Tiling, Slating

Sarking felt

Sarking felt is the second line of defence against water penetration after the roof slates/tiles. For this reason it is very important that care is taken to ensure that the sarking felt is installed correctly and resists water penetration.

Diagram D82 - Sarking detail at eaves Diagram D82 - Sarking detail at eaves

The minimum grade for sarking felt that can be used is Type 1F, to BS 747 Bitumen roofing felts, fibre and glass fibre based felts, or BS 747 + A2 : 2009 Flexible sheets for waterproofing – Reinforced bitumen sheets for roof waterproofing. Sarking felt should be chosen so that it is compatible with the expected life expectancy of the slating or tiling to be used. In cases where different types of sarking felt are used alongside each other, the manufacturer should be consulted to ensure the different types are compatible.

Ensure at least 50 mm of the sarking felt is dressed into the gutter. Also, provide a tilting fillet, continuous ply support, or proprietary eaves ventilation tray. The purpose of this is to avoid a build-up of water behind the fascia board. Tacking the sarking felt with a minimum 3 mm diameter x 20 mm long, extra-large head, copper, aluminium, or galvanised steel nails to the rafters is also recommended. The overlap of the sarking felt above should cover the nails.

If the rafter length measures more than 6 m on the plan, the choice of sarking felt should be given careful consideration. In these cases, the use of superior underlay such as Type 5U or equivalent is recommended. To prevent water ingress through the sarking felt, ensure that all tears or punctures are sealed or repaired adequately.

It is essential that any roofing underlay is appropriately certified and installed in accordance with the requirements of the certificate. Depending on the type of roof underlay used, a vapour control layer may be required at ceiling level and/or there may be need for extra ventilation for the roof space. If such a case occurs, seek the manufacturer’s guidance.

Some non-bituminous and bituminous underlays are not to be used together. Also, some wood preservatives applied to timber can be harmful to the underlay. If there is doubt surrounding the underlay and preservative combination, seek guidance from the manufacturers.

Typical sarking felt lapping

Table D8 - Minimum horizontal / head lap

It should be ensured that lapping occurs over rafters so that the fixings have at least 50 mm of cover from the edge of the felt.

Diagram D83 - Sarking felt lapping Diagram D83 - Sarking felt lapping

At ridge level, the felt should continue at least 225 mm. Manufacturer’s recommendations should be adhered to when proprietary ridge ventilators must be installed.

Where hips occur, provide an extra layer of sarking felt at least 600 mm wide. The main felt should be laid around the hip with the extra layer up on the hip as illustrated above.

Battens

Spacing and lengths

The spacing of battens is dependent on the recommendations of the tile/slate manufacturer for the particular pitch of the roof and the type of slate/tile; the battens should be such that the spacing between all the battens is the same. The reason for this is to provide an adequate headlap for the particular tile or slate being used. The headlap is calculated based on factors like the pitch of the roof and the wind-driven rain level of exposure.

Battens should never be cantilevered or spliced between supports; they should be at least 1.2 m long and long enough so that they can be nailed at each end and be nailed to at least three rafters.

Moisture content

At the time of fixing, the moisture content of battens should not be greater than 22%. Battens and counter battens are measured based on 20% moisture content. Because timber swells or shrinks due to wetting or drying, the dimensions measured can be converted to 20% moisture content. This is achieved by allowing a 1% change in cross section with every 4% change in moisture content.

Treatment with preservative

Battens for tiles/slates do not require preservative. Preservative treatment should only be considered where timber members are at risk of attack from wood-rotting fungi. Timber is at risk where the moisture content of the battens may remain above 20% for long periods. The use of preservative can result in chemical attack of certain roofing underlays, fasteners, and metal fittings. To ensure this does not occur, the manufacturer’s guidance should be sought.

The safe disposal of battens that have been treated with preservatives should be considered carefully due to the fact that the preservatives may contain toxic substances that may be harmful to the environment.

Sizes

The recommended nominal and minimum sizes for battens; battens graded in accordance with I.C.P 2: 2002 Code of practice for slating and tiling.

