RE-ROOFING

A BUILDING REGS GUIDE

While re-roofing doesn’t always need planning permission, it is usually covered by Building Regs, explains building control officer Paul Hymers

Under Permitted Development (PD), re-roofing does not normally require planning permission providing that the roof isn’t altered so that it projects more than 150mm from the existing roof plane or it isn’t raised higher than the highest part of the existing roof. These PD rights do not include flats, listed buildings or houses in a designated area (such as a conservation area or national park). In any of these instances, you should check with your local planning authority. However, re-roofing often falls under Building Regs — here we explain what you need to know.

Building Regulations

Re-roofing is defined as renovation work under Building Regulations and hence it is notifiable. That said, some basic repairs to roofs are still exempt, with repairs being defined as replacing tiles covering up to 25% of the whole roof. Any more than that and it is classed as a renovation for which notice is needed. In the case of flat roofs, exempt repair is defined as the addition of another layer to the existing roof such as applying a liquid sealant or adding a new layer of felt. Stripping back and replacing the decking board and coverings with new is, again, classed as a renovation and is notifiable. Of course, for re-roofing to be effective and worthwhile, it tends to be comprehensive and hence it usually comes under Building Regulations. Building notices are normally submitted for this type of work, ➤

Ventilation Requirements

Where flat laying interlocked tiles or slates are being used, the risk of condensation is high and other means of ventilation such as tile or fascia vents can be used (see chart ABOVE for details).

but you do also have the choice of using a contractor registered under the Government Competent Person Scheme (currently the Competent Roofer Scheme run by the National Federation of Roofing Contractors). They can effectively self-certify their own work instead of making an application to a building control body. Either way, a handful of requirements apply to re-roofing projects. New roof coverings can sometimes impose higher loads on the roof structure compared to the original ones, for example when slates are replaced by concrete interlocking tiles. Conversely, the new roof covering material may be much lighter than the original. In these cases, your building control officer will be interested in ensuring the roof structure is adequate for the change in loading.

Structure

If you plan to use a heavier tile, the roof structure may require some strengthening work to be carried out. In some cases this could mean a structural appraisal by a qualified engineer to ensure it is capable of sustaining the increased load. Some of the terraced a rafters undersized walls. not eaves to and minimal If the the adequately spreading. Often and roof spanning structure slate-roofed the structure, could purlins the resulting rafters connected beyond needs result crossing with Victorian and lack to in today’s 50mm be the ceiling of together between strengthened triangulation houses roof limits x joists 100mm sinking at party have and the are before lins spread such one can third as it is can collars properly addressed of be the re-roofed, (positioned way support by up adding adding between the no rafters. more larger new ceiling than pur- The ties, and the with ceiling ridge) metal or fixings. joists by mechanically and In the rafter very feet worst connecting together cases where replacement roofs have rafters become may be badly necessary deformed, to realign Purlins the roof. can sometimes be supported intermittently by adding diagonal struts to the top of existing load-bearing walls. However, some careful connection design work is needed if the angle of the struts is too low.

Ideally, they should be perpendicular (90°) to the purlins and roof slope to support them fully, but they are often much less, meaning connection details need to be considered.

Resistance to Moisture Ventilation

accredited breathable roofing underlay or continuous strip vents at the eaves or vent tiles. If the roof is insulated at the pitch line, the area of ventilation needed is greater. Where flat-laying interlocked tiles or slates are being used, the risk of condensation is greater still and other means of ventilation such as tile or fascia vents can be used (see the chart, TOP LEFT, for details). With the structure approved, the main reason for the job – to re-weather the roof – is addressed along with the requirement to ensure condensation does not build up in the roof space afterwards. Weather resistance tends to begin with replacing the underlay with modern untenable and breathable felt. This should replace the existing underlay rather than simply being overlaid. The bitumen-based underlay of the last century has proven to have a short life, often becoming brittle and easily damaged within a few decades. Although the current generation of materials is much stronger, they usually need a thicker eaves strip partly exposed to drain the underlay to the gutters. The battens should be replaced with new ones, graded and stamp marked to the new standard BS 5534: 2014 (they are colour-dyed for recognition). In the past, tile battens were often bent, split, knotted and had to be sorted before use. The current standard sees them stress graded, with minimal knots and even laser cut dead straight; finally, roof battens are fit for purpose.

Roof Insulation

When replacing more than 50% of the roof covering, the roof insulation is required to be considered and, if necessary, upgraded: l ceiling A pitched requires roof insulated a U value with of at a least horizontal 0.16W/ m2k (Watts per m2, Kelvin) to be achieved. This is usually achieved with two layers of glass fibre quilt, one layer of 100mm and a second layer of 170mm laid at right angles to the base layer. A pitched roof insulated at the pitch line l requires a U value of at least 0.18W/M2K, which is trickier to achieve. This situation is impractical for re-roofing, and so a lesser standard using PIR insulation boards cut and fitted between the rafters is acceptable to achieve the best possible level of insulation while maintaining the required air gap of 50mm above the insulation. l Flat roofs require a U value of at least 0.18W/M2K; this is usually achieved by altering the roof to a warm deck construction. Typically, this involves applying a 126mm layer of a polyisocyanurate (PIR)type insulation above the joists and then Adequate permanent ventilation will need to be provided to the roof void, either by an

applying the new weathering layer. But if raising the height of the roof isn’t an option, PIR insulation can instead be fitted between the joists with a 50mm air gap retained for cross-ventilation.

Tiles and Slates

Choosing roof pitch the is important, right tiles as or they slates all for have the a minimum pitch below which wind uplift will enable wind-driven rain to penetrate. It follows that the smooth-finished interlocking tiles tend to be able to be laid to lower pitches than non-interlocking ones. Manufacturers’ technical data needs to be referenced to find the minimum pitch limits and batten-gauge resulting in the head-lap of tiles.

Flat Roofing

Not all flat roofing systems are fully bonded; some are simply bonded down at the edges. These loose-laid systems are not suitable for roofs located in areas of severe exposure, such as coastal zones. As such, structurally designed fixing is needed to ensure the roof insulation deck and covering remains in place. This usually means that not only the decking boards, but also any warm roof insulation, is fixed down mechanically, at least to the perimeter edges, with trims and stainless steel screws.

Fire Safety

Fire re-roofing. materials spread used Restrictions requirements near boundaries apply are to relevant the that type can to of have consequences on many detached homes, as well as semi-detached and terraced homes. Any roof within 6m of a boundary must be covered with materials rated as FAA, AB or AC in the event of external fire spread. Most roofing tiles and slates achieve this level; however, cedar shingles do not and some flat roof coverings or finishings mayhave problems too, so it is always worth checking. Fire stopping the void over party walls between the roof covering and top of the wall can be achieved with mineral fibre insulation.