在铺设瓷砖之前，我们需要先安装一层防潮垫层。

We need to install a moisture-resistant ___underlayment before laying the tiles.

安装屋顶前，工人先铺设了一层防水垫层以防止漏雨。

Workers installed waterproof ___underlayment before putting on the roof tiles to prevent leaks.