How to Install Slate Roof Flashing

Use copper flashing on slate roofs; this is the most expensive but longest-lasting of flashing metals and will last the lifetime of a slate roof. Buy copper flashing in metal rolls to be formed on the spot -- copper is easily bent into any needed shape -- or pre-formed flashing strips or elements, such as vent flashing. Get at least 16-ounce copper, which weighs 16 ounces per square foot.

Install edge flashing first. Get pre-formed edge flashing if possible or bend copper at a 90-degree angle; one edge will go up the roof under the shingles and the other down the fascia or facing board, with a lip bent outward to direct water away from the wood and into a gutter. Put flashing under the roofing paper on the bottom or eave; for raked or sloped edges, install flashing over them. Fasten the flashing in place with copper nails and a hammer.

Put base flashing against a wall or similar intersection with at least 4 inches of copper up the side and 4 inches over the roof. Fasten copper flashing to wall sheathing with copper nails; never nail into the roof side. Use a step technique against most walls, by installing a strip of flashing, covering the edge with a shingle, putting flashing over that edge then adding another shingle; every flashing and shingle edge should be covered.

Use pre-formed flashing for vent pipes and similar openings. Use step flashing around chimneys but install counter-flashing above it. Counter-flashing is bent to go into the seams of a mortared joint and extends down over the top of the base flashing. Fold copper flashing over the junction of two roof lines at a peak or down the valley where two roof lines join. Use solid strips for peaks and step flashing for valleys. Fold peak or cap flashing back 1/2-inch at the bottom to block wind pressure.

Make a kickout or cricket at the top edge of a chimney or dormer or at the bottom edge of a roof to wall intersection. Cut the end of the flashing with tin snips at a 45-degree angle to the width of the flashing from edge to bend line, which forms the 90-degree angle and divides the flashing in half. Bend the wall or vertical side of the flashing around the wall end or chimney side and bend the other side out to divert water from above and away from the junction.

Connect copper flashing, as necessary, by soldering it in place with either a straight or overlapped joint. "Tin" the flashing with a 1 1/2-inch layer of solder on either side of the joint then "sweat" the seams by heating the copper so the tinned solder melts onto both elements to seal them. Solder all joints where there is a likelihood of deep snow or ice accumulation.