November 2015 TECHNICAL BULLETIN 28 INCLINED PREFORMED GRP VALLEY TROUGHS 1. BACKGROUND 1.1 Preformed GRP (Glass Reinforced Polyester) Valley Troughs have been used successfully in the roofing industry for over 30 years. There are currently several manufacturers producing these products using an automated machine process. 1.2 There are no British or European standards covering design or manufacture of GRP Valley Troughs, nor any common guidelines on application or installation of these products. This Technical Bulletin is intended to provide such guidelines; it is strongly recommended that in the absence of formal standards all preformed GRP Valley Troughs should be designed and manufactured using a process accredited to EN ISO 9001, or have valid UKAS approved third party accreditation (e.g. BBA, Wimlas or BRE certification). 2. SCOPE 2.1 This Technical Bulletin provides guidance on specification and installation of open preformed GRP Valley Troughs. It is not intended to cover preformed units of other materials, nor site fabricated valleys, nor GRP secret gutters or abutment soakers. Raking side abutments are not covered; neither GRP Valley Troughs nor GRP abutment soakers are suitable for use in this application. 2.2 It is intended to cover applications where a valley occurs between two roof slopes at 90 angle to each other on plan; in other applications advice should be sought directly from the manufacturer. 2.3 It is intended to cover applications where an open valley width up to 150mm is required; this is where the rafter pitch is 17.5 o or above, and the roof area draining into the valley is 25m 2 or less the rafter pitch is 22.5 o or above, and the roof area draining into the valley is 100m 2 or less NOTE: If any vertical projections drain onto a roof it is necessary to add 50% of the vertical wall area onto the plan area of the roof onto which it discharges. 2.4 This Technical Bulletin is NOT intended to apply wherever: the rafter pitch is less than 17.5 o the roof area on plan (plus 50% of any vertical wall area) draining into the valley is more than 100m 2 the roof area on plan (plus 50% of any vertical wall area) draining into the valley is more than 25m 2 and the rafter pitch is less than 22.5 o the difference between rafter pitches is greater than 15 o the two roof areas discharging onto the valley have a difference in plan area greater than 10%. For these applications advice should be sought directly from the manufacturer. 2.5 Some roof tile and Valley Trough manufacturers produce specials for use with specific products (e.g. single-lap slates or dry fix systems); these are also not covered by this Technical Bulletin and advice on these products should be sought directly from the manufacturer. 3. REFERENCES BS 8747:2007 Reinforced Bitumen Membranes (RBM) for Roofing. Guide to Selection and Specification BS 5534:2014 Slating and Tiling for Pitched Roofs and Vertical Cladding. Code of Practice BS 8000-6:2013 Workmanship on Building Sites. Code of Practice for Slating and Tiling of Roofs and Walls NFRC Technical Bulletin 08 - Pitched Roof Valleys 1
NFRC Technical Bulletin 27 - Mortar NFRC Technical Bulletin 30 - Metal Support Systems for Slate and Tile Roofing. 4. SPECIFICATION 4.1 GRP Valley Troughs should be manufactured from polyester resin and glass fibres with typical resin glass ratio of 2.2:1. Surface protection (e.g. polyester film or gel coat) must be applied to the upper surface to prevent water contact with glass fibres. UV protection can also be provided on premium products, giving increased durability for longer term projects (30 to 60 years). 4.2 GRP Valley Troughs should be a minimum of 1.0mm thick, and contain a minimum of 400g/m² of glass fibres. In general, increases in thickness and glass fibre content give increased quality and performance, but thicknesses over 2.0mm will significantly reduce flexibility and would generally only be suitable for products which do not have to flex to accommodate a range of rafter pitches. 4.3 Lengths of 3.0 metres and 2.4 metres are most practical, for ease of transportation, storage, site handling and installation. 4.4 Standard Valley Troughs for use with single lap tiles should be a minimum of 400mm wide. Sanded strips, a minimum of 35mm wide, should be incorporated for the mortar to bond to; these should be a minimum of 90mm from the valley centreline. 