Lead Gutters — Common Defects

Lead Gutters - Common Defects

Common Defects

Simple parapet gutter at Corpus Christi College

Lead linings are commonly found in the valleys between pitched roofs and behind masonry parapets. Defects here often seep quietly for months or even years, providing the ideal conditions for dry rot, wet rot, insect infestations and other forms of decay. The correct use of lead is crucial to their success, and if correctly detailed, these linings can last for centuries.

Lead sheet is the earliest known metal to be used as a roofing material, and with a proven lifespan in excess of 100 years it continues to be used today. From early medieval times many churches and fine houses used sheet lead which had been cast on a bed of sand for covering their roofs, and sand cast sheet continued to be used exclusively until the 18th century, when milled lead (now known as rolled lead) began to be used for roofing.

Today, there are three types of lead sheet manufactured for use in construction, with significant differences in each end product that can affect performance.

Rolled lead sheet (previously referred to as milled lead) is the most commonly used and widely produced. As its name suggests, it is made on a rolling mill that controls the thickness of the finished sheet to within fine tolerances (any point on the sheet must be within +/- 5% of the standard thickness). Rolled lead is the only type of sheet that is produced to a European Standard, BSEN12588, which as well as regulating dimensional tolerances also states the amount of permitted inclusions (of copper, tin, antimony, etc) in the lead that provides the feedstock to the rolling mill. Thickness are defined by codes with Code 4 (1.8mm) to Code 8 (3.55mm) the most commonly used. Rolled leads uniform metallurgical grain structure helps it to resist fatigue cracking from thermal movement.

Sand cast shee t is still made using the oldest method of manufacture, as it remains in demand for restoration and refurbishment work where like-for-like replacement is required. This traditional method cannot produce the fine thickness tolerances achieved by modern rolling mills and is normally produced in the thicker codes of sheet. Nevertheless, the British Standards Institute recognises the product in BS6915 (Design and construction of fully supported lead sheet roof and wall coverings Code of Practice ) as it has proved during its centuries of use to perform just as well as rolled lead sheet.

Machine cast sheet is the latest manufacturing form, originally introduced to the UK as a sound attenuation product and adapted for roofing in the 1980s. Machine cast sheet is not made to a European Standard as it is not produced to within the +/-5 per cent thickness tolerances required of BS/EN12588 material. In addition, the metallurgical consistency of machine cast is different to rolled sheet as it does not have a uniformly distributed grain structure within the sheet.

Users of machine cast sheet should seek working and fixing recommendations from the manufacturers which have been shown to be adequate through evidence of use.

PROPERTIES

Lead is a silvery metal which rapidly turns a dull grey when exposed to the atmosphere. This patina is relatively inert and extremely durable, and is one of the two main reasons why lead sheet makes such an excellent roof covering. The other is that the sheets are easily shaped by basic hand tools. The result is an unrivalled weatherproofing material which can, in the hands of a skilled and knowledgeable craftsman be shaped to fit, protect and preserve any detail of architectural significance, no matter how complex.

However, this same malleability creates problems for the unwary. Being a soft metal, lead sheet reacts to thermal changes, expanding in warm sunlight and contracting when temperatures cool. Properly allowing for this thermal movement when sizing and fixing any detail is critical to the long term performance of the sheet, which is why only a skilled installer should be used.

DESIGN

Lead sheet in general has an excellent performance record, but when failures do occur, it is normally the result of incorrect sizing and fixing, restricting natural thermal movement.

Most lead details involve a piece of lead sheet hanging in some way on its fixings; even a roof bay fitted to a fall of only 1 in 80 tries to creep down the slope, restrained by the fixings at the head of the panel. Head fixings prevent creep, and intermediate fixings (clips) prevent wind lift, both of which need to be fitted correctly to allow free thermal movement.

Common Defects

Simple parapet gutter at Corpus Christi College

Lead linings are commonly found in the valleys between pitched roofs and behind masonry parapets. Defects here often seep quietly for months or even years, providing the ideal conditions for dry rot, wet rot, insect infestations and other forms of decay. The correct use of lead is crucial to their success, and if correctly detailed, these linings can last for centuries.

Lead sheet is the earliest known metal to be used as a roofing material, and with a proven lifespan in excess of 100 years it continues to be used today. From early medieval times many churches and fine houses used sheet lead which had been cast on a bed of sand for covering their roofs, and sand cast sheet continued to be used exclusively until the 18th century, when milled lead (now known as rolled lead) began to be used for roofing.

Today, there are three types of lead sheet manufactured for use in construction, with significant differences in each end product that can affect performance.

Lead Gutters - Common Defects

Rolled lead sheet (previously referred to as milled lead) is the most commonly used and widely produced. As its name suggests, it is made on a rolling mill that controls the thickness of the finished sheet to within fine tolerances (any point on the sheet must be within +/- 5% of the standard thickness). Rolled lead is the only type of sheet that is produced to a European Standard, BSEN12588, which as well as regulating dimensional tolerances also states the amount of permitted inclusions (of copper, tin, antimony, etc) in the lead that provides the feedstock to the rolling mill. Thickness are defined by codes with Code 4 (1.8mm) to Code 8 (3.55mm) the most commonly used. Rolled leads uniform metallurgical grain structure helps it to resist fatigue cracking from thermal movement.

Sand cast shee t is still made using the oldest method of manufacture, as it remains in demand for restoration and refurbishment work where like-for-like replacement is required. This traditional method cannot produce the fine thickness tolerances achieved by modern rolling mills and is normally produced in the thicker codes of sheet. Nevertheless, the British Standards Institute recognises the product in BS6915 (Design and construction of fully supported lead sheet roof and wall coverings Code of Practice ) as it has proved during its centuries of use to perform just as well as rolled lead sheet.

Machine cast sheet is the latest manufacturing form, originally introduced to the UK as a sound attenuation product and adapted for roofing in the 1980s. Machine cast sheet is not made to a European Standard as it is not produced to within the +/-5 per cent thickness tolerances required of BS/EN12588 material. In addition, the metallurgical consistency of machine cast is different to rolled sheet as it does not have a uniformly distributed grain structure within the sheet.

Users of machine cast sheet should seek working and fixing recommendations from the manufacturers which have been shown to be adequate through evidence of use.

PROPERTIES

Lead is a silvery metal which rapidly turns a dull grey when exposed to the atmosphere. This patina is relatively inert and extremely durable, and is one of the two main reasons why lead sheet makes such an excellent roof covering. The other is that the sheets are easily shaped by basic hand tools. The result is an unrivalled weatherproofing material which can, in the hands of a skilled and knowledgeable craftsman be shaped to fit, protect and preserve any detail of architectural significance, no matter how complex.

However, this same malleability creates problems for the unwary. Being a soft metal, lead sheet reacts to thermal changes, expanding in warm sunlight and contracting when temperatures cool. Properly allowing for this thermal movement when sizing and fixing any detail is critical to the long term performance of the sheet, which is why only a skilled installer should be used.

DESIGN

Lead sheet in general has an excellent performance record, but when failures do occur, it is normally the result of incorrect sizing and fixing, restricting natural thermal movement.

Most lead details involve a piece of lead sheet hanging in some way on its fixings; even a roof bay fitted to a fall of only 1 in 80 tries to creep down the slope, restrained by the fixings at the head of the panel. Head fixings prevent creep, and intermediate fixings (clips) prevent wind lift, both of which need to be fitted correctly to allow free thermal movement.


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