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

ROOFS A GUIDE TO THE REPAIR OF HISTORIC ROOFSConstant saturation and frost action can severelyundermine the structural integrity of a masonrychimney, particularly if there is any loss of mortar orspalling of bricks (the surface of the brick detaching). Itis important that chimney flashings be inspected anddefects remedied promptly. A common flashing detailevident in Dublin at roof level is the use of slatesbedded in lime to protect the chimney or the masonryabutment. Where these cut-slate flashings have beenlost or damaged, the original detail should be repaired,bedding the slate in lime.stack. Infrequently, fireplace hearth slabs were builtdirectly on top of joists, which can cause charringwithin the floor structure and is of particular concernin historic structures with thinner hearths. Old charredtimbers are susceptible to catching fire within, oradjoining, flues and their condition and location needto be ascertained within redundant fireplaces or fluesif they are to be reused. If there is evidence of a pastchimney fire it is quite likely the condition of the fluewill need to be investigated further, and relining maybe necessary.The installation of television aerials and satellite dishesto chimneystacks can cause problems, as extrapressure and movement is exerted on the structure inwindy conditions. Where possible, alternative locationsfor such installations should be sought and no newaerials or dishes fixed to historic chimneystacks.Because of their exposed location and inaccessibility,chimneystacks are vulnerable to the effects of severeweathering and poor maintenance. Their conditionshould be monitored to avoid structural failureWater penetration through damaged or lost stonecopings or flaunchings (the profiled fillet between thecoping and the chimney pot) can cause considerabledamage to the top courses of a masonry stack.Flashings commonly crack where damage is caused bymovement in the masonry. On brick chimneys wherethe brickwork has distorted and the mortar joints haveeroded, the lead or copper flashings may becomedetached.Shared stacks should be inspected and any structuraldefects addressed as a priority. This is best dealt withwith the cooperation of the adjoining owner, as adamaged or cracked stack can affect both properties.Sometimes, in mud-walled structures, chimneystacksare built from brick set on top of a rammed earthchimneybreast comprising a large hood at fireplacelevel and supported by a large timber beam throughthe upper floor level. In all cases, careful inspection willbe necessary to determine the nature of constructionand the condition of any timbers adjoining the flue orPrevious inappropriate repairs to damagedchimneystacks may be the cause of problems, such astying the stacks with metal straps or applying acoating of cement render. Problems will arise as thecement render, in time, will crack allowing water tosaturate the softer brick or stone beneath. As cementrenders are impermeable, they prevent the drying outof the masonry, causing further deterioration of thestack. Superficial examination of a chimneystackrepaired in this way may suggest just minor crackingin the render. However, removal of the metal strappingand cement render will usually reveal greaterstructural damage beneath.Stacks and pots should not be removed, even if theybecome redundant. Redundant flues should beproperly capped and ventilated. Ventilation can beachieved using a vented pot or brick to the flue,venting to the external air and not into the attic, andalso at fireplace level, to ensure airflow sufficient tokeep the stack dry.A damaged fin wall separating a flue, if dislodged, canfall into adjoining flues and cause a blockage. Telltalesigns that this failure has occurred include smokerising from more than one chimney pot when onlyone fire is lit, or by smoke leaking into an adjacentroom or even into an adjoining property.From about 1900 onwards, the insertion of chimneytrays into brick stacks became standard buildingpractice. A chimney tray is a lead tray inserted at rooflevel into a chimneystack, forming a barrier tomoisture ingress. In combination with a DPC below thecoping and with flashings at the base of the stack,water is prevented from passing through thebrickwork and entering the attic space.62