Home Additions & Remodeling

Two things are true at a tie-in that are not true anywhere else on the house. First, it concentrates more flashing per linear foot than any other detail — new valleys, a roof-to-wall run, a head condition, and often a cricket, all within a few feet of each other. Second, the addition and the original house sit on different foundations poured decades apart, and they settle at different rates. A rigid, sealant-dependent detail at a moving joint has a predictable ending.

The cleanest addition roof continues an existing plane at the same pitch, so the new field simply becomes part of the old one. When the pitch changes, the transition has to be built and flashed as a deliberate detail rather than trimmed to fit — a pitch change creates a break where water changes speed and direction, and that break needs metal under it.

When the addition runs in as a lower roof against an existing wall, the detail is roof-to-wall: step flashing woven course by course up the wall, counterflashing or the siding’s weather barrier lapped correctly over it, and — critically — a kickout flashing at the bottom where the roof edge dies out above the gutter. Without the kickout, water runs off the end of the roof and straight down inside the wall. That failure is invisible from outside until the sheathing is gone.

Additions frequently end up with a shallower roof than the house, because the addition roof has to duck under existing windows or eaves. Asphalt shingles have a minimum slope: standard application is generally limited to 4:12 and steeper, and low-slope application down to 2:12 requires a doubled underlayment. Below 2:12, shingles are the wrong product no matter how well they are installed, and the addition needs a low-slope membrane instead. Building a shallow addition roof out of shingles is one of the most common and most avoidable failures in residential remodeling.

Where new decking meets old, the layers have to shingle correctly — each upslope layer lapping over the one below it, with the new underlayment tucked under the existing