Your question raises the issue of concurrent causation (treated comprehensively in the Fire volume, Commercial Property Colb-). Concurrent causation is a court-developed doctrine that operates to hold as covered losses that involve an excluded peril and a peril not excluded by the policy. ISO subsequently amended its forms to exclude certain perils (earth movement, flood, operation of ordinance or law, etc.) regardless of contributing non-excluded perils operating in any sequence to the loss.

Apparently you make the argument that it was not the peril of weight of ice or snow that caused damage to your insured's down spouts, but instead was the change in temperature that allowed the ice pack to break off and slide down the roof, causing damage to drain spouts along the way.

In the right litigation circumstances (a liberal judge in a jurisdiction favorable to the concurrent causation doctrine), your proximate cause argument might find favor. ISO's concurrent causation exclusionary language does not specifically apply to the weight of ice or snow limitation. However, the weight of ice limitation contains the language "caused by or resulting from" weight of ice or snow. If the phrase was simply "caused by," your argument would be strengthened. However, as the policy excludes coverage to gutters or spouts "caused by or resulting from" weight of ice or snow, it is hard to argue that the damage to the spouts did not result from the weight of ice moving against the roof.

Although some argument for coverage might be made, we do not think the policy ambiguous, and feel the insurance company is within the contract in denying coverage.