IMPORTANT INFORMATION ON THE USE OF INTERNALLYINSTALLED ROOF UNDERLAY

LAP VENTILATORS

CI/SfB (47)Uniclass Pr_30_59_29August 2018

As recent winters have become milder, the lessons learnt about the importance of adequateventilation to avoid the risk of harmfulcondensation during the prolonged cold spell in2010 are in danger of being forgotten.

Research has shown that about 20% of the air thatenters buildings, and in particular dwellings, leavesvia the roof with an additional moisture load.Typically 80% of any water vapour transfer into theroof is by convection through the ceiling via gapsand cracks, especially in older properties.

IMPORTANT INFORMATION ON THE USE OF INTERNALLY INSTALLED ROOF UNDERLAY LAP VENTILATORS

In existing buildings there is a growing interest in the useof internally fitted ‘ventilators’ designed to open the laps ofthe underlay to transfer the condensation risk to the battencavity to reduce the costs of external works retro-fitting tileand slate ventilators. But do they actually work?

All such devices sufferfrom a lack of guaranteedventilation performance.

Whilst some may have aknown ventilation freearea of the unit itself, thismay well not becontinued to the outsideatmosphere as required by BS 5250. It will be verydependent upon the type/style of roof covering and thedepth of available batten space. BS 5250 describesdifferent types of roof covering and whether they aredefined as ‘sufficiently air open’ or not and it is criticalthat this be considered during the specification processotherwise insufficient ventilation may well be theconsequence.

This is because the ventilation provision will bedependent on the equivalent free area through the tiles orslates in a single course. Whilst one may think that airwould escape above and below the tile battens, it is verytile or slate dependent and the battens will provide asufficient restriction to this flow to make it virtuallyirrelevant for calculation purposes.

A typical interlocking tile tested by Building ProductDesign was found to have 874mm2 of ventilation aroundits perimeter. Therefore in a single course of tiles aninternally fitted ventilator, even one with a free area ofaround 11,000mm2, would need between 10-12 tiles toventilate out, a spacing of about 3.6m. This will onlyachieve 3,000mm2 per metre. With close fitting roofcoverings such as interlocking and double lap slates andplain tiles, this would be even less. Whilst plain tiles mayseem to have a significant number of joints from whichair can escape it must be remembered that they are seton 38mm battens at 100mm centres which willsignificantly restrict a large portion of the ‘ventilation’area. Closer spacing of the internal units won’t help asthe air cannot escape through the roof covering insufficient quantity and one cannot count the sameventilation gap twice.

Impermeableunderlay (type HR)with flockedundersideshowingcondensatebeing held

Vapourpermeableunderlay(type LR)with smoothundersideshowingcondensate run-off anddripping

Condensation will occur as soon as the moist airmeets a cold surface. This can occur even inproperties that have been built for a number ofyears without experiencing any previousproblems.

Ensuring adequate roofspace ventilation is installedin both new build and in particular refurbishedproperties (insulation upgrade schemes where thebalance of thermal and vapour characteristics havebeen altered) can prevent the formation of harmfulcondensation and problems for the future.

Considering the life of abuilding andthe potentialnumber of hard winters, it is a verysmall price to pay.

INTRODUCTION INTERNALLY INSTALLEDUNDERLAY ‘VENTILATORS’

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IMPORTANT INFORMATION ON THE USE OF INTERNALLY INSTALLED ROOF UNDERLAY LAP VENTILATORS

As these products are designed for installation in the lapsof the underlay, the next opportunity to install up-slope would be between 1m and 1.5m depending onthe width of the underlay. Ventilation provision at lowlevel should be 10,000mm2 per metre.

Another practical issue is that the first opportunity toinstall in an underlay lap would be at least 1m up fromthe eaves. At this point it could be practically impossiblefrom inside owing to the restricted headroom in the loftspace, not to mention balancing on ceiling joists in anon-boarded roofspace with the associated safe workingissues it creates. The next opportunity would be at leastanother metre up-slope so the ventilation provision is notnow at low level where it needs to be, to effectivelyventilate at eaves in line with Building Regulationrequirements.

Implications of wind loading on theroof coveringIt is also important to understand the issues that usinginternally installed roof underlay lap ventilators can createon the management of wind loading and weatherresistance of the roof covering.

Building Regulation Approved Document A1:2004 states:

(1) The building shall be constructed so that thecombined dead, imposed and wind loads are sustainedand transmitted by it to the ground:

safely; and…

(2) In assessing whether a building complies with subparagraph (1) regard shall be had to the imposed andwind loads to which it is likely to be subjected in theordinary course of its use for the purpose for which it isintended.

Section 0.2b Loading states that ‘…and wind loadshould be in accordance with the current Codes ofpractice referred to in Section 1’.

The wind loading on a dwelling is calculated to BS EN 1991-1-4:2005 + A1:2010 and each dwelling willhave had a specific wind load calculation for the roofbased on its location, orientation, roof pitch, height etc.From the expected wind load results, the roof tilemanufacturer will provide a fixing pattern or specificationto ensure that the roof covering will be able to remain inplace in the occurence of the worst case 1 in 50 yearwind event.

The roof covering fixing specification is based on BS 5534: 2014+A1:2015, which assumes that the roofcovering and the underlay manage the wind loadpressures equally. Previously (BS 5534: 2003) it wasassumed that the roof underlay only managed 33% ofthe wind load; this change places yet more onus on thecorrect function of the roof underlay and its overlaps.

The underlay contributes to the wind load on the roofbecause the laps which are not sealed under normalconditions effectively close when under wind gustconditions, preventing transfer of wind load pressures tothe loft space.

