SR is a useful concept to use in assessing the structural stability of buildings under review. It is recommended that the standard to apply to old buildings is Regulation D15
FIG 4.2 Raking shores used to support bay suff ering from subsidence pending repair. Note the tree.
Slenderness Ratio
and Schedule 7 of the old Building Regulations for structural works of bricks, blocks or plain concrete, and using Schedule 7 it is not necessary to calculate loads or wall thick-nesses required, the rules of the schedule being suffi cient as a check on the stability of the construction.
Schedule 7 may be applied to walls which form part of any storey of a building other than a basement, basement storey walls needing a structural check by calculation in accordance with BSI CP 111:1970 should this be considered necessary.
It is assumed for this purpose that point-loads are properly distributed and that walls are properly bonded and solidly put together. There is an overall height limit of 12 m for the use of these rules (above which CP 111 should be used) and also a limit of 3 kN/
m 2 as an imposed fl oor load. This is well above the imposed loads likely in residential construction but which could well be exceeded in other types of building, in which case CP 111 and special calculation would be required.
The table to Rule 7 sets out certain minimum thicknesses for external walls and separat-ing walls, in each case assumseparat-ing that walls will have at each end either a pier, buttress, FIG 4.3 Stucco render has been applied to the fl ank wall of this London terrace. Poor preparation of the wall behind with inadequate keying, trapped moisture and the wrong mix, badly applied, combine to produce this unfortunate result. The builders need to hack it all off back to the brickwork and then start again. Many of these fl ank walls in London have resulted from Second World War bomb damage, so that part of a terrace has been demolished and something has to be done to weather-proof the exposed wall.
61
required for a solid wall under Rule 7, plus the width of the cavity, except in the case of the upper storey of a two-storey dwelling house where the inner skin may be 75 mm (giving an overall width of 225 w mm) subject to certain special rules including a dou-bling of the normal number of wall ties, roof loads being carried partly by the outer skin, a strong mortar being used, and the wall being not more than 8 m long and 3 m high (5 m high to a gable).For the purpose of Rule 7, the heights are measured from the base of the wall immedi-ately above the foundation or footing and not from ground or fi nished fl oor levels.
Walls are regarded for this purpose as being divided into separate lengths by piers, but-tresses, chimneys and buttressing walls. These separate lengths are measured centre-to-centre of the piers, etc. provided that piers and buttressing walls themselves meet the necessary requirements and are fully bonded.
Figure 4.4 shows part of an end-of-terrace Victorian cottage. Note that the brick cross-wall
construction is separated from the fl ank wall on both levels by door openings so that little or no eff ective cross-wall bonding exists. The stud cross-wall is of no benefi t to the stiff ness of the fl ank wall either. The fl ank wall is 225 mm thick and divided into three lengths, these being 3, 5 and 3 m, respectively, this fl ank wall being carried up to form a gable-end with an eff ective height exceeding 3.6 m.
If the chimney breasts are removed a single length of 11 m of wall exceeding 3.6 m high results which should be a minimum of 300 mm thick at ground fl oor level. Since this
Slenderness Ratio
buttressing wall or chimney unless the wall concerned forms part of a bay window and is less than 2.5 m in both height and length. This table is set out below:
is not so, the removal of the chimney breasts is not recommended; they add essential stiff ness to the fl ank wall construction. If soft lime mortars are used, the removal of but-tressing from the fl ank wall may still be ill-advised, even if a check by reference to Rule 7 raises no technical objection, because the SR by reference to CP 111 is excessive.
SRs should be calculated in accordance with the formula laid down in CP 111. For two-storey residential buildings the Code recommends that the SR for walls built in cement mortars should not exceed 24 or 18 if built in the older type of lime mortars (12 if more than two storeys).
The SR is determined by the length and height of the wall, the lateral restraint provided and the nature of the construction. A superfi cial examination of the overall construc-tion will show if a building ‘ feels ’ right. Experience will tell whether a problem may arise having regard to the layout of the cross-wall construction and age of the structure.
Newer practitioners may fi nd it necessary to check such details by approximate calcu-lation until they have acquired the necessary experience.
Having assessed the condition of the main walls and the manner in which they are likely to cope with the forces imposed upon them, the surveyor should then consider the other factors determining whether those walls are likely to be satisfactory or not.
