(To be read in conjunction with our General Information Leaflet No.1)
Note to Tenderers (if applicable)
Please note that this specification is what is considered necessary for the control of an outbreak of dry rot or, with suitable adjustments, of wet rot. Tendering should be done on the basis of this Specification with the exception of any extra works that tenderers require in order to be able to issue guarantees. All such works should be specified and the costs itemized separately.
This specification is written for the purpose of specifying dry rot control techniques. With due alteration, as described in Gil No.1, some of the techniques also apply to wet rot control.
DRY ROT TREATMENT
1. THE REMOVAL OF STRUCTURALLY DEFECTIVE TIMBERS
All structurally defective timber which is built into brickwork should be removed in its entirety. In the vast majority of cases, this removal should be extended 1m beyond the last visible sign of infestation either of the timber or in adjacent brickwork. Wherever possible, for built-in timbers at risk such as wall plates, if it is at all practicable, the complete wall plate should be removed.
Where timbers are not built in, it is not necessary to remove timbers 1m beyond. Instead, they should be cut back progressively from the visible area of attack until the whole of the cut-off cross-section reveals sound timber. In the case of built-in joist ends, which are structurally sound but are within the 1m danger zone beyond the perimeter of visible infestation, these joist ends can be isolated with a single layer of a damp-proof membrane or preferably cut flush with the brickwork and hung in metal fixings, such as joist hangers or, preferably, joist angles (such as those that used to be made by BAT and which can now be purpose made).
For the purpose of inspection by the surveyor representing the client, all cut-off joist ends removed in this manner should be retained in clear polythene bags, nailed to the joist from which they have been removed or, alternatively, adequately labelled and stored.
2. THE REMOVAL OF PLASTER OR RENDERING
Where the outbreak is diagnosed as active, plaster on brickwork should be removed in all directions 1m beyond the last visible evidence of infestation in the brickwork and possibly further if the plaster shows evidence of excessive dampness. Tendering companies should ensure this means up, down, left, right, front and rear. Where the outbreak is dead or dormant, plaster removal can be eliminated or reduced at the discretion of the supervising expert.
Indications for exposure works may be given by means of accompanying sketch plans. Where exposure is known to be required, normally this will be indicated but, because of the sequential nature of discovery of the fungus, exposure works may require extending.
Following plaster removal, a further inspection by the timber surveyor or specifier should be made before the brickwork is brushed free of obvious evidence of mycelium.
Plaster removal from timbers not in contact of brickwork should be minimal, if at all, but sufficient to enable thorough examination of that timber, but does not, for example, in the case of ceiling plaster abutting a brick wall, require removal '1m beyond'. It is sufficient merely to gain access to inspect, to determine the extent of damage or wetting, and possibly to carry out the application of chemicals, if necessary.
3. CHEMICAL TREATMENT
The considerable requirements of the Food and Environmental Protection Act 1985 (FEPA) - in particular the Control of Pesticides Regulations 1986 (CPR) - and Control of Substances Hazardous to Health 1988 (COSHH) and all pertinent subsequent amendments should be noted both by contractors and specifiers.
3a. Chemical Treatment of Timber
If this is to be done it should be carried out only after an assessment of the hazard class as described in British Standard 5268 (1989) The Code of Practice of the Preservative Treatment of Structural Timber and its subsequent European equivalents.
Following assessment of the hazard category, the required chemical treatment can be determined by reference to both BS5268 and also the BWPA Manual 1986.
The contents of the two following publications should be noted when carrying out these works: Remedial Timber Treatment in Buildings :A guide to good practice and the safe use of wood preservatives HSE 1991 and The Safe Use of Pesticides for Non-Agricultural Purposes APPROVED CODE OF PRACTICE, HSC 1991.
The practical limitations of in situ chemical treatment of timber should be noted.
Whilst there is available a broad spectrum of a response, it is likely that in the majority of cases requiring treatment in normal buildings, the response would be restricted to:-
(i) treatment with a fungicide to very wet timbers not likely to dry out quickly. This probably would require a solid, water-based or monoethylene glycol-based fungicide such as disodium octaborate;
(ii) preservative paste treatment;
(iii) since the scope for penetration of organic solvent-borne fungicide by this means in wet wood is limited and there is no need to apply fungicide to dry wood, organic solvent-borne application is only likely to be useful for insecticidal treatments.
If organic solvent based preservative pastes are used, these should not be used in such a manner that their creep causes problems to decorations and finishes. Sufficient precautions must be taken, for example by isolating with joinery liners, to prevent staining.