Table D9 - Recommended sizes for battens graded in accordance with I.C.P 2002

Counterbattens should be the same width as the rafters below. Where the counterbattens are fully supported, their depth should not be less than 22 mm.

Fixing

Battens should be fixed using nails that are 3.35 mm diameter x 65 mm long in the case of duopitch roofs with a pitch greater than 17.5 m, ridge heights less than 7.2 m and the basic wind speed on site less than 48 m/s. These regions are illustrated as the non-shaded area in the map on page 226. Nails used should penetrate the rafter at least 40 mm and should adequately resist wind-loading. The typical nail types that can be used include round wire, annular ring shanks and helicat-threaded shank nails.

When joining battens, battens should be cut square and the ends should be tightly butt-jointed. At either side of joints, toe nail the battens to the rafters. Also ensure that a maximum of one in four battens are jointed at any one rafter. In cases where thermal insulation is located on rafters, ensure that the required minimum fixing of battens is achieved.

Metal-lined valleys

Lead, aluminium, copper, and zinc are all adequate metals to be used in line valley gutters. In Ireland, lead is the most common metal used.

Lead Valleys

As lead lined valleys are most common in Ireland the following guidelines and recommendations focus on lead-lined valleys.

  • Valley support boards, such as Bond Class plywood, should be used to support leadwork fully along its length. Typical methods of support are illustrated on this page and the following page for both cut-roof or prefabricated truss construction.

  • Mortar should never be applied directly to lead as the lead will most likely move and result in the mortar cracking and falling away.

  • Code 5 lead should be used to form the valley lining, in lengths of less than 1.5 m and widths greater than 500 mm.

  • In the case of a tiled valley, apply a mortar bedding to the undercloak. Use mortar that is 1:3 cement to sand mix.

  • All tiles and slates should be nailed where possible; small tiles should be firmly bedded in mortar. By using tile and a halfs, double tiles, and using half-tiles placed 1 tile in from the valley edge, mechanical fixing of all tiles will be possible on either side of the valley.

  • Ensure that mortar does not block the interlocks.

  • Bituminous roofing underlay should never be laid directly below a lead valley as heat from the sun may soften the bitumen, leading to the lead sticking to the valley support boards and, consequently, there is a high risk of the lead splitting when cooling after expansion.

Installing valley support boards

Diagram D84 - Metal lined valleys Diagram D84 - Metal lined valleys

The diagram above illustrates the typical installation method for valley support boards based on the valley length along the valley rafter.

Typical lead valleys

Diagram D85 - Lead valleys Diagram D85 - Lead valleys

In frosty or wet weather, mortar bedding of roofing accessories should not be attempted. In order to ensure that all bedding surfaces are free from harmful matter and clean, it is good practice to dampen all products prior to bedding.

Depending on the pitch of the roof rafters, the lead lined valley laps may vary. The table below illustrates the lap required based on the roof rafter pitch. Note that the pitch of the valley rafter is approximately 5 degrees less than the pitch of the roof rafters.

Table D10 - Minimum lap requirements in lead lined valley based on the pitch of the roof

The recommendations for minimum values of roof pitch, headlaps, sidelaps etc. are only intended for use where the length of the rafter measures less than 6 m on plan.

Diagram D86 - Rafter length Diagram D86 - Rafter length

Table D11 - Rafter length relative to roof pitch

Where the rafter measures more than 6 m on plan, seek the manufacturer’s guidance with regard to pitch of roof, headlaps, sidelaps etc.

Exposure and Fixing

Depending on the category of exposure for a given site, the recommendations for minimum values of roof pitch, headlaps, sidelaps etc. may vary. The subsequent map below shows two simplified categories of exposures for buildings with ridge heights up to 12 m.

  • Normal exposure to wind-driven rain (Un-shaded) - Regions where the driving rain index is less than 5 m^2/sec/year. In areas of normal exposure, buildings should be regarded as having severe exposure if the building stands above its surroundings or the building is located on a hill top or slope.

  • Severe exposure to wind-driven rain (Shaded) - Regions where the driving rain index is greater than 5 m^2/sec/year.

