EMD Walthamstow - facade repairs - appointment

Tender description

4.1.2 – RENDER The external main façade of the Cinema is made of three different volumes, which includes a taller central body, where the main entrance to the building is located, and two lower ones, on each side. These facades are finished in render, painted white. There is another small section of the south façade, comprising the depth of the Victoria Pub building, which follows the same finish. The condition of the finishes are relatively poor, with some areas of loss, staining and previous patchy repairs. The ground floor will be subject to considerable renovation as part of the works, but the intention is for the areas above to be retained and repaired, as especially the window openings are particularly significant. Most importantly, the lost pediment on the top of the entrance volume is proposed to be reinstated following the well documented design through historic images. This element was lost following the bombing of the building during WWII, so it is assumed considerable repairs were carried out afterwards to the entire façade. It is unclear whether there are areas of surviving original render, or if complete replacement was carried out. In any case the significance lies more on the overall aspect of the façade rather than the composition of the render mixture. The proposals are for retaining as much of the render as possible, and as such, understanding what is currently in place is required prior to finalising the composition of the materials to be introduced. It is recommended that a few samples are taken from selected areas and analysed. The proposed render should have either a similar composition or performance, as well as thickness/coats, finished texture, etc, so that in time differences in the façade are avoided. These locations can be agreed on site, but it would be advisable to select an area of the existing pediment (which would reflect the works carried out post-war) but also around windows and in other areas of the façade where disruption is less likely to have taken place in the past. From photographic evidence it is assumed the wall was originally painted white or of a light colour, and as such a paint analysis exercise would also be relevant. Assessment and preparation of these surfaces prior to the works would follow and initial cleaning and potential paint removal, so that the substrate is as stable and even as possible. The extent of the repairs can be then agreed following a condition survey of this façade. Visual inspection from the ground has allowed for the identification of some defects, which are described below. Defect 1 – Cracks Render defects that have a crack like appearance could have different origins and could even relate to structural problems or movement in the substrate. In these cases, surface repairs might not be enough to overcome the problem, and should concerns be raised over such defects, a structural assessment should be carried out prior to repairs. In the majority of the cases, though, these can be repaired locally, as long as the render is in fair condition and stabilized. Very fine hairline cracks in painted surfaces will probably disappear with the application of the new finishes, as traditionally you would apply several coats of limewash. Cracks which are wider than 0.5mm, but not accompanied by detachment, may be surface filled. Externally, the use of a colour-matched fine grained lime mortar, slightly weaker than the surrounding material, is most suitable. Existing cracks may need to be opened out towards the back by scraping with a knife or spatula to form a parallel sided crack, in order to improve the key for repair. Loose material should be removed and the surface dampened down, unless using a proprietary filler, in which case the surface must be dry. New mortar should be pushed firmly into the back of the crack using appropriate tools, and letting it protrude slightly from the surface, which after curing can be textured back by scraping or tamping. Whilst curing, the repairs need to be kept damp and protected from wind and sunshine. Defect 2 – Loss, delamination or detachment Where cracking is due to inadequate keying of the substrate or differential movement underneath, bridging with a mesh can provide additional adhesion and dissipate localised stress. Ordinary stainless steel and galvanised steel wire mesh are not suitable for lime renders as they have very different thermal properties and are so smooth that lime mortars cannot adhere firmly. The best materials are non-metallic meshes from alkali-resistant mineral fibres or stainless steel wire mesh, such as brick mesh, brick lath or clay lath. The method of surface repair will depend on whether loss is restricted to the finishing coat or extends into the lower layers. As for the other repairs, the reinstated material should ideally be slightly more porous and lower flexural strength, whilst being a close match as much as possible. Appropriate tools should be used to cut back the areas of damage carefully, avoiding the risk to lose adjacent material in fair condition. Small irregular areas can often be patched successfully with minimal visual impact, especially when painted over. Depending on the case it could be useful to cut back to adjacent elements such as window openings, string course, etc, in order to achieve a better finish. Ideally, the remaining surrounding render should be firmly adhered to the backing. The application of the repair should follow the same guidelines described for Defect type 1. Large areas of detachment have not been identified from visual inspection from ground level. However, mechanical means of retaining the render in place could be considered, if there is a preference for salvaging the material. If the decision is made for removal of large areas of render, due recording of materiality and condition should be carried out prior to removing. Defect 3 – Soiling/staining Given the intention to re-paint the façade, existing staining defects will be less of a concern. Preparation of the surfaces could encompass the removal of the existing paint, once analysis has been carried out, in order to obtain a better finish. Trials should be carried out for cleaning and paint removal in order to assess the best methodology. Gentler methods should be tested first, such as dry brushes with soft bristles or low pressure heated water jets, which need to be carefully controlled. It is not expected that more abrasive/mechanical methods, such as scraping and/or picking with spatulas and scalpels, will need to be used, but could be considered after further assessment. 4.1.3 – WINDOWS The main façade windows located on the first floor of the building are of high significance, given that they should be original – whereas other areas have been significantly altered, particularly at ground level. No evidence has been found of replacement post-war, although this is a possibility given the damages and repairs carried out at the time. Some of these windows have been boarded up and are in a particular state of disrepair, having also suffered a considerable number of alterations (glazing panels, locks, finishes, etc). They are, however, a notable element of the building and an effort should be made towards retention. Expected thermal performance improvements to the building directly impact the proposed works to the windows. Initial proposals were for full replacement throughout, but it is expected that such route is duly accompanied by a thorough report on the options considered for retention, and why (if) this is not a possibility. As it stands, the option for retention is still a consideration. An initial condition report was prepared by WDI, which has been attached as an Appendix to this document. Specialist involvement will be required in order to consider whether the condition of the frames is salvageable (once all the layers of paint have been analysed and removed) as well as options for improving its thermal performance by replacing the glass panels – products such as Histoglass should be considered as are single glaze but can achieve lower U values. Secondary glazing options are less desirable as will have some visual impact internally but might still be preferable to full replacement, but further discussions following specialist receipt of specialist advice will need to take place with the local authority and HE. Most mild steel windows from the interwar period were not galvanized and as such susceptible to rust – evidence of this is found in the preliminary condition report. The most common form of surface protection given to steel was the application of paint, and from observation overpainting is evident, probably as a way to repair damages and degradation by weather and light. Paint analysis is recommended to assess the nature of these finishes and whether evidence of original materials can be found. If present, original coatings should be preserved, although this might prove challenging or economically unviable. Cleaning methods suitable for mild steel must be selected in order to provide a stable surface to which new paint application can adhere – the use of hand and power tools, together with washing treatments are probably most suitable. Cleaned metal should be primed with a rust inhibitor immediately after, to avoid reformation of rust on the cleaned and exposed surfaces. Careful repainting, incorporating a system of primer, mid and top coats, should be carried out, upon advice of specialist contractor. Paint coating usually require re-application within 7-10 years, subject to manufacturer’s recommendations and depending on quality, application method, and the surrounding environment. If required, repairs will require using the correct alloys and equipment and should only be undertaken by qualified and experienced operatives. Although mild steel alloys are still readily available, the original standardized sections common to all manufactures of the interwar period are no longer in production. Repairs that require the splicing in of new metal are therefore reliant on the availability of salvaged sound material or the fabrication of new which match the original sections. If these windows end up being removed and replaced with new, a proper disposal strategy should be prepared and carried out as some of the material could be salvaged for other uses. Trial samples of paint stripping, repair techniques and cleaning methods should be undertaken and recorded, including strength and dwell time and conditions for stripping cleaning, in order for proper replication once the most suitable and effective method has been agreed.