Building Pathology: Poor Specification of Leadwork

Lead Flashings are often the only form of defence against water penetrating the horizontal and sloping junctions formed where a roof meets a wall or a parapet. These abutments are generally amongst the most exposed parts of a building and therefore should be detailed in such a way to ensure longevity.

On a recent new roof inspection I found an inadequate detail on a lead secret gutter. At first glance it would be quite easy to miss this poor design as the sealant can hide the problem. An appropriate design would have been to raggle the lead into the wall or provide a cover flashing, with the latter being the best choice.  This detail had neither and was simply run up the wall and sealed.

I have peeled back the leadwork in the picture below to exemplify the fault. The lead had been taken up the wall and sealant was placed along the top edge which would not have been enough to give the detail a long term weather seal. The only means of protection from water ingress was a single bead of silicone. The fix in this instance was to get the contractor back to install a cover flashing which would have been an easier job to do before the tiles had been laid.

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Specifying Secret Gutter Details

The guidelines for constructing a secret gutter are laid out clearly in the lead sheet associations manual. Generally speaking, the secret gutter should be constructed from code 4 lead as a minimum and cut in lengths not exceeding 1.5m. The end to end lap joints should be calculated in relation to the pitch of the roof, for a 30° roof a lap of 150mm would be required.

The secret gutter should be a minimum of 25mm deep and have a vertical upstand of no less than 65mm above the top surface of the tiles. The gutter should be at least 75mm in width and the tiles should be laid to leave a gap of 15mm to the side of the abutment.

The gutter should then have a cover or stepped flashing, again in lengths not exceeding 1.5m. Cover flashings must be turned-in to joints by a minimum of 25mm and secured by folded wedges or fixing clips at 450mm centres. I prefer the use of fixing clips as lead wedges have the propensity of becoming loose and falling out. The flashing should overlap at joins by 100mm minimum.

Clips can be used to secure the free edges of cover flashings, these clips should be placed at 300 to 500mm centres with lead clips being used for sheltered exposures and tinned copper or stainless steel for most other situations.

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Secret Gutter after Cover Flashing Applied

Leadwork after cover flashing applied

Finishing the Leadwork

Pointing can be carried out with mortar although lead sheet sealant which provides a long term flexible joint which can absorb temperature changes and is quicker and cleaner to use than mortar.

In rainy or damp conditions new lead will quickly develop an uneven powdery white coating of lead carbonate which can run and stain the roof tiles (although not in our example of a secret gutter). Patination oil would minimise this as it gives a more pleasing appearance to the leadwork.

Smooth brown Marley Eternit Ludlow Major tiles were used in this project so I have provided some of the specification guidelines for them also.

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