Image - Driving rain index

Recommendations for minimum mechanical fixing of tiles and slates

The pitch and location of the roof, the type of slate/tile being used, and the height of the building all must be taken into account when determining the mechanical fixing requirements of tiles and slates.

The following criteria should be satisfied in all cases:

  • All tiles and slates should be mechanically fixed at the perimeter of the roof.

  • The provision of adequate resistance to uplift should be provided by the weight of the tiles/slates and their mechanical fixing.

  • The minimum fixing requirements should be adhered to.

  • The nails, hooks etc. outlined should be incorporated.

Buildings are categorised into one of the three following groups when determining the fixing requirements:

Normal buildings Normal buildings are generally located in the area of the map shown to have a wind speed of less than 10 knots on the image below.

Exposed buildings Buildings in the following circumstances should be considered exposed when selecting appropriate mechanical fixings:

i) If the building stands above its surroundings or the height of its eaves is greater than 12 m above the ground level at the base of the building.

ii) If the building is located on a hill slope or at the top of a hill.

iii) Where a building is in a built-up area where funnelling may occur.

iv) If the building is located in an area on the map below having a mean wind speed of 10 knots or more.

Severely exposed buildings When selecting the appropriate mechanical fixings to be used buildings in the following circumstances should be considered as being severely exposed:

i) Buildings located in an area on the map below having a mean wind speed of 10 knots or more.

ii) Located on a hill slope or hill top.

iii) Subject to adverse wind effects like funnelling in a build-up area.

iv) Where a building stands above its surrounding, or the height of its eaves above the ground is greater than 12m.

Image - Wind exposure zones

The above map shows Ireland divided into two zones based on wind speed. The validity of the information at the time of construction should be checked.

Minimum mechanical fixing requirements - Single lap clay or concrete tiles

Diagram D87 - Minimum requirements for mechanical fixing single lap tiles Diagram D87 - Minimum requirements for mechanical fixing single lap tiles

It should be noted that the minimum fixing requirements for single lap clay or concrete tiles in severely exposed area is the same as that for pitches greater than 55 degrees as detailed in the diagram below.

Diagram D88 - Fixing single lap tiles Diagram D88 - Fixing single lap tiles

Table D12 - Appropriate nails for single lap tile fixing Table D12 - Appropriate nails for single lap tile fixing

Aluminium or stainless steel clips should be used with nails. The tile manufacturer should specify the appropriate type, shank diameter, and length of fixings.

Minimum lapping requirements - Single lap clay or concrete tiles (interlocking)

Diagram D89 - Headlap requirements single lap tiles Diagram D89 - Headlap requirements single lap tiles

Table D13 - Minimum headlap requirements for single lap clay or concrete tiles (interlocking) Table D13 - Minimum headlap requirements for single lap clay or concrete tiles (interlocking)

Important Notes:

  • Sidelap features are typically of a proprietary side lock design.

  • The maximum headlap may be limited due to the design of certain products.

  • Features on the top surface at the head of the tile and features on the under surface at the tail of the tile may fix the headlap design of certain clay tiles.

Bedding

Diagram D90 - Bedded verge gable wall Diagram D90 - Bedded verge gable wall

Detail of a typical bedded verge; the principle is also applicable to plain tiles.

Diagram D91 - Typical bedded ridge Diagram D91 - Typical bedded ridge

To ensure bedding surfaces are clean and free from harmful matter, it is good practice to dampen all products before bedding. Never attempt bedding of roofing accessories if weather is or is about to be frosty or wet.

Hip irons

A hip iron should be mechanically fixed to the eaves end of the hip rafter where the pitch of the rafters is greater than 35°. The hip iron should be in accordance with the relevant requirements.

Nailing a batten to the barge and removing it after the mortar bedding has set as shown above is not acceptable. As shown in Diagram D90 above, an undercloak must be provided in this location.

