I built a very large bungalow in 1980. Forty-three years later the felt has started to crack and leak in places, allowing water ingress in places, resulting in staining of the ceiling. It is a slated roof that was cleaned and spray-painted two years ago. I have been advised that the felt needs to be replaced and to do this the slates have to be lifted, which could result in many of them breaking as they have adhered to one another following the spray painting. The cost of carrying out such work will be substantial. I had hoped that the existing old felt, either in its entirety, or alternatively where it is cracked and compromised, could be sprayed with a sealant, which would prevent the water ingress and obviate the necessity of removing the slates, replacing the felt and relaying the slates, which would be very expensive.
With older slate-covered roofs, the slates were laid directly on timber battens with no provision for any underfelt. However, slates are relatively small, and there are numerous joints where water can seep in, particularly in times of wind-driven rain and/or snow. The risk of water penetration will be dependent on several factors, including the degree of exposure and the pitch of the roof.
Underfelts, typically known as sarking felts, were introduced in the 1960s as a secondary barrier to rainwater penetration by collecting and directing any water that penetrated the slates to the base of the roof. Whilst the modern sarking felts are very good-quality material, the earlier traditional felts are quite basic and would tear easily and were often damaged during the installation process (I anticipate that the cracking that you are referring to are the actual tears/holes in the sarking felt). Furthermore, the felt tend to become brittle with age and tends to break down or perish, particularly in hollows or along the base where any water that does penetrate is most likely going to settle. Accordingly, a secondary layer with holes/tears will not be particularly effective at keeping the water out and remedial works will be required.
Depending on the extent of the holes/general condition of the felt, there may be scope for repairing the felt from the underside ie from within the roof void by localised patching, and typically by incorporating a small strip of new felt under the bottom course of slates along the roof edge to ensure that any rainwater that does penetrate through the slates is properly discharged in to the eaves’ gutters. Unfortunately, in some cases, the felt will be so bad that it is beyond repair. I would generally not recommend sealing the felt by spraying the underside of it, as the typical spray on products are not waterproof and many of these are not breathable and this could lead to greater problems.
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While the above comments focus on the underfelt or sarking felt, it must be remembered that the sarking felt is only a secondary layer and you need to bear in mind that older-slate covered roofs did not have any sarking felt at all. The real area to focus on is the primary protection layer, the slates. In this respect, a well-laid modern slate covered roof is unlikely to let much water in anyway and thus the condition of the felt would not be as critical.
You do not say what type of slate the roof is covered with, and I am mindful that the issue will be dependent on many factors including the degree of exposure and the roof pitch. For example, a more exposed roof in say a coastal region or on a hilltop or a lower pitched roof will be more prone to water seeping through the slates.
You say that you had the slates cleaned and painted about two years ago. The need for cleaning would suggest a combination of either overhanging or large trees close by and a relatively low pitch. Furthermore, and more alarmingly, the “need” for painting suggests discolouration of the slate which typically arises either due to high exposure or due to a poor-quality fibre in the slate make up.
One potential issue to consider is whether or not the slates are an asbestos-fibre cement slate, which would typically start to break down after 30-40 years. For the avoidance of doubt, you should have the slates checked to see if they are asbestos-based. If this is the case, you will need to have the works carried out by specialists in accordance with all current asbestos and health-and-safety regulations.
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However even if the fibres are not asbestos-based, it is likely that they are of poor quality. If my assumptions are correct, then you are wasting your time concentrating on the repairs to the sarking felt and the focus needs to be on the actual slate covering. All the symptoms suggest to me that the only effective remedy here is to strip off the roof coverings including the slates, the timber battens and the sarking felt and to re-cover the roof in its entirety.
I am mindful however that other factors need to be considered before reaching this conclusion, and there is no real substitute for having the roof inspected by an expert so that a proper diagnosis can be made based on all the facts. I have no doubt that your local chartered building surveyor will be able to undertake an inspection of the roof and advise you on the most appropriate course of action.
Val O’Brien is a chartered building surveyor and member of the Society of Chartered Surveyors Ireland
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