The roof is the home's primary defense against water intrusion from rain, snow, and ice. Water that seeps into the house through the roof can quickly ruin insulation, create conditions for mold growth and pest invasion, and even set into motion structural rot. Valleys and penetrations through the roof decking are some of the most vulnerable areas for water intrusion. To keep out water, the roofer will install a peel-and-stick membrane in all roof valleys and wherever the roof abuts a wall. Metal flashing is usually installed over the peel-and-stick membrane at wall intersections. Flashing is also installed around chimneys, vent stacks, and any other roof penetrations. Ice and water shield is installed along the edges of the roof. Some builders install peel-and-stick or a paint-on damp-proofing over the whole roof. Others will use roofing felt over the remainder of the roof.