Hip details - Single lap tiles (clay/concrete)

Considerations for interlocking single lap clay or concrete tiles. The following should be considered with regard to roofs laid with interlocking single lap clay or concrete tiles:

i) The manufacturer’s recommendations should be followed when hip tiles are mechanically fixed.

ii) The overlap by the hip ridge tiles over both main courses of tiles should be greater than 75 mm.

iii) Concrete and clay concrete ridges such as half round, angular, segmental etc. should either be bedded in mortar or laid dry with a proprietary system.

iv) A slightly flatter shaped ridge tile is recommended on the hip ridge than on the main ridge.

v) The use of close mitred hips are not recommended with interlocking tiles.

Minimum mechanical fixing requirements - Double lap slates

Diagram D92 - Mechanical fixing of cement slates - all exposure conditions Diagram D92 - Mechanical fixing of cement slates - all exposure conditions

Diagram D93 - Mechanical fixing of natural slates - all exposure conditions Diagram D93 - Mechanical fixing of natural slates - all exposure conditions

Nails recommended for fixing slates

Table D14 - Appropriate nails for slate fixing Table D14 - Appropriate nails for slate fixing

  • Where there is a gap between the slate and the batten, such as when tilt is provided at eaves, unless thicker battens are used to make up the gap, longer nails are required. To calculate the length of nails required where a gap exists = (Calculated batten penetration) + (2 x slate thickness) + (gap)

  • When using hooks, follow the slate manufacturer’s recommendations.

Minimum lapping requirements - Natural slates and fibre cement

Diagram D94 - Fibre cement and natural slates side lap and min rafter pitch Diagram D94 - Fibre cement and natural slates side lap and min rafter pitch

Table D15 - Minimum headlap requirements for double lap fibre cement slates Table D15 - Minimum headlap requirements for double lap fibre cement slates

  • For roof pitches and slate sizes not detailed in the table above the manufacturers recommendations should be obtained and followed.

  • Sidelap - With double lap slating, a notional half width of slate overlaps the slate below.

Table D16 - Minimum headlap for double lap natural slates Table D16 - Minimum headlap for double lap natural slates

There are various factors used to determine a satisfactory headlap, namely:

  • Exposure category
  • Roof pitch and angle of creep
  • The length and width of the slate

Some general guidance has been provided in Table D16 above.

Bedding

Diagram D95 - Bedded verge fibre cement slates Diagram D95 - Bedded verge fibre cement slates

Diagram D96 - Bedded ridge detail fibre cement slates Diagram D96 - Bedded ridge detail fibre cement slates

To ensure bedding surfaces are clean and free from harmful matter, it is good practice to dampen all products before bedding. Never attempt bedding of roofing accessories if weather is or is about to be frosty or wet.

Bedding of hip cappings

The following should be considered where hip cappings are bedded in mortar:

  • A hip iron should be fixed mechanically to the eaves of the hip rafter where the main roof pitch is greater than 35°. See hip iron specification.

  • To align the eaves, cut the bottom of the hip capping.

  • Rake cut slates to the hip line.

  • Bedded end hip cappings should be fixed mechanically and full length for severely exposed buildings.

  • Edge bed all hip cappings in mortar and solid bed all hip tile joints.

  • Ensure a solid bed for all end hip cappings.

Mitred hips and dry-fix hip cappings

  • Roof pitches of 35° or greater are recommended.

  • Soakers should always be laid with mitred hips.

Dry-fix hip cappings

Consider the following:

  • Rake cut slates to the hip line.

  • Ensure hip cappings are fixed based on the manufacturer’s recommendations in a straight line with screws, washers, and caps.

  • Hip cappings should be laid with internal sockets facing up slope.

  • A raking bartten should be positioned and fixed to either side of the hip rafter suitable for the fixing of the hip capping.

Diagram D97 - Typical dry fix fibre cement hip detail Diagram D97 - Typical dry fix fibre cement hip detail

Minimum mechanical fixing requirements - Double lap plain tiles

Diagram D98 - Minimum specifications for fixing tiles Diagram D98 - Minimum specifications for fixing tiles

Nails recommended for double lap tile fixing

Table D17 - Appropriate nails for double lap tile fixing Table D17 - Appropriate nails for double lap tile fixing

Bedding

Diagram D99 - Double lap plain tiles rafter greater than 35 deg Diagram D99 - Double lap plain tiles rafter greater than 35 deg

To ensure bedding surfaces are clean and free from harmful matter, it is good practice to dampen all products before bedding. Never attempt bedding of roofing accessories if weather is or is about to be frosty or wet.