3b. Chemical Treatment of the Brickwork with Fungicides
In addition to the aforementioned COSHH (1988) requirements, surveyors and/or specialists professing expertise recommending treatment of the brickwork, or builders and/or specialist companies executing such treatment, should familiarise themselves and their company technicians with British Wood Preserving Association (now the PCA) leaflet No.1 Fungal Decay in Buildings - Dry Rot and Wet Rot, pages 3 and 4; Treatment of Dry Rot, the penultimate and last paragraphs of page 3 and the first two paragraphs of page 4. They should also familiarise themselves with BRE Digest 299, the British Wood Preserving Association News Sheet No.160, issued May 1980, from the sixth paragraph entitled 'Containment of the Fungus within a Wall' through to the ninth paragraph entitled 'Irrigation'.
Where the fungus is alive or potentially alive, in general, the guidelines laid out in BRE Digest 299 (whereby only surface treatment is considered necessary) should be followed but chemical treatment of the brickwork may be on the 'toxic box' principle utilising, wherever possible, window reveals and cavities caused by the removal of timbers. Holes should be drilled around the perimeter of the toxic box at approximately 300mm staggered centres and care taken to ensure each box is complete. Surface application of fluids can be at the recommended levels of the tendering remedial treatment specialist companies but should be sufficient to conform with their supposed efficacy but not so excessive as to be hazardous or not to comply with the Legislation Control of Substances Hazardous to Health (1988).
If, upon exposure, the brickwork appears to be very wet, a water-borne masonry biocide should be used for surface treatment. Provided the treatment company can ensure, by use of an adequate plastering specification, that no efflorescent salts will appear on the decorated surface of the plaster, the choice of quantity and type of fluid can be left to the tenderer's expert adviser.
In the light of the above COSHH (1988) Regulations, where the brickwork is dry, the specifier should consider carefully whether or not the use of a fungicide can be justified.
4. REPLASTERING
4 a. Brickwork
Where very wet brickwork is to be replastered, the following specification, which is not mandatory, is suggested:-
1. a 6mm splatter coat of 1:1 sand:cement containing SIKA in accordance with the manufacturer's specifications;
2. when set but still wet, a floating coat not greater than 12mm of Limelite Renovating;
3. any further such coats to make out the thickness, no coat being greater than 12mm maximum;
4. a finish coat of Limelite Finish.
The purpose of this specification is to provide an initial water-proof splatter coat as a check against water penetration and staining of the new plaster from the outside, followed by a lightweight cement-based plaster with a lower 'U' value than sand:cement to minimise the risk of subsequent condensation from the inside.
Alternative specifications, for example the use of allegedly fungicidal specialist sand:cement formulations, should be submitted and priced separately and are acceptable provided they are correctly formulated, fit for their purpose and comply with COSHH.
Any gypsum-based backing plasters, even those claiming to have specialist properties, are not acceptable as backing plasters.
4b. Ceilings and Stud Walls
These should be plastered as recommended by the architect. Care should be taken that no gypsum-based plaster is in direct contact with both brickwork and timber.
5. REPLACEMENT STRUCTURAL TIMBERS
As with in situ application of chemicals in section 3 above, an assessment needs to be made as described in BS5268 (1989) part 5, tables 2 and 3. This can then allow the required timber properly to be specified.
In most cases replacement structural timbers should be treated with a Copper Chrome Arsenic water-borne pressure treatment, such as `Tanalith' or `Celcure'. They should be less than 18% weight by weight moisture content and all worked or cut ends should be treated by immersion (dip treatment) in an organic solvent-based fluid for 10 to 15 minutes at a minimum, preferably for 60 minutes.
All replacement of timbers should be carried out in such a manner that timber/masonry interfaces are eliminated and there is no danger of plaster or cement mortar droppings bridging any gaps. A suggested technique involving the use of a thin gauge polythene bag and a joist angle is shown in the diagram accompanying this text.
Where a continuous row of joists has been removed from the brickwork, subject to the opinion of a qualified chartered building engineer, every third joist should have a right-angled BAT strap screwed to its side and adequately be embedded in the brick wall. These straps should be 5x30mm, approximately 1m long, bent at right angles 100mm from one end (the end buried into the brick wall) and screwed into the neutral axis of the joists with three heavy gauge suitable plated screws.
6. REPLACEMENT OF JOINERY TIMBERS
It is preferable to prefabricate and then to impregnate with a double vacuum organic solvent-borne process. Such impregnation processes are described in the BWPA manual 1986.
Where it is not possible to pressure-treat at a plant, in situ treatment should be by the immersion methods also described in the manual.