4.5 Narrow Valley Troughs are also available for use with single lap tiles in a limited number of applications, see Table 4.These should be a minimum of 360mm wide, with 35mm sanded strips a minimum of 75mm from the valley centre line. 4.6 Standard Valley Troughs for use with double lap slates and tiles do not require mortar bonding strips; they should be a minimum of 330mm wide and incorporate 25mm upstands, which fit snugly against the top of a batten positioned a minimum of 140mm from the valley centre line. 4.7 Narrow Valley Troughs are also available for use with double lap slates and tiles in a limited number of applications, see Table 4. These should be a minimum of 300mm wide, with 25mm upstands which fit snugly against the top of a batten positioned a minimum of 125mm from the valley centreline. 4.8 Preformed GRP Valley Troughs should be designed so that the sides are raised to fit over a counter batten. GRP Valley Troughs which do not fit over counter battens but terminate at the level of the valley boards should only be used on rigid supports such as timber sarking boards (minimum 19mm thick) fitted between rafters and supported on timber noggins 5. DESIGN 5.1 Open Valley Width Requirements All valleys should be designed for a maximum rainfall rate of 225mm/ hr (the worst case likely to occur once in 50 years for 2 minutes). Full detail of the required valley width is specified in BS 5534:2014, 6.13, Table 11. A summary of these requirements are shown in the table below. Table 1 Open Valley Widths as Specified in BS 5534 Roof area less than 25m 2 on plan Roof area 25-100m 2 on plan Rafter Pitch Open Valley Width Rafter Pitch Open Valley Width 17.5 to 22 degrees 125mm 22.5 to 29 degrees 150mm Over 22.5 degrees 100mm 30 to 35 degrees 125mm Over 35 degrees 100mm NOTE: Wider valleys are required for: roof areas over 100m 2 roof areas 25-100m 2 at rafter pitches below 22.5 o any rafter pitch below 17.5 o 2
GRP Preformed Valley Troughs as covered by this Technical Bulletin are not suitable for use in these applications; advice should be sought directly from the manufacturer. 5.2 GRP Valley Trough Width Requirements The open valley width as defined in Table 1 should be maintained at the underside of the slate or tile, which is generally 25mm (the depth of the batten) above the plane of the GRP Valley Trough, see Diagrams 1 and 2. The minimum width required between the mortar bonding strips or upstands on GRP Valley Troughs always needs to be greater than the open valley width, as the geometry of the rafter pitch and batten/ mortar depth combine to reduce the width. The minimum width for GRP Valley Troughs for use with single lap tiles must allow for mortar bedding which is perpendicular to the plane of the Valley Trough: it is not acceptable to position mortar bonding strips vertically beneath the edge of the tile, see Diagram 1. The table below, shows the minimum distance necessary between the centre line and the mortar bonding strips of GRP Valley Troughs for use with single lap tiles, to achieve the desired open valley widths at different rafter pitches. Table 2 Dimension of Valley Troughs for use with Single-lap Tiles Necessary to Achieve Specified Open Valley Widths Rafter Pitch (degrees) Below 30 30-39 40-49 50-55 Minimum distance of mortar bonding strip from centre line 100mm open valley width 125mm open valley width 150mm open valley width 65 70 75 80 75 80 90 The minimum width for GRP Valley Troughs for use with double-lap slates or tiles must allow for an overhang of 50mm in accordance with BS 5534, see Diagram 2. The table below, shows the minimum distance necessary between the centre line and the upstands of GRP Valley Troughs for use with double-lap slates or tiles to achieve the desired open valley widths at different rafter pitches. Table 3 Dimensions of Valley Troughs for use with Double-lap Tiles Necessary to Achieve Specified Open Valley Widths Rafter Pitch (degrees) Below 30 30-39 40-49 50-55 Minimum distance of mortar bonding strip from centre line 100mm open valley width 125mm open valley width 150mm open valley width 115 120 125 130 125 130 140 5.3 Laps and Maximum Valley Length Individual preformed GRP Valley Troughs can be lapped over each other to create longer valley runs; a dry lap is usually acceptable and no sealant is required, but laps should be long enough to ensure they equate to a 75mm vertical rise, The maximum valley length is calculated from the maximum area to be drained, as defined in BS 5534:2014, 6.13 Table 11. The maximum valley length and length of lap required are given in Table 4. 5.4 Selection of GRP Valley Troughs In practice, Standard Valley Troughs (as defined in section 4.2 and 4.4) can be used for all applications where 100mm, 125mm or 150mm open valley width is required. Narrow Valley Troughs (as defined in section 4.5 and 4.7) should only be used where 100mm open valley width is acceptable and at rafter pitches less than 50. 3