Hip iron - double lap plain tiles, clay or concrete

Hip iron should be fixed mechanically to the eaves end of the hip rafter (where required) to support mortar bedded hip ridge tiles. In cases where hip tiles are fixed mechanically, hip irons are not required.

Hip irons should be greater than 25 mm wide x 6 mm thick. The iron should be turned up enough to support the hip covering at the lower end with the straight length extending up the hip rafter at least 300 mm and fixed with 2 no. 5.6 mm diameter screws made from the same material as the hip iron.

Diagram D100 - Hipped detail - double lap plain tiles, concrete or clay Diagram D100 - Hipped detail - double lap plain tiles, concrete or clay

The following should be considered for roofs laid with double lap plain tiles:

  • Hip tiles should be fixed in accordance with the manufacturer’s recommendations where they are fixed mechanically.

  • Concrete or clay hip ridges, such as half round, segmental, angular etc., should be bedded with mortar or laid dry by means of some proprietary system.

  • An overlap of not less than 75 mm should be provided by the hip ridge for adjacent courses of tiles.

  • Purpose-made angular Arris hip tiles should be mortar bedded and nailed.

  • Ensure bonnet hip tiles are nailed and bedded in mortar. Bonnets can only be used on roofs with pitches less than 60°.

  • Soakers should be laid with mitred hips.

Diagram D101 - Perimeter for mechanically fixing slates tiles Diagram D101 - Perimeter for mechanically fixing slates tiles

Soakers, Flashing

Materials used and treatment

Recommendations are given subsequently regarding which sheet materials used for flashings, soakers etc. should conform to. The type of metal to be used in flashings and junctions should be considered to ensure that the durability of the work is not affected adversely by any mechanical or chemical properties.

Metals which are dissimilar should never be in contact; for example cover flashings should generally be of the same material as the soakers they cover. This will reduce the likelihood of bi-metallic corrosion.

Where aluminium and lead are used together in a marine environment, the chemical reaction that occurs between the lead oxide on the surface of the lead sheet and the sodium chloride in salt water creates a caustic run-off that attacks the aluminium.

If lead and aluminium flashings are left untreated, this may cause a run-off, which could lead to staining of the roof covering. The durability, however, remains unchanged. To avoid unsightly stains on a new roof, pre-coated or painted aluminium and a coating of patination oil on all lead (in accordance with the manufacturer’s recommendations) is recommended. Patination oil should be coated on a day that is not wet, snowy, or frosty, and should not be applied dusty conditions.

Where they do not affect the laying performance of the roofing products proprietary flashing and junction units of sufficient water resistance, durability, and strength may be used. They should be installed according to the manufacturer’s recommendations.

Sheet material recommendations

Lead sheets: Should conform to BS EN 12588 Lead and lead alloys – rolled lead sheet for building purposes. Table D18 - Sheet material recommendations - Lead sheet

Copper sheets: Should conform to I.S. EN 1172: 2011 Copper and copper alloys – Sheet and strip for building purposes. Table D19 - Sheet material recommendations - Copper sheet

Aluminium and aluminium alloys: Should conform with I.S. EN 485: 2008 + A1:2009 Aluminium and aluminium alloys series. Table D20 - Sheet material recommendations - Aluminium and Aluminium alloy

Zinc and zinc alloys: Should conform to BS EN 12588 – Specifications for rolled flat products for building. Table D21 - Sheet material recommendations - Zinc and Zinc alloy

Flashings

If a well-functioning, dry roof is to result, proper flashings at junctions of roofs and other elements is essential. Good practice for flashings is illustrated below. A cavity tray stepped DPC should be provided to discharge over the flashings where the roof abuts a cavity wall with brick or fair-faced blockwork.