Replacement of box frames, skirtings, door linings, etc should be such that a thin gauge polythene membrane or joinery liner is installed to protect the timber from contact with all coats of backing plaster or render and, in the case of a window, all exterior cement:lime mortar pointing. This should be done by encasing the timber in question with a thin gauge joinery liner (say 63 μ), leaving the joinery liner in situ during all applications of plaster except the finish coat. Prior to the application of the finish coat, after all the backing coats have set, surplus joinery liner should be cut off with a sharp `Stanley' knife or similar and the junction covered internally with the finish coat of plaster or architrave and externally on the window frames with a bead of mastic in accordance with Defect Action Sheets 68 and 69 from the Building Research Establishment. Care should be taken when carrying out this operation to ensure than thin gauge polythene is used to avoid any rucking. Plasterers must understand the sequence of operations.
7. EXTERNAL JOINERY TIMBERS
All external joinery timbers (such as windows) subject to weathering should be assessed for risk, the hazard category determined, treated as described above and then have two coats of suitable primer applied which is allowed to dry prior to their installation. We suggest the Dulux Weathershield clear primer and paint system.
8. SKIRTINGS ABUTTING SOLID FLOORS
Because of their hazard category, any such timber skirtings should be double vacuum impregnated timber. All subsequent worked or cut ends should be treated with a 60 minute immersion prior to fitting. There should be a gap of approximately 25mm between the top of the solid floor screed and the bottom of the wall plaster and this should be entirely free of any form of debris, plaster droppings etc. Optionally, the exposed masonry in the gap between the bottom of the plaster and the floor should be treated with two liberal coats of `Synthaprufe' or a similar product, allowing the first coat to dry before application of the second. The skirting boards then should be fixed in position using a non-timber fixing such as a `Fischer' or a `Rawlplug', and with a joinery liner applied to the back of the skirting which extends over its full width from the top to underneath . The liner should be cut off at the front of the base flush with the floor and the top only after the backing coats prior to the application of the finish coat as described in Section 6 above.
Below ground level since the plaster is likely to be a `tanking' specification specialist techniques are required which avoid puncturing the plaster. For example, a specification for gluing with epoxy resins, such as 'Sikadur', treated timbers to tanking plaster is obtainable from SIKA Limited.
9. Specification for Window repairs
Remove the sashes or the casement and set aside for repair, if necessary, possible treatment with a fungicide and thereafter for later reuse.
EITHER (and this is generally to be preferred)
The decayed parts only of the window frame should be removed, leaving the remainder in situ.
Whilst the frame is in situ, cut replacement timbers exactly to size, check the fit, complete any workings and then treat by the double vacuum impregnation process. Allow the solvent to dry, suitably prime and refit into place by gluing with a suitable glue (Resorcinol or Phenol-Resorcinol Formaldehyde) such as Bison Resorcin and/or non-ferrous screws covered in timber plugs. All reinstatement of framing timbers to be as described above.
OR (less satisfactorily, but sometimes a more practical approach)
The affected window frame should be removed in its entirety and the resultant opening protected from the elements.
The frame can then be repaired at a joinery works or on site. If, on removal, it is seen to be necessary, a new frame can be constructed.
Ensure that;
* there is a drip groove in the base of the sill,
* that there is an adequate slope on the upper surface of the sill and
* that the arris is suitably rounded to accept paint.
For `whole element repairs',eg a cill or a complete style, or `replacement' use Scots pine (Pinus sylvestris). Only suitable, very durable, hardwoods are acceptable alternatives. These are expensive. Oak and luan are not very durable (as defined by BRE Digest 296).
For 'patch repairs' eg the bottom 100mm of a style use the same species of timber as closely matched as possible with the original for grain angle, density, distribution of growth rings, etc.,
If the assembled window frame will be too large to fit into the treatment plant, prior to assembly the components of the repaired or replacement frame should be treated with an organic solvent borne insecticide/fungicide by the double vacuum impregnation method at a pressure treatment plant. If the window frame is small enough, it may be fabricated, using suitable fixings and glue, and then the whole unit double vacuum treated.
If any double vacuum treated timber has to be cut to size, or further worked, on site, ensure the cut ends are treated by immersion in a suitable [generally organic solvent borne] fungicidal fluid for a minimum period of 60 minutes.
Prior to reinstatement of the window frame in the building, chemical treatment of the brickwork may need to be carried out (see the section on Chemical Treatment above).
Reinstate the frame isolating from masonry contact with the use of joinery liners, as described in our "Description of Treatment Techniques".
Apply a suitable paint to the external exposed part of the frame.
Apply sealants to the exterior interface between the frame and brickwork as described in BRE Defect Action Sheets 68 and 69 and as described above.
An essential associated reference to be read in conjunction with this leaflet is the Technical Pamphlet No 13 "The Repair of Wood Windows" obtainable from the SPAB, 37 Spital Square, London E1 tel. 071 377 1644.