IntroductionPhotovoltaic panels are being increasingly addedto many existing roofs, often in order to benefitfrom the government’s ‘Feed-in-Tariff’ scheme.Solar thermal collector panels are also gaining inpopularity, although not presently part of theFeed-in-Tariff.

This best practice note is intended to provideguidance as to the effect of retro-fit installation ofpanels on existing domestic-scale roof structuresand how this affects compliance with thefunctional requirement A1 of Schedule 1 to theBuilding Regulations 2010.

ConsiderationsCompetent Persons SchemeSchedule 3 of the Building Regulations 2010makes allowance for installers to gainmembership of a Competent Persons Scheme(CPS) and thereby perform self-certification ofcertain defined-scope building works includingsolar-thermal and PV installation.

For Photovoltaic (PV) panels only, theMicrogeneration Certification Scheme (MCS) existsto register installers for membership of the Feed-in-Tariff scheme, but the MCS is NOT a CPS.

Guidance published by certain CPSadministrators indicates that membership of aCPS will allow self-certification of Part Arequirements and installers will be deemedcompetent to carry out structural assessments ofexisting roof structures, if credits have beengained in City and Guilds 2372 - PV.

Building RegulationsRegulation 3 defines Building Work which needsto secure compliance with the regulations. Theinstallation of solar panels primarily falls within the

category of ‘Installation of a Controlled Service orFitting’; additionally it will also involve a ‘MaterialAlteration’, for the structural alterations elementwith regard to an existing building. ApprovedDocument A gives guidance on achievingcompliance with this aspect and suggests thatadditional loading to a roof structure wouldconstitute a material alteration if the loading to theroof is increased by 15% or more.

CPS membership allows installers to assesswhether compliance with Part A will be achieved,and reference to the 15% increase in loadingthreshold may be a suitable benchmark, i.e. anincrease of load that is less than 15% can beassumed to fall outside the remit of Part A.However, it is very important to bear in mind thatnot all CPS scheme operators encompass Part Aissues.

Panel loads and their effect on the roofPanels currently in production have a fitted massof around 20 kg/m2 (both PV and Solar Thermal).Considering an average total rafter load of around140 kg/m2, the addition of 20 kg/m2 wouldrepresent an increase in loading of some 15%. It isthe duty of an installer to assess the effect of thisincrease in loading in order to maintain anadequate factor of safety against failure.

Individual roof structures will vary byconstruction/type, workmanship, materials,maintenance/decay, exposure etc. Some roofsmay already be overstressed, if carrying heavyroof coverings or in exposed locations.

Typical roof constructionsa.Trussed rafters: Modern trussed rafters have been in common usesince the 1970s. Assessment by leading structuralconsultants and the BRE has established that

Best Practice note on application of Building Regulations

Retrofitting Solar Panels

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LABC represents Local Authority Building Control in England and Wales.

small dead load increases to standardconfiguration fink trusses (of up to 9m span) willnot overstress truss members or their connectorplates to any significant degree. Hence, an“allowed” installation of a single row of solarthermal or PV panels is considered acceptable,without further structural investigation.

An installer should always carry out a basicassessment to establish a minimum level ofrobustness in the construction, which includestruss fixings to wall plate; ensuring bracings tointernal members are in place; centrality ofconnector plates at node points; general timberdegradation or metal fastener corrosion.

b. Traditional cut roofs – purlins, binders,principal trusses:

Historic roof structures, which are generallydefined as pre-Victorian, will often comprise anarrangement of principal trusses, with supportingbeams spanning between them (purlins andbinders). The secondary members supporting thebuilding fabric (rafters and ceiling joists) are in turnsupported by the primary members. These roofsare usually constructed of locally sourcedhardwood and section sizes tend to beconservative. Connections may be traditional (e.g.mortice and tenon, dove-tailed etc) with timberdowels, bolted with iron straps or fixed with largeiron nails. Kept dry, the timber will tend to gain inshear strength over time, as seasoning continues.Valley and hip members are sized accordingly,depending on their degree of support and restraint.

Victorian and 20th century roofs saw thewidespread use of softwood construction. In the1950s the Timber Development Association (TDA)produced standard pattern roof arrangements,comprising softwood principal trusses, purlins,binders etc.