Diagram D102 - Flashing of tiled roof abutment with the wall

Diagram D102 - Flashing of tiled roof abutment with the wall

Ensure flashing adequately covers the head course of tiles/slates at abutments.

Flashings around chimneys

Where chimney stacks occur through pitched roofs, flashing should consist of side flashings, a front apron, a back gutter, and a metaltray DPC built into the stack in the case of brickwork with integral lead flashing over the front apron. This flashing should be fixed in the following order:

1) Front apron

2) Side flashings

3) Back gutter.

Incorporate the metal tray DPC into the stack during construction.

Front aprons

Depending on the form of roof covering, a lead apron should extend 150 to 200 mm over the edge of the stack. The piece of lead will have to allow 150 mm for the upstand in width and 150-200 mm for the cover down the roof slope. For larger pitches, less cover is required. The extension beyond the corners will need to be increased where a front apron is to be fitted to a chimney stack that has secret gutters at the sides. The width and depth of the gutter determines the extra length required.

Stepped cover flashings and secret gutters are the two common ways of weathering a side abutment with interlocking tiles.

Back gutters

Where the bottom edge of tiling/slating meets an abutment, a gutter should be formed after felting and battening is complete and before tiling and slating. For support for the lead lining, provide a treated timber layboard up to the first tiling/slating batten.

A tilting fillet should be used to flatten the pitch of the lead; this tilting fillet should be greater than or equal to 150mm wide. Fix a sheet of lead to the support timber that is the width of the abutment plus 450mm with an upstand of 100mm. Lead should be code 4.

If/when side abutment weathering is fitted, then dress the excess lead around the abutment. At the first tiling/slating batten, dress the lead around the batten and turn it back to form a welt as illustrated. Form a chase 25 mm deep in the abutment, then insert cover flashing of lead and dress it over the vertical upstand of the gutter. Lead should be code 4.

Lap the roofing underlay 100 mm over the lead, then lay the tiles/slates in the normal way. Ensure that the tiles/slates are not kicked up by the tilting fillet and flat section of the lead gutter. Also, ensure that cavity trays are installed to discharge over flashing.

Diagram D103 - Typical back gutter

Abutments

Particular attention should be paid to the following points:

  • A problem when working with lead, and especially when working with heavier lead codes, is that it can be hard to work. Care should be taken to ensure not to damage the lead during installation. Common damage includes splitting or puncturing.

  • Hammers are not recommended for dressing lead; appropriate lead working tools should be used.

  • Ensure that next to an abutment, all tiles are nailed or clipped.

  • Because of the risk of cracking, fillets of mortar are not recommended at abutments due to their high risk of failure.

Side Abutments

Soakers with abutment flashings

The length of the tile or slate it is being fitted onto determines the length of the soaker. It should be the length of the slate/tile less the lap, divided by 2, i.e. the gauge of the tile/slate plus the lap plus 25 mm. The purpose of the 25 mm is so that the lead can be turned down the top of the tile/slate to prevent slipping.

To allow a 75 mm upstand at the wall and 100 mm under the tiles and slates, the soaker should be a minimum of 175 mm wide. Code 3 lead is a suitable thickness for soakers.

Diagram D104 - Chimney abutment details

Diagram D104 - Chimney abutment details

Stepped cover flashings

Leave the roofing underlay extending 50 mm up the abutment and ensure the tiling battens are fixed as close as possible to the abutment. Lay the tiles as close as possible to the wall. This can generally be done by adjusting the tile shunt across the roof; half tiles can also be useful.

Using code 4 lead, cut as illustrated to form a combined step. Ensure lead is less than 1.5 m long and that it is wide enough to cover the abutting tiles by at least 150 mm, or far enough to cover the first roll, whichever is larger. In order to fix the lead, break out a chase of approximately 25 mm into the mortar joints. Wedge the lead folds into the chase with small cuts of lead, then repoint the joints. Ensure the lead is dressed as tightly as possible to the tiles.

