Lead Replacement Due to Underside Corrosion
In this project, we were tasked with replacing a relatively new lead flat roof. Our customer’s existing roof was causing damp and leaks that needed attention. We have found the cause of their problems, and have increased their lead flat roof longevity in excess of 100 years.
On inspection of the existing roof, there were splits in the lead (such as those shown below). This type of split shows signs of underside corrosion.
(Clockwise from top left) – Split in lead close to the roll; Split in the centre of a bay; lack of an adequate upstand under tiles; lack of upstand.
Removing the Existing
Once we had inspected the roof, our next step was to strip the existing lead. Due to sizes of the bays, and the mass of lead, the bays were cut in half and rolled up. The underside of the lead is white which is a clear indication of corrosion.
Immediately after removing the lead, we noticed a lot of moisture underneath. This is likely to be due to condensation problems that can happen. The existing geotextile underlay was soaked through and was retaining the moisture under the lead.
We discovered that there was no airflow through the roof, which is the likely cause of the condensation. Therefore, we had to create an airflow.
Lead underlay showing signs of moisture under the existing lead.
Corroded underside of lead.
Deteriorated lead underlay, and underside of lead.
Remedying the problem
In order to create an adequate airflow, we created a cross-flow of air by removing the existing decking, placing timbers perpendicular to the existing joists, and refitting the decking. This ensured an airflow through the whole roof. As you can see by the stains on the ply, there was a lot of moisture within the roof. We left the roof open for it to air-dry naturally.
Now that the decking was installed, we started replacing the roof. The first step in this is to lay a building paper to isolate the lead from the decking, and allow the lead to expand and contract freely with temperature changes (LSTA, 2018). On top of the building paper, we installed wood core rolls, cutting the end to a taper. Wood core rolls are used to divide the lead flat roof into bays of the correct size, and to make an upstand on the joints of the two lead bays. The lead is then dressed over the wood core roll, including dressing the roll ends to create a neater finish.
The final step is to apply a coat of Patination oil, which helps to prevent the lead from staining due to rain. The customer was very happy with the finish provided by the lead, and this was increased by the fact that a lead flat roof has a longevity of over 100 years.