Studies of these roofs have shown them to benear capacity at present, with the principalvariable being the standard of timber used (quality,stress-grading, seasoning etc). Bungalow roofs,on estates constructed in the 1960s and 1970s,

tend to push these roof types to their limit, asspans increase.

Connections are critical in principal trusses, manyof which have been found to comprise heavilycorroded bolts, varying sizes of washer andinadequate projection of bolt threads. Internalmembers are often only connected by 2 no. nails!

Structural modelling of the addition of solar panelshas indicated that bending stresses wouldapproach 100% capacity and more. This erodesany factor of safety and presents the possibility ofan ultimate failure condition (collapse). Memberdeflections in TDA roofs currently run at around150% of that recommended for brittle finishes, i.e. plasterboard and plaster skim. Imposition offurther dead load only exacerbates this.

Any proposed loading increase in both historicand more recent cut roofs should therefore beinvestigated by a Structural Engineer.

Wind upliftThe addition of solar panels should not affect thepositive wind pressure acting on roofs, as they arealigned to the profile of the roof.

The unit size of the panel may be sufficient to actas a wind-suction collector and thereby generateconcentrated uplift forces at certain locations onthe existing roof. An average wind load to apply toall but the most exposed areas of England andWales would assume a Dynamic Pressure, q = 1.2 kN/m2. Further specific guidance is givenin BRE Digest 489.

It is unlikely that such forces would have any neteffect on the overall negative wind load on theroof, as the roof dead load will remain thedominant effect. However, a localisedconcentration of uplift force will be expected atfixing points. The critical element is the connectionof the clamp brackets to the rafters; fixing to tilingbattens is not considered a suitably robustsolution. On trussed rafter roofs, individual trussfixings may need strengthening, in exposedlocations.

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LABC represents Local Authority Building Control in England and Wales.

Rafters may be as little as 35mm wide in pre-fabricated roof trusses, thus carefulconsideration must be given to the specificationof fixings into this timber in order to achieve thenecessary edge distances required by the designcodes. The absolute minimum acceptable edgedistance is 4 x screw diameter and the minimumscrew spacing is 7 x screw diameter. The installerof the clamps must ensure that these minimumdistances are achieved. Brackets and fixingsshould also be suitably durable for their exposure(e.g. galvanised).

Snow loadIt is suggested that snow will be less likely to buildup on PV panels, due to their thermal property asa “black body”, flat profile and low coefficient offriction. Solar thermal collectors are different inprofile, however, and may encourage a localisedaccumulation of drifting snow. Current researchdata suggests that this effect is not significant, butinstallers should make an assessment of any riskof snow accumulation.

Conclusion• The snow loading on roofs is not thought at

this time to be significantly affected by theinstallation of PV or solar-thermal panels.Installers will have specialist knowledge ofsolar-thermal panel profiles and any likelihoodof localised drifting, caused by evacuatedtubes which have a raised profile, comparedto flat PV panels.

• Wind effects on the overall roof structure willnot be changed significantly. However,adequate fixing of panels to rafters isnecessary to prevent panel uplift.

• The predominant effect of installing panels isan increase in dead load of around 15%,therefore all roof structures should beassessed for their strength and robustnessto accommodate additional dead load.

• Modern trussed rafters (of up to 9m span) aregenerally considered suitably robust for limitedadditional loads, comprising a single row ofpanels only. Proposed loadings beyond thisscale would require a survey by a StructuralEngineer.

• Older roofs, of traditional cut-roof construction(including principal trusses, purlins, binders etc)are often operating at near maximum capacity.Where there are concerns about the structureof these roofs, they should be assessed by aStructural Engineer, in order to ascertainsuitability to receive additional loading.

• The installer remains responsible for thecommissioning of all surveys and any remedialdesign work.

Competent Person Scheme members may self-certify all aspects of installation, and would beexpected to address all the issues discussedabove. BUT not all CPS administrators require, orexpect their members to carry out a structuralassessment of roof structures; in some instancesthey merely deal with the electrical aspects ofinstallation. So it may sometimes be necessary fora Building Regulation application to be made forthe structural aspects of solar installations.

Non-members of a CPS should always obtainBuilding Regulations consent.

www.labc.uk.com

LABC represents Local Authority Building Control in England and Wales.

Reference code 0001-05-2011