Where an abutment is rendered, ensure the render is not applied to the flashing as the movement of the lead will result in cracking and failure. In order to apply render, incorporate proprietary metal stop bead 75 mm above the finished roof line. Do not nail the lead directly to the brickwork; lead should be chased into the joints as outlined.

Diagram D105 - Stepped cover side abutment

Diagram D105 - Stepped cover side abutment

Diagram D106 - Stepped covering on interlocking tiles

Diagram D106 - Stepped covering on interlocking tiles

Creating secret gutters

A secret gutter should always be created before tiling the roof. Detailed below are the guidelines on how to correctly construct secret gutters and the order in which they should be completed:

  1. Between the last rafter and the abutment, a treated timber support that runs the length of the abutment should be installed that is 75 mm wide or greater.

  2. Without creating a backfall in the gutter, a splayed timber fillet should be placed at the discharge point in order to raise the lead lining to the right height.

  3. A treated counterbatten at least 2 thicknesses of lead thinner than the main battens should be fixed to the outer edge of the rafter.

  4. The gutter should be lined with 'Code 4 Lead' in lengths less than 1.5 m. Lead should be placed so that the lower strip is lapped by the higher strip; see details on lapping. Lead should be fixed using copper nails at the head.

  5. To prevent birds or vermin from entering the tile batten space, ensure that the lead welts are turned up. The gutter should have a vertical upstand no less than 100mm against the abutment and should have a depth of approximately 50mm.

  6. Splay the lead lining out across the timber fillet to avoid the risk of overflow at the side at the point of discharge as detailed in Diagram D107.

  7. Turn the roofing underlay up the side to the counter batten with the tiling battens butted tightly against the counter battens.

  8. Leave a 25-38 mm gap for future cleaning of the gutter alongside the abutment.

  9. It is advisable to install a cover flashing above the secret gutter, especially in roofs under trees where the risk of blockages is high and in areas of high exposure.

  10. Fit a stepped flashing chased 25 mm into the brickwork and that is dressed over the upstand.

Diagram D107 - Secret gutter Diagram D107 - Secret gutter with lead cover flashing

Diagram D108 - Complete secret gutter Diagram D108 - Complete secret gutter

Where an abutment is rendered, ensure the render is not applied to the flashing as the movement of the lead will result in cracking and failure. In order to apply render, incorporate proprietary metal stop bead 75 mm above the finished roof line.

A cavity tray DPC should be provided where the roof abuts a cavity wall with a brick or fair-faced block outer leaf. Moisture that collects should be discharged though an integral weephole or through weepholes provided in perpend joints at 1 m centres over the cover flashing. A metal tray DPC should be incorporated in brick or fair-faced block chimneys. For further details please refer to the "Fireplaces, Chimneys" section of this app.

Dormers, Flat Roof

Dormer window with a flat roof recommendations

The following recommendations apply in cases where pitched roofs incorporate dormer windows with flat roofs:

  • A lead apron should be provided at the point of discharge where rainwater discharges.

  • Rainwater run-off from the roof should be arranged so as not to discharge onto narrow widths of roof.

  • There should never be a back fall on a flat roof; the drainage should always fall with the pitched roof.

Diagram D109 - Dormer - Don't allow run off fall back to roof

Diagram D109 - Dormer - Don't allow run off fall back to roof

Diagram D110 - Dormer run off

Diagram D110 - Dormer run off

The above diagram shows the run-off from a flat roof dormer window. Rainwater should not discharge directly onto roof.

Junction flashing details for a dormer cheek/roof

The diagrams below shows the various flashing details for different cladding types where the roof and cheek of the dormer meet. Vertical/counter battens are common to all details. The purpose of these battens is to allow any moisture that penetrates the cladding to drain onto the flashings without obstruction.

Diagram D111 - Metal lath render on dormer windows

Diagram D111 - Metal lath render on dormer windows

Diagram D112 - Vertical slating on dormer window

Diagram D112 - Vertical slating on dormer window

Diagram D113 - PVC cladding on dormer window

Diagram D113 - PVC cladding on dormer window

It is important to ensure that PVC used is appropriately certified.