Do I Need Felt Paper on My Roof Text

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Multiply the width by the height for the square footage of each section, then add the totals. The answer is the number of square feet of roofing tar paper you need to purchase. Dampness and mould growth on a tie beam caused by the use of impervious roofing felt it is now commonly accepted, at least within conservation circles, that it is important not to restrict the ability of traditional building materials and structures to lsquo breathe rsquo. However, attention has tended to focus on the damage caused by the use of impervious modern paint systems and cement rich mortars and renders, and the one part of an old building where the assessment of performance and attention to detail is often neglected is the roof.

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Yet this is one of the principal areas where ventilation could readily take place, particularly in traditionally detailed tiled or slated roofs. Today, the performance of many historic roofs has been impaired by the introduction of roofing felts and insulation. Although many roofing felts are now marketed as being vapour permeable, until recently almost all felts were impervious.

Roofing felt was introduced primarily to act as a secondary barrier against wind driven snow and rain, but its use also causes a reduction in air movement within the roof space, particularly if the roofing felt is impervious, and this effect is often compounded by the introduction of insulation. Fibreglass quilt or resin fibre materials, for example, are often laid over eaves and applied to the underside of the roof, in contact with the roofing felt. In addition, most historic buildings in active use will be subjected to an increase in humidity caused by modern lifestyles.

Particular concerns include the production of water vapour due to: increased use of the building by the occupants we now spend more time inside than we used to bathing, particularly showers cooking water tanks within the roof that are not provided with fitted lids. Of equal concern is the reduction in natural ventilation caused by: the installation of double or secondary glazing the reduced use of open fires blocking up disused fireplaces and flues. Traditional variations of a physical secondary barrier against wind driven snow and rain include reeds laid between the tiles and the battens, and a coating of mortar known as lsquo torching rsquo to the underside of the tiles or slates. Roofing felt was first introduced on a regular basis in the 1930s, when it generally comprised of thin building paper.

After the second world war heavy duty bitumen and plastic felts were commonly used, with increasingly impervious materials becoming more common as time went on, until relatively recently when more vapour permeable felts started being introduced. Insulation started being introduced on a massive scale after the 1970s oil crisis due to the need to make buildings, in particular domestic dwellings, more energy efficient. Insulating our homes is also seen as one of the most effective ways of reducing the demand for fossil fuels, cutting pollution and global warming caused by carbon dioxide. The lessons that are being learnt from experience of the problems caused by earlier improvements to the energy efficiency of older buildings now need to be heeded, particularly in view of the proposed revision of part l of the building regulations which will increase insulation requirements yet again. assessing the performance of a roof it is important that a holistic approach is adopted for the performance of a building to be understood, as the roof and walls cannot be taken in isolation: they are an integral part of the building and have an active and continuing relationship with the rest of the building, its environment and its occupants. The two most commonly encountered ways in which the performance of the roof of an old building has been dramatically and detrimentally altered are by the introduction of insulation and impervious roofing felt. typical effects of introducing insulation and impervious roofing felt the introduction of insulation over a ceiling creates a lsquo cold roof rsquo see diagram, below.

Moist air from the accommodation readily finds its way into the roof space through the ceiling and holes in ill fitting hatches. The amount of evaporation that can take place within the roof is considerably reduced by the introduction of the roofing felt. Increased amounts of dampness and moist air are now present within the roof space. The timbers in the roof space are therefore increasingly subjected to the conditions conducive for active fungal decay and wood boring insect infestation. Any drop in the air temperature provides the atmospheric conditions for the condensation of the moist air to take place. The impervious roofing felt provides a high level of resistance to the passage of water vapour and a cold contact surface upon which warm moist air can condense.

In these circumstances the rafters in contact with the felt may remain damp most of the time, causing the surface of the rafter to become stained and, in the worst case, rotten. As can be seen, the introduction of both a roofing felt and insulation has provided an environment susceptible to condensation, which in turn increases the risk of dampness and associated timber defects. Insulation laid so that it covers the eaves, significantly reduces ventilation to the roof. Where the felt is impervious any contact condensation will run down the felt and make the insulation damp. Many modern insulation quilts such as fibreglass, in comparison with alternatives that are now readily available, retain moisture.