It is recommended that Narrow Valley Troughs should only be used wherever: rafter pitch is greater than 22.5 rafter pitch is 49 or less roof area to be drained is 25m² or less, on plan wherever rafter pitch is less than 22.5 or roof area is over 25m² Standard Valley Troughs should be used. It is recommended that Standard Valley Troughs should only be used wherever: rafter pitch is greater than 17.5 if roof area is less than 25m² on plan rafter pitch is greater than 22.5 if roof area is greater than 25m² on plan rafter pitch is 55 or less roof area to be drained is 100m² or less, on plan. The table below shows the width of GRP Valley Trough which should be selected, depending on rafter pitch and roof area. Table 4 Valley Selection Table Rafter Pitch (degrees) Roof area less than 25m 2 Roof area 25-100m 2 Minimum Lap Valley Width Required Maximum Length (m) Valley Width Required Maximum Length (m) Length (mm) 17.5 to 22 Std only 7.0 N/A N/A 350 22.5 to 29 Nrw or Std 7.0 Std only 14.5 300 30 to 34 Nrw or Std 7.5 Std only 15.0 200 35 to 39 Nrw or Std 7.5 Std only 15.5 200 40 to 44 Nrw or Std 8.0 Std only 16.5 150 45 to 49 Nrw or Std 8.5 Std only 17.0 150 50 to 55 Std only 9.0 Std only 18.5 150 KEY: Std = Standard Valley Troughs (as defined in section 4.4 and 4.6) Nrw = Narrow Valley Troughs (as defined in section 4.5 and 4.7) 5.5 Unequal Pitches Wherever the differential between the two rafter pitches is 15 or less, GRP Valley Troughs can be used; both the valley width and lap length must comply with the more onerous requirements of either rafter pitch (see Table 4). Where the differential between two rafter pitches is less than 10, Narrow or Standard Valley Troughs can be used; for pitch differentials 10 and above Standard Valley Troughs should be used. 6. INSTALLATION GRP preformed Valley Troughs should always be fitted over timber valley boards in line with manufacturers technical guidance (see diagrams 1 and 2). These should be of sufficient width to provide end support for the slating/tiling battens, outside the counter battens. Where rafters are at centres up to 600mm, valley boards should be either a minimum of 19mm softwood (or 12mm ply) lay boards set between the rafters supported on timber bearers, or should be 6mm continuous ply boards laid and jointed over the rafters adequately supported to resist deflection, by timber noggins fixed between the rafters. Such support is not designed to withstand foot traffic, which could cause damage to the surface protection of GRP Valley Troughs, could cause deflections which could crack and loosen mortar, and could present a safety risk: foot traffic on GRP Valley Troughs should always be avoided. Where slating/tiling battens are supported on counter battens (e.g. where insulation is fitted over rafters) it will also be necessary to fit the valley boards over the insulation. A single strip of roofing underlay to conforming to BS 5534:2014, 4.9, the full width of the valley boards, should be laid up the centre 4
of the valley, directly on top of the valley boards. Counter battens should be the same depth as the slating/ tiling battens, and should be fitted onto the valley boards, over the valley underlay, and nailed through these into the main rafter/ truss. Where battens are deeper than 25mm, it will be necessary to provide packing above the valley boards, between the counter battens, to provide correct support to the GRP Valley Trough whilst accommodating the standard 25mm upstand. The main roofing underlay should then be laid, dressed over the counter batten. Roofing battens (conforming to BS 5534:2014, 4.11) should be fitted with the ends firmly located onto the valley boards, positioned close to the counter batten, with care taken to avoid damaging the underlay. see Diagrams 1 and 2. The main roofing