which, if not of water, are usually smoke particles – solids, very largely of soot (carbon) although tarry substances may be carried up into the air when tarry material is incompletely burnt.
  
  The clean air act has helped considerably to reduce smoke, i.e., solid particles, though sulphur gases, which are invisible, are still produced by most forms of combustion. Carbon monoxide is another pollutant given out in motorcar fumes. It is invisible and odourless, but poisonous to human beings, and must be allowed to disperse into the air: it appears to do no harm – at least to buildings. Industries that produce it burn most of it, e.g., the ironworks blast furnaces burn the very large amount of CO products in “Cowper Stoves” for preheating the air used, and for other works uses.  
     
 
SULPHUR GASES & ACIDS (top)  
     
1.
 Sulphur dioxide, SO² is the normal product of burning sulphur. Solid fuels like coal and coke contain sulphur. The gas is very pungent, sharp smelling, acidic and soluble in water to form sulphurous acid, H² SO³:  
  SO² + H²O = H² SO³  
  Though the gas is twice as heavy as air when cold it is carried up flues and chimneys with other gases when hot. What happens then to the SO² depends on the conditions: from high chimneys it may disperse into the upper atmosphere and be carried far away – accusations have been made by Norway that Britains tall chimneys have polluted and poisoned some of her trout and salmon streams by producing “acid rain” – or without much wind, SO² and H² SO³ will tend to sink to ground or building levels somewhere down wind. In wet air it forms sulphurous acid, which partially oxidises on exposure in air, forming sulphuric acid, H² SO  
  2H² SO³ + O² = 2H² SO4  
2.
 Sulphur trioxide, SO³: a small amount of this is directly obtained when sulphur is burnt. When cold it would be a white solid, but having very deliquescent properties – that is, affinity for water in which it readily dissolves. It would be converted to sulphuric acid in ordinary open atmosphere quickly:  
  SO³ + H² O4 = H² SO  
  This amounts to saying that sulphur pollutants are largely sulphurous and sulphuric acids. Apart from the effect on people with weak lungs, the long-term effect on buildings is seen mostly in connection with limestone. CaCo³. The effect on calcium carbonate of these two acids is to from calcium sulphite and calcium sulphate.  
  CaCO³ + H² SO³ = CaSO³ +
HO² + CO²
CaCO³ + H²SO4 = CaSO4 +
HO² + CO²		  
  Consider where calcium carbonate is found externally in buildings. All limestone’s are or contain a major proportion of CaCO³. Marble is CaCO³. Much of the cement at the face of concrete exposed to the air becomes CaCO³. Mortar contained lime, and exposed lime carbonates to CaCO³. Calcareous sandstone’s e.g., White Mansfield, contain CaCO³ in their matrix.  
  Calcium sulphite is soluble in water and is likely to be washed down by driving rain from its place of formation to lower levels. In sheltered limestone facades there is less likelihood of its removal. Calcium sulphate (gypsum when crystallised) is less soluble, being very sparingly soluble (about 0.24 parts per 100 of water). It tends to stay where it is formed between the particles of CaCO³ in a limestone building, unless it is well rain-washed. This is one rare case where plenty of washing by rain is a good thing, because if calcium sulphate is allowed to build up in the pores of the stones it causes exfoliation.  
     
 
EXFOLIATION (top)  
     
  As calcium sulphate builds up in the pores of limestone in place of CaCO³, it gradually causes stress in the hardened surface layer (which may be 10mm thick). First a braking away near the edges of the stone around the joints occurs, with slight hollowing of the face left. Eventually, the whole face may peel off in sizeable layers or lumps. This action is called exfoliation. The amount of stone lost by this mechanism is much greater than the amount dissolved by the same amount of acid.  
     
     
         

1 | 2 | 3 | 4 | NEXT>