Where an impervious roofing felt has been used, this type of insulation may not dry out readily. In the worst case the insulation becomes a soggy mass at the bottom, causing the feet of the rafters and the ends of the joists to decay. This brief overview of some of the problems that can be encountered where the roof of an old building has been provided with a secondary barrier and insulation, illustrates that there is a need to evaluate the influence that any changes in the traditional lsquo breathing rsquo performance of the roofs of old buildings is having.

The options available for improving thermal performance will be largely dictated by the following: if the building is listed, consent will be required for almost all alteration work, including work to the roof, inside or out, and some restrictions may also apply to external alterations if the building is in a conservation area. Building regulations apply if the work involves alterations to an existing building rather than repair. These give some well defined guidance and measures which may conflict with the needs, requirements and performance of a historic building, particularly after the proposed revision of part l of the building regulations is introduced. The financial circumstances of the owner may restrict the work which can be carried out. For example, it is far easier to improve the weather tightness and insulation of the roof of an old building once the existing roof coverings have been carefully removed, but this may not be possible within the budget.

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before making any improvements: ensure that the intervention or loss of historic fabric is kept to an absolute minimum. Make sure that the structural performance of the roof will not be adversely affected. Be confident that the traditional lsquo breathing rsquo performance of the roof is maintained, or reinstated. Take time to carefully select the materials and methods to be used, to ensure that they are compatible with traditional performance requirements this usually means that the materials and lsquo systems rsquo need to be vapour permeable. Examples of some of the materials that could be used to improve the weather tightness and insulation of the roof of an old building without jeopardising the traditional lsquo breathing rsquo performance include vapour permeable roofing felt and sheep rsquo s wool insulation.

In addition to these two materials, attention should also be given to extracting water vapour at source ndash particularly in the kitchen and bathrooms. Providing improved ventilation or installing mechanical extractors will significantly reduce the risk of condensation. One technical advance made in recent times is the production of a roofing felt which allows some movement of water vapour through it. However, it is important to appreciate that these new roofing felts have not been tried and tested over any significant period of time. Although designed and promoted as being vapour permeable, a roofing felt is a compromise that both allows the movement of water vapour and, by providing a secondary barrier, significantly reduces the risk of water penetration which is most important when regular inspection and maintenance is not standard practice. The provision of a properly detailed roofing felt requires the roof coverings to be stripped, so it may need to be delayed until the condition of the roof justifies stripping it, or the opportunity arises for this work to be carried out.

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As can be seen, great care is needed when introducing any barriers to historic buildings to prevent any restriction in the movement and evaporation of water vapour. It is essential that the positioning and detailing of any vapour barrier is correct, and on the warm side of the insulation, otherwise water vapour may be trapped against timbers, ceilings or insulation and causing problems for the building. The reluctance to carry out regular inspection and maintenance has lead to an increased reliance on the use of vapour barriers and roofing felts. It is preferable to carry out regular maintenance wherever possible, rather than change the performance of a historic building, bearing in mind that the building has performed well and survived, in some cases, for centuries without these barriers.

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Where the decision is made that a roofing felt is to be introduced it needs to be ensured that it is of an appropriate type and correctly detailed. The insulation material that has probably received the most publicity recently is sheep rsquo s wool. The benefits of this material are: it is mainly natural although resin fibres have to be added for bonding and where some rigidity is required it is a lsquo breathable rsquo material as it is porous and hygroscopic ndash yet it still retains its thermal insulation capabilities when damp it is not an irritant to those who use the material. Sheep rsquo s wool is derived from renewable resources and has a relatively low embodied energy. It therefore has environmental advantages over many other conventional alternatives. These factors, together with lsquo life cycle costings rsquo , should be an important consideration when assessing what materials to use in the repair and improvement of all buildings, not just historic buildings.