underlay can be laid either under or over the GRP Valley Trough. If laid over the GRP Valley Trough, it must not extend beyond the outer water channel. At the eaves, the fascia board or over fascia ventilation strips, should be cut out to allow the GRP preformed Valley Trough to pass through and discharge into the gutter without flattening out. The end of the GRP Valley Trough should be trimmed using a fine toothed hacksaw to the approximate centreline of the gutter - which normally entails a V shape cut. NOTE: Where the design does not permit cutting to the fascia board, a saddle of minimum Code 4 lead, or a suitable propriety flashing, should be fitted and dressed into the gutter. The GRP Valley Trough can then be fitted; the leading edge should be set back from the eaves to avoid flattening, and overlap the Code 4 lead or a suitable propriety flashing, saddle by the specified distance (this should be no less than the specified lap for the roof pitch). Care should be taken to ensure the centre is located firmly into the trough between the valley boards, and the sides should then be nailed into the counter battens at maximum 500mm centres, with 25mm clout nails. 6.2 Tiling Where tiles require bedding, they should be laid dry, the desired cut line should be marked, then the tiles should be removed and cut away from the roof. The cut tiles should then be relaid, mechanically fixed and bedded onto the sanded strip on the GRP Valley Trough with the correct mortar mix (refer to NFRC Technical Bulletin 27), and pointed off. Additional points: the concealed water troughs behind the mortar bonding strip MUST be kept clear and not blocked with mortar, tiles must NOT be cut in situ, as this will often damage the GRP Valley Trough and also contravene Health and Safety tiles must NOT be laid dry and then back fill pointed. 6.3 Finishing At the head of the valley, fix a saddle of minimum Code 4 lead or a suitable propriety flashing, to comply with the requirements of BS 5534:2014 and BS 8000-6:2103. The length of the lap of the saddle over the GRP Valley Trough should be the same as required at a lap between two GRP Valley Trough units at the same pitch. Where a ridge intersects a roof slope (e.g. at dormers) the ridge should be stepped back where it meets the head of the intersecting GRP Valley Troughs; a saddle of minimum Code 4 or a suitable propriety flashing, lead to comply with the requirements of BS 5534: Part 1 and BS 8000: Part 6 will be required to dress under the adjacent tiling, and over both Valley Troughs. When a valley discharges onto a roof slope (e.g. at dormers) a lead saddle will be required at the base of the GRP Valley Trough to dress onto the adjacent tiling or slating. At any significant change in pitch (e.g. at sprocketted eaves, or mansards) a single continuous GRP Valley Trough should not be fitted over the change in pitch, but separate lengths should be fitted above and below, and a lead saddle used to link the two parts with sufficient lap to suit each Valley Trough pitch. 6.4 Handling Maintenance and Repair GRP Valley Troughs can have sharp edges and should be handled with care; gloves should be worn. Mechanical cutting equipment should always be used in accordance with Health and Safety Regulations. 5
Periodic maintenance should be carried out to ensure the valley is kept clear, and to remove any growth or build-up of mosses, lichen or other debris; manufacturers advice should be sought. TYPICAL TILE VALLEY DETAIL Diagram 1 (DIAGRAMMATIC ONLY) Diagram 2 TYPICAL SLATE VALLEY DETAIL (DIAGRAMMATIC ONLY) NOTE: Although care has been taken to ensure, to the best of our knowledge, that all data and information contained herein is accurate to the extent that they relate to either matters of fact or accepted practice or matters of opinion at the time of publication, NFRC, the authors and the reviewers assume no responsibility for any errors or misrepresentations of such data and/or information or any loss or damage arising from or related to their use. Data and information are provided for general guidance only and readers must always take specific advice in relation to the use of materials, techniques and/or applications. Published by The National Federation of Roofing Contractors Ltd Roofing House, 31 Worship Street, London, EC2A 2DY Tel. 020 7638 7663 Fax. 020 7256 2125 Email. info@nfrc.co.uk www.nfrc.co.uk 2