Does Tyvek go over flashing?

Whether Tyvek goes over or under flashing depends on the type of flashing and location, following the principle of water shedding where upper layers overlap lower layers to direct water outward. Tyvek is a house wrap water-resistive barrier installed over wall sheathing behind siding. For step flashing along sidewalls, Tyvek should go over the vertical leg of step flashing, not under it, so that water running down the house wrap goes onto the flashing and then onto the roof, not behind the flashing into the wall. The correct sequence for new construction is to install house wrap, then install step flashing interwoven with shingles, with the house wrap overlapping the step flashing. However, for counter flashing on chimneys, the house wrap or building paper should go under the counter flashing. For window and door flashing, Tyvek is typically cut and integrated with flashing using proper shingling technique. At roof to wall intersections, some builders install a strip of self-adhering membrane over the house wrap and over the step flashing for extra protection. The key principle is that water should always be directed outward and downward, so upper layers of water-resistive barrier overlap lower layers of flashing. Improper overlap where Tyvek goes under step flashing can allow water to run behind flashing into the wall cavity, causing rot and mold. Always follow manufacturer instructions for specific flashing details.