Hail is probably the most important peril along the populated south-eastern seaboard, but no regulations govern roofing materials.
冰雹可能是人口集中的东南沿海一带最重要的风险，但还没有规定对屋顶材料加以控制。
They came into more widespread use in the 19th century, when new waterproof roofing materials and the use of structural steel and concrete made them more practical.
19世纪出现了防水的新型屋顶材料，使得平屋顶更为普遍，钢筋混凝土的使用也使得平屋顶更加实用。
If valley flashings are too narrow, backed-up water can find its way under the roofing materials.
如果天沟过于窄的话，积水将渗透屋面材料。
If valley flashings are too narrow, backed-up water can find its way under the roofing materials.
如果天沟过于窄的话，积水将渗透屋面材料。
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