These are: (a) their resistance to dampness rising up, penetrating through or percolat-ing down the walls, (b) their thermal effi ciency and (c) in the case of separating walls (or party walls), their soundproofi ng properties. It is proposed to deal with each of these factors in turn.
225 mm
L1
L2
L3
Chimney breast
Chimney breast Stud wall Brick wall
First floor Ground floor
225 mm
112 mm
FIG 4.4 Ground and fi rst fl oor plans of end-of-terrace Victorian cottage.
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Dampness
The building under review should be examined and compared with an equivalent modern building constructed to current standards. The extent that it falls short of mod-ern standards should then be considered and the client advised accordingly. A good standard would be cavity-wall construction with clear, unobstructed cavities, having damp-proof courses at the base of walls at least 150 mm (two brick courses) above surrounding ground levels and with vertical and horizontal damp-proof courses at all points of contact between the outer and inner skin, and damp-proof cavity trays at all points of bridging. Additionally, there should be an impervious damp-proof membrane in solid fl oors contiguous with the damp-proof courses in all walls. By such means the passage of moisture into the interior from the exterior, or the site, is avoided.
The fact that a particular building is modern will not, of itself, ensure that a good stand-ard is met since, in practice, poor detailing of cavity walls, obstructed cavities, punc-tured membranes and other faults abound. Nevertheless, a good yardstick to apply is that described. In so far as any construction under review fails to meet this standard, then this should be pointed out to the client, and many older buildings will not reach this standard.
Before about 1938, much residential construction was in solid brickwork or stonework and the main barrier against weather penetration in such construction is the condition and pointing of the external brick or stone face. In such circumstances this should be explained to the client, who should be advised to maintain the exterior surfaces in good order in future years and make a particular point of checking periodically for gutter and downpipe leaks. Leaking overfl ows will also cause considerable damage to internal plas-terwork if neglected since the moisture can easily penetrate directly to the inside.
The clearance of 150 mm between exterior levels and damp-proof courses is recom-mended in order to prevent gutter-splash reaching the wall above damp-proof course and to allow some margin of clearance. Experience indicates that the ground levels around houses tend to rise by about 300 mm every 100 years, due to the addition of soil to gardens and the laying of new paving over old. It is essential to resist this trend.
When there is insuffi cient clearance to damp-proof courses it is recommended that ground levels be lowered. A general lowering of levels is best with the surplus soil or paving being carted away. Alternatively, a channel may be formed alongside the wall, at least 150 mm wide, suitably formed with cementwork sides and benching, and hav-ing a fall-away from the main walls and adequate drainage to suitable storm gullies. A channel which fails to drain adequately is worse than useless since it will actually exac-erbate any dampness. The brickwork then exposed at the base of the wall should be made good as necessary by pointing, cutting in new bricks or rendering to a suitable standard.
Vertical damp-proof courses of slates set into a rendered plinth are a traditional rem-edy. They are inserted between the high exterior levels and the walls where damp-proof course clearances are inadequate or the courses themselves are bridged. Such vertical courses often prove ineff ective in the long run as the slates become porous
Dampness
and the bedding mortar between slates is rarely impervious to moisture. Any verti-cal slate damp-proof courses found between high exterior ground levels and the external walls of older buildings should be regarded with a critical eye and consid-eration given to a general lowering of levels or tanking applied to any at basement or semi-basement.
Damp-proof courses become porous with age and rising damp results. Specialist con-tractors now provide remedial services either by chemical injection of the walls or electro-osmosis to prevent capillary attraction in the walls, or by inserting new damp-proof courses in sections using a bricksaw to cut out chases in the walls, generally of about 1 m at a time. Electro-osmosis is now generally considered to be less eff ective compared with other methods and one large specialist fi rm has discontinued its use.
Electro-osmotic damp-proof courses should be given special consideration if encoun-tered during a survey.
Before allowing a client to employ such a specialist contractor it should be confi rmed that the dampness found actually is due to rising damp. Many cases of diagnosed ris-ing damp are not due to failure of the damp-proof courses at all but from penetration or bridging. It is signifi cant that guarantees off ered by specialist contractors will nor-mally exclude liability for dampness due to causes other than moisture rising up the walls by capillary action. Before employing a specialist contractor it is recommended that obvious defects likely to cause dampness should fi rst be eliminated and the struc-ture subjected to a period of observation, if this is possible. It will often be found that dampness can be cured by such simple steps alone.
