Pitched Roof Coverings Series - NHBC

Pitched roof coverings series1. Where is it all going wrong?2. Wet mortar work essentials3. Weathering details & fixingThis is the third session of a three sessionseries on pitched roof coverings.The series provides a practical approachto raising the standards of pitched roofcoverings on-site.This session provides guidance onweathering details and fixingrequirements.Key learning points are identified with thissymbol:for Site Managers

Aims of this e-learning seriesAfter each individual e-learning sessionyou should:Session 1Where is it all going wrong? Know the extent of pitched roof claims Be clear where to focus your attention as asupervisorSession 2Wet mortar work essentials Understand the correct use of mortar on roofs Appreciate the benefits of adopting a teamapproach on site Be aware of alternative solutionsThis sessionWeathering details & fixing Have a good understanding of typical roofweathering details Appreciate the importance of correct fixingAdopting a team approach can bebeneficial for all

AcknowledgementsNHBC would like to acknowledge and thank the following for theirinput and support associated with this e-learning session: The National Federation of Roofing Contractors Limited Forticrete Marley Eternit Redland SandtoftThis e-learning session takes a non product specific, generalapproach for the following reason, manufacturer’s may have theirown very specific specification or fixing requirements.Therefore please make use of the manufacturer’s technical productsupport relevant to the products used on your site.NHBC would like to thank The Crowood Press Ltd for giving permission touse their images in this learning series. These images are from the book:Taylor, K., 2009. Roof Tiling and Slating a practical guideMarlborough: The Crowood Press Ltd.

Flashing and weathering detailingNHBC continually highlights the importance ofdetailing in ensuring that claims related to flashingsand weathering are avoided. Typical exampleswhere claims are being experienced and detailing isalso found to be deficient during recent inspectionsinclude: abutments dormers projections through the roof chimneys cavity tray/flashing interface head of valleyIf weathering details are unclear then seek the appropriate clarification and avoid ad-hoc details.

Images from NHBC claims/inspection services1. This ad-hoc approachwill lead to failure!12342. This damage to theunderlay can lead toproblems with windassisted rain\snowingress.3. The underlay detail atabutments is importantif weather ingressissues are to beavoided.4. Clearly detailing likethis must be avoided.If in doubt seekappropriate advice.

Flashing and weathering detailing - head of valley detailAvoid running ridge tiles over the valley head asshown in the top image. The mortar depth becomesexcessive and prone to failure. This details oftenconceals the lack of a lead saddle to the head of thevalley.The bottom right-hand image shows the detail whichrepresents good craftsmanship.A saddleflashing shouldbe used wherea ridge meetsthe main roof.GRP valley gutter

Flashing and weathering detailing - abutmentsThe following applies to small/bay roofs as well asmain roofs:The underlay should be turned up at least 100mm atall abutments to prevent wind assisted rain andsnow being blown into the roof space. The underlaywill need to be adequately supported at abutmentsand consideration should be given to the position ofthe last rafter relative to the wall.Where a pitched roof abuts the wall at an angle, astepped cavity tray linked to a stepped flashingshould be used. Stepped flashings should be cutfrom a strip at least 150mm wide.Soakers or a secret gutter should be installed atabutments where plain or flat interlocking tiles areused.Cover flashings alone can only be used where theydischarge over a roll on a profiled tile.Soakers beneath eachtile overlapped by leadflashingStepped lead flashingheld in mortar jointswith lead wedgesUnderlay turned up at abutmentTop images from the publication; Roof tiling and slating by Kevin Taylor

Flashing and weathering - cavity tray interface at abutmentsThe main issues of concern here are:1. The position of the weep hole(circled) suggests that the cavity tray isinstalled too high!2. The bed joints have been ‘ground out’to accommodate the correct flashingposition relative to the roof covering, butwill the flashings link with the cavitytrays?3. If the cavity trays are in the correctposition, have they been damaged bythe grinding out process?4. The underlay does not appear tohave sufficient upstand towards the topof the inclined roof abutment!Where flashings link with cavity trays, rake out 25mm of mortar below the dpc before the mortarsets, to allow for the flashing to be tucked in. This is the preferred approach.

Flashing and weathering detailing - abutmentsIf the cavity tray has been installed at the wrongheight to link with the flashing, this will need to beaddressed. Where flashings link with cavity trays,rake out 25mm of mortar below the cavity tray toallow for the flashing to be tucked in.In the top image the flashing should have been takenup higher to meet with the cavity tray.Weep holesAll abutments should be weatherproofed using nonferrous metal flashings. Lead flashings should be atleast code 4 (colour coded blue). Where required,soakers are normally code 3 (colour coded green).Normally, lead flashings should not exceed 1.5m inlength, with laps of not less than 100mm.Flashing should be tucked into a mortar joint 25mmdeep and at least 75mm above the tiling level andlead wedged into place. The joint should then bepointed with mortar or using suitable exterior gradesealant .Lead flashingwedged intojoint belowcavity trayUnderlayturnedupbehindflashingAt least 75mmAt least 150mmClip free edge offlashingThe use of disc cutters to create the recess for the flashing can damage the cavity tray and shouldtherefore be avoided.

