Roof Runoff

Roof Runoff, which is generally created by precipitation flow from roof surfaces, represents one of the sub-flows of urban runoff, which on the other hand is the urban subset of surface runoff.

Roof runoff can be a significant source of water pollution, particularly in Europe because of the widespread use of metal roofing materials such as copper and zinc. In urbanized areas most of the roof runoff enters the sewer system and is either directly transported to the recieving waters or, in case of combined sewer systems, sent to waste water treatment facilities. While in the first scenario roof runoff is not treated at all, the combined sewer system on the other hand bears the risk of sewer overflow due to limited sewer capacity during heavy precipitation events.

 Content Table

• Main Constituents
• Runoff Properties
  • Corrosion and corrosion rate
  • Runoff rate
• Related Articles

Main Constituents

It took about twenty years after the first studies on road runoff quality to spark scientific interest for roof runoff, because pollution loads had been underestimated for a long time. In a study investigating pollution sources into the mixed sewer system of Paris in 2001 it was found that roof runoff was the major source for the metals copper, zinc, cadmium and lead.¹ The number of metal roofs in Paris is especially high due to its many historic buildings.

In contrast to other urban runoff sources where the main source of contamination originates from deposition on the washed-off surfaces in the case of roof runoff the roofing material itself is mostly responsible for the presence of contaminants in runoff. Therefore the spectrum of contaminants is also much smaller and generally limited to heavy metals as the main components.The presence of other major contaminants is generally due to locally or temporally specific atmospheric impairment and will be reflected by similar amounts in runoff from adjacent areas due to dry or wet deposition on these.

Even though other conventional roofing materials like tile, polyester, shingle can also contribute measurable concentrations of copper and zinc their release of heavy metals is marginal compared to zinc and copper roofs.

Runoff Properties

Runoff from metal roofs is special as for aged roofs the presence of the according metal ions is independent of the total precipitation volume or duration. After reaching its maximum thickness the metal oxide layer represents an almost unlimited reservoir of metal ions. Thus, there are two rates that define the amount of metal washed off: the corrosion rate and the runoff rate

Corrosion and corrosion rate

Corrosion is simply the conversion of pure metal into its oxides. For copper the first step is commonly the formation of yellow-reddish Cuprite (Cu₂O). With the presence of CO₂, SO₂, or chlorides (coastal areas) and moisture these are transformed into the basic, blue salts CuCO₃∙Cu(OH)₂. CuSO₄∙Cu(OH)₂, or CuCl₂∙3Cu(OH)2. This blue salts in combination with the yellow color of the Cuprite make the roof appear it is distinctive green color.

For Zinc this conversion into its oxo-salts is not as apparent.The oxidation starts Immediately on contact with carbon dioxide of the air to the softer, colorless carbonates Zn₅(OH)₃(CO₃)₂ and Zn₄CO₃(OH)₃∙H₂O or in the presence of SO₂  the sulfates Zn₄SO₄(OH)₆∙xH₂O².

Runoff rate

Runoff rates depend on the applied metallic material, the climatic and pollution situation and on roof properties such as age, height, inclination and direction. They can be calculated when all runoff events are collected as follows:³ 

with R_p is the runoff of zinc per m² roof area a in g/m²; ρ_p is the mass concentration of zinc ions mg/L; ν_p is the sample volume in L. R_m is the runoff rate if m is equal to the number of rain events of one year.

Published runoff rates for zinc roofs⁴⁵⁶⁷vary between 2.3 to 7.5 g/m²a and between 1.0 and 3.9 g/m²a for copper roofs⁸⁹¹⁰in Europe.

Related Articles

• Surface Runoff
• Urban Runoff
• Stormwater 
• Storm Drain

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2. ^ S. Jouen, B. Hannoyer, A. Barbier, J. Kasperek, M. Jean: A comparison of runoff rates between Cu, Ni, Sn and Zn in the first steps of exposition in a French industrial atmosphere. Materials Chemistry and Physics 85, 73-80 (2004)
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