Having said this, occasions will undoubtedly arise when dampness may be diagnosed as rising damp due to failure of damp-proof courses. In such circumstances all main walls will need attention since the damp-proof courses will be of the same age and type unless parts of the structure date from diff erent periods. The provision of new damp-proof courses in only a part of a building and leaving the original damp-proof courses in other parts may be unwise since further work in the other areas will be nec-essary in due course if the damp-proof courses are failing due to their type and age.
Specialist contractors are commercial men seeking work and there is nothing wrong with that. As a professional, however, the surveyor should be able to say whether or not the specialist’s proposals are unnecessary or premature.
Bridging of damp-proof courses can arise from inside the building as well as from outside. Frequently there is a build-up of rubble and debris under suspended timber fl oors. Whenever such fl oors have to be exposed for building works or services it is rec-ommended that the over-site areas be cleared out and swept, and a check be made that all vents are free from blockage. Solid fl oors in older buildings may also bridge the damp-proof courses. The use of membranes in solid fl oors was not general practice in pre-1939 construction, and in halls, kitchens and sculleries quarry or ceramic tiles were often laid directly on the screed and thus provided a passage for moisture to rise up into the base of adjoining walls. It will often be found that plaster at the base of walls next to such fl oors has been aff ected by damp and a high moisture-meter reading obtained due to this damp transference.
65
In modern construction the failure to ensure that membranes are lapped over or under damp-proof courses and sealed at all joints can give rise to local bridging. Polythene can also tear or puncture, especially if laid under a concrete slab on gritty or stony blindings. Often, 500-grade polythene has been used as a cheaper substitute for the more durable 1200-grade which the writer would recommend for this purpose.
Thermoplastic tiles and woodblocks, commonly employed in houses of the 1950s, were laid in a bituminous backing which was itself the damp-proof membrane. Sometimes such tiles or blocks are removed by a do-it-yourself owner and replaced with other sur-faces, the bituminous backing being disturbed or removed altogether. Modern vinyl tiles or welded sheet vinyl fl ooring cannot be laid with spot adhesive on a fl oor surface unless this surface rests over a suitable membrane. Failure to observe this will result in the fl oor screed sweating under fl ooring and causing it to lift and loosen. The bitu-minous backing to tiles and woodblocks should be contiguous with the damp-proof course in the walls but this may not always be so and can also be a source of bridging and moisture transference into the wall above damp-proof course level.
In older houses with damp solid fl oors or with timber fl oors in poor condition below recommended levels a useful remedy can be to have them replaced by new solid fl oors at a higher level. This can often be done usefully in kitchen and scullery areas where ceiling heights are suffi cient. The ceiling height should not be reduced below 2.3 mm (the old Building Regulation minimum), preferably higher. The new fl oor can
FIG 4.5 Rising damp due to failure of the damp-proof courses will result not only in high readings from a moisture meter but also invisible deterioration to the plaster and decorations. If you cannot see it then it is probably not rising damp. Obviously, there is a problem here which will require investigation, treatment and replastering using a special plaster mix. The condition of any adjacent areas will also need to be checked, especially any timber fl oors.
Dampness
then incorporate a damp-proof membrane lapped at the sides and contiguous with the damp-proof courses in the walls. These should fi rst be exposed by chasing the wall plaster and removing skirting boards as necessary. The client will then be able to lay vinyl tiles, sheet fl ooring or similar surfaces on the new screed in the knowledge that the fl oor will be dry and will not transfer moisture into the surrounding walls.
Apart from normal moisture-meter tests which should be made at selected points around the base of the main walls and at other points at risk, the wall surfaces should also be examined for signs of deterioration in the plaster, staining to wallpaper and mould growth, all signs of dampness (although not necessarily rising damp). Rust stains from skirting board nail heads are a good indicator of moisture behind the skirt-ing board.
One fi nal point regarding dampness in modern buildings is that in many post-1945 houses the pipework for plumbing and heating systems has been buried under solid fl oor screeds above the membranes and if an isolated point of dampness is found at the base of a wall, especially an internal wall, it is well worth while investigating the possibility of a leak in the system under the fl oor. If a serious leak is taking place from a heating system this can be confi rmed by tying up the header tank ball valve, marking the side of the header tank at the existing level and then checking this level periodically to see if it drops. Any rapid loss of water from the header tank will indi-cate a leak to the central heating pipework in the fl oors which will have to be traced and remedied.
FIG 4.6 The Victorians used special-shaped bricks as wall ties in their cavity walls.