Flashing and weathering - Dutch gable detailThe top image is of a Dutch gable, and whilst Initiallythis looks like a neat job, the long term durabilitymust be considered. NHBC looks for saddleflashings to be provided here to prevent waterrunning off the main roof areas and behind the hiptiles through the inevitable cracks that will formbetween the mortar fillet and hip tile.Reliance upon the underlay for weather proofing isnot a robust solution. Instead a lead saddle shouldbe neatly dressed and fitted as shown in the lowerimage.Lead welded saddle

Flashing and weathering detailing - roof projectionsParticular care is needed where soil ventilationpipes/proprietary ‘ventilation tiles’, project throughthe underlay. Torn underlay around soil ventilationpipes/proprietary ‘ventilation tiles’ can lead toweather ingress resulting in the ceiling underneathbecoming wet and stained.With soil ventilation pipe projections; cut a smallcross in the underlay and sleeve over the pipe sothat the tongues of the underlay turn up. A purposemade one-piece flashing and upstand should beused around pipes projections through tile/slatecoverings.Proprietary ‘ventilation tiles’ usually come suppliedwith purpose made soaker tray.

Flashing and weathering detailing - dealing with featuresCareful setting out will improve the finishedappearance of the roof and helps to avoid problemssuch as unequal overhangs at verges and oftenmakes it possible to avoid excessive tile cutting atabutments, chimneys and similar obstructions.Small sections of cut tiles are difficult to fix andshould be avoided. This can be achieved byincorporating tile sizes such as double size tiles, tileand a half or half tiles where these are available.Where small tiles cuts are unavoidable, seektechnical advice from the manufacturer. Many offerspecial clipping arrangements, additional mechanicalfixings or specialist adhesives to bond tiles together.Where appropriate, establish the tolerance range ofthe flashing detail to determine if the cut line can beadjusted.

Flashing and weathering detailing - chimneysLead flashings associated with chimney structuresrequire careful detailing of the masonry work inorder to ensure that the dpc’s are incorporated at thecorrect heights above the roof covering. Chimneyflashings should link with the chimney dpc trays.Underlay should have a minimum upstand of 100mmat the chimney abutments.In addition, timber support to the back gutter leadwork is equally important. Adequate support willprevent the lead work becoming depressed and\orsagging and causing potential weather ingressissues.Again it is vitally important that the lead work is firmlyfixed/wedged into place in line with good practice,due to its exposed nature.

Flashing and weathering detailing - underlaySlight sag required to under felt toenable water to drain to gutter, ratherthan collect at the battens.If the underlay is too tight, any waterwill run down the underlay until itreaches a batten, from there it willcollect until it eventually seepsthrough the batten nail holes in theunderlay. The image below is anexample of this failure.Slight sag

Nailing and fixing - specificationsThe tile fixing specification should be either; As per the manufacturer's calculated fixingschedule (based on BS 5534)Or As per the contractor's fixing specification basedon the ‘zonal method'. The zonal fixing methodwas devised to avoid having to go to themanufacturer for a fixing specification and isacceptable to NHBCTypical factors affecting fixing specifications include: tile type site address and postcode roof pitch height of building to eaves width of buildingSome tiles require specific fixing schedules(manufacturers to advise).

Nailing and fixing - generallySlates should be fully nailed over the whole roof.The fixing specification should specify the number offixings for clay and concrete tiles.Careful setting out will improve the finishedappearance of the roof and helps to avoid problemssuch as unequal overhangs at verges and oftenmakes it possible to avoid excessive tile cutting atabutments, chimneys and similar obstructions. Smallsections of cut tiles are difficult to fix and should beavoided. This can be achieved by incorporating tilesizes such as double size tiles, tile and a half or halftiles where these are available.Single lap Interlocking tiles have a tolerance ofapproximately 3mm in the joint. For double lappedplain tiles and slates, joints should be slightly open.This allows some flexibility in setting out and shouldavoid tile cutting.

Nailing and fixing - tile clipsIn addition to standardnailing, and verge clips,many fixingspecifications will requireadditional clipping to allor certain specified tiles.121. A typical general tileclip.2. A typical eaves tileclip.3. A typical tile to tileclip.3Images from the publication; Roof tiling and slating by Kevin Taylor

Materials - roof coveringsThe following roof coverings are acceptable: clay tiles and fittings to BS EN 1304 concrete tiles and fittings to BS EN 490 andBS EN 491 fibre cement slates and fittings to BS EN 492 natural slates- only certain grades to BS EN 12326are acceptable to NHBC (see Appendix 7.2– G)Natural stone should be used in accordance withestablished custom and practice.Use of reclaimed materials is covered in the NHBCStandards; Chapter 7.2 clause M6.Proprietary coverings should be assessed in accordancewith the NHBC Standards Technical Requirements R3.Slate packing crate showing BS EN reference