A Guide to the Repair of Historic Roofs - Dublin City Council

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ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSCHIMNEY POTSChimney pots suffer similar exposure and weatheringto chimneystacks. They may sometimes be toodamaged to successfully repair and, in such cases,should be replaced with new pots of a matchingdesign, as they are important visually and provideventilation to redundant flues. Many traditionaldesigns of chimney pots are still widely available.Vented potsThe use of wire crow guards prevents birds fromnesting in chimney pots, but does not stop plantsrooting in blocked fluesMany older flues have little more than half-brick finwalls called ‘feathers’ within the stack separating theflues from each other. These can be poorly tied in oreven entirely missing as the result of chemical attackfrom soot, heat damage or over-vigorous cleaningwith soot brushesOn rare occasions, a chimneystack may be eroded ordamaged to the extent that it presents a public safetyissue or would damage a roof should it collapse. Insuch cases, following expert advice, it may benecessary to dismantle and rebuild the chimneystack.Prior to dismantling, a full photographic and measuredsurvey should be compiled, with all decorativefeatures and profiles recorded to aid the accuratereconstruction. Depending on the level to which thechimneystack has to be taken down, the insertion of aDPC tray should be considered when reconstructing.Where more than one fireplace gather to combine intoa single flue and one flue only is to remain in use, thenall adjoining flues should be sealed off. An alternativesolution, dependent on there being sufficient spaceavailable within the original flue, is the installation ofadditional liners for the fireplaces. The choice of flueliner will depend on the following: the openingrequired within the room; the route and size of theexisting flue; and the fuels proposed to be used in thefireplace/stove. The redundant flues should be ventedwherever possible.In predominantly straight flues, a slate shelf wassometimes incorporated within the flue. This shelf wasinstalled so that rain did not fall directly onto the fire.Unfortunately, within a disused flue this shelf tends togather debris, soot and the material from birds’ nests.63
ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSFLUE LININGMany flues were constructed of stone or, moregenerally, well-fired brick, finished out with lime, whichfrom the 1960s onwards was superseded by the use ofceramic or clay flue liners as the standard constructiondetails. The insertion of clay liners into the flueprevents the worst effects of the hostile sulphateenvironment on the masonry within the chimney.Most old flues are sufficiently large in size, and if theyare to remain in use, the installation of liners should beconsidered as a precautionary measure, particularly forchimneys on gable walls where the outer skin of theflue is exposed to greater environmental extremes. Ifthe chimneystack has not been in use for a while, thebrickwork in the flue can be in poor condition. Thisoccurs when moisture acts on the sulphate content ofsoot deposits and converts it to an acid, which iscapable of eroding the bricks. Damaged bricks shouldbe carefully removed. Sulphate-damaged brick shouldnot be used in reconstruction work. Solid clay bricksshould be used for repair purposes. The stack shouldbe consolidated internally to prevent the escape offlue gases into the roof space, and lime-renderedexternally, where appropriate, to improve weathering.There are several ways of lining a flue. Most methodsare carried out by specialist companies. If the flue isstraight and sufficiently short in height, a stainlesssteeldouble-lined flue can be installed within theexisting flue. In this method, short sections of pipe arefitted to one another. The second method is to uselightweight pumice liners with joints sealed at eachsection of liner. The sections of liner are fitted from theroof top and require the chimney pot to betemporarily removed. Access is also required internallyat the base of the chimneybreast. At the base of theflue, support is needed to hold the flue liner and theinsulated backfill in place. It can take the form of aconcrete gather or an aluminium plate (depending onthe proposed use of the flue).A third choice, a flexible stainless-steel flue liner, isprobably the simplest to install and can be used ifthere are many bends in the masonry flue. Accesspoints have to be made at the change in direction ofthe original flue to ensure the proper installation ofthe metal tube. This may be unacceptable where thereis a fine interior. There are two flue liner options of thisstainless-steel type. The first type, which is for use witha gas appliance, is a single-skin construction. Thesecond is a spirally wound double skin made ofstainless steel with a smooth inner surface which isparticularly resistant to corrosion and can be used forall fuel types.Flexible stainless steel flue being inserted into ahistoric masonry flueIf an existing flue is required to serve a gas boiler, itwill require a flue terminal at roof level and the flue tobe lined (approved by the utility provider) because ofthe higher temperature omissions. These stainlesssteelterminals can be unsightly, though there is also aterracotta gas terminal available which may besuitable in some situations. If the visibility of theterminal location is an issue, consideration should begiven to an alternative location for the boiler flue orthe use of a balanced flue (a horizontal flue combiningair intake and extract at low level through a wall).PARAPETSParapets were designed to conceal the exposed roofslopes from view. There are inherent difficulties withparapets arising from construction methods, includingpoor tying-in to the structural walls and high levels ofexposure. These difficulties are often worsened bytheir inaccessibility and the consequent lack ofmaintenance, giving rise to the potential for seriouswater penetration and deterioration of the fabric. Forfurther information on the repair of brick parapet wallssee Bricks – a guide to the repair of historic brickwork inthis Advice Series.64
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A Guide to the Repair of Historic Roofs - Dublin City Council

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