Installing products that permanently open the laps of theunderlay irrespective of external conditions, in an attemptto disperse moisture vapour from the loftspace,effectively stops the underlay acting as a wind deflector.Opening the laps of the underlay reduces the pressuredifferential either side of the underlay to zero, whichmeans that all of the wind pressure has to be dealt withby the roof covering.

The original fixing specification of the roof covering willhave been determined based on closed underlay laps.With open laps, the roof covering itself has a muchgreater risk of being forcefully removed by the increasedwind loads causing localised damage and possibly fatalinjuries.

In addition, there is a greater risk of tiles chattering atlower wind speeds which will increase the incidence oftiles cracking / shearing and falling off the roof slope.This danger will occur far more frequently than the 1 in50 year occurrence, and should not be ignored.

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Implications on weather resistance of the roof covering

The driving rain resistance of themajority of roof coverings especiallyinterlocking tiles and slates is verydependent upon the integrity of theunderlay barrier underneath. A changein the batten space pressure byopening up the underlay laps,especially on the windward side of aroof, will detrimentally affect theability of the roof covering to resistdriving rain.

BS 5534: 2014+A1:2015, section4.15.7 states that:Ventilation products should:

a) Provide an effectiveweatherproof air path from the external environment

into and / or out of the roofspace or roof constructionzone for the design life of the building;b) Provide the free air space as recommended in BS 5250 for eaves, slate / tile or ridge applications; c) Prevent access by wildlife, e.g. birds, rodents or large insects.

Users should pay particular attention to the resistance towind loads, deluge and driving rain, durability andeffective performance of ventilation products.

It is for this reason that BS 5534 specificallyrecommends against such products. Section 5.9.2states:

Devices that form holes through the underlay, sarkingor insulation to promote ventilation from the roof spaceto the batten space (in contrast to ventilation tiles orslates which ventilate the roof to the outside air) canadversely affect the driving rain resistance of slates,tiles or shingles, as well as increase the wind upliftforces acting on them.

The specifier should confirm with the devicemanufacturer and the slate or tile manufacturer thatthe performance of the roof will not beunacceptably impaired. In particular, the specifier oruser should ensure that the relevant wind upliftcalculations demonstrate that slate or tile fixingswill be adequate.’

BS 5534 and manufacturer’s literature all put the onuson the specifier to ensure that the device will not affectthe weather and wind load resistance of the roofcovering.

IMPORTANT INFORMATION ON THE USE OF INTERNALLY INSTALLED ROOF UNDERLAY LAP VENTILATORS

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Installation issuesThe increase in popularity of internallyinstalled devices is a combination ofease and cost of installation andperceived advantages over working withexternal ladders which many consider to be a problemunder health and safety.

Safe and practical guidance on the use of ladders can befound in The Work at Height Regulation 2005: A briefguide produced by the HSE. Routine maintenance tasksregularly require the use of ladders - good workingpractices and a simple risk assessment policy documenteasily overcome any health and safety issues.

It is sensible to consider the longer-term risk associatedwith condensation, wind uplift and weather resistanceissues which can potentially cause serious and lastingdamage over the life of the building; not to mention thepossible damage or injury caused by falling tiles or slates.

The safe solutionThe safe, secure solution is to install purpose designedslate or tile ventilators with integral throat or purposedesigned extension sleeve to ensure the hole is sleevedfor the depth of the roof covering, underlay and battensto meet the design requirement of BS 5250: clauseH.5.4. Correctly installed slate or tile ventilators willremain weatherproof and fit for function for the life of thebuilding - simply fit and forget. They can be easilyinstalled as part of the normal slating or tiling process orretrofitted.

Glidevale offers a wide range of universal and profilededicated slate and tile ventilators which are designed toventilate through the roof slope and form an integral partof the roof covering. They are supplied with underlayopening protector, integral 4mm insect screen andventilator throat or extension sleeve to meet all the designrequirements of BS 5250 and can be colour matched.They are driving and deluge rain resistant.

IMPORTANT INFORMATION ON THE USE OF INTERNALLY INSTALLED ROOF UNDERLAY LAP VENTILATORS

Glidevale has over 30 years experience in roofingproducts in the UK and Ireland. Our aim is tomanufacture and supply only products that are fit forfunction and to help educate the industry of the issuesthey should bear in mind when drawing up specificationsor purchasing products.

Glidevale’s slate and tile ventilators offer one simple,unobtrusive method of providing adequate roofventilation to comply with current Building Regulations.As part of our free site survey service we can provide afull specification that itemises quantities and types ofvents required, broken down by individual property.Glidevale’s unique site survey service is designed to savevaluable time and resources for property maintenancemanagers involved in energy upgrade schemes.

Further informationFor further advice or detailed information regarding aspecific project, please contact our Technical ServicesTeam on 0161 905 5700

CPD seminars on BS 5534 wind uplift and BS 5250control of condensation can be provided on request.Please contact our sales team for further information.

ReferencesBuilding Regulations Approved Document A1:2004(incorporating 2010 and 2013 amendments)

BS 5534:2014+A1:2015

BS 5250:2011 +A1 2016 Code of Practice for control ofcondensation in buildings

Safe use of Ladders, HSE

GLIDEVALE2 Brooklands Road, Sale, Cheshire M33 3SS Tel: 0161 905 5700 Fax: 0161 905 2085

Email: info@glidevale.com Web: www.glidevale.com

Glidevale maintains a policy of continuous development and reserves the right to amend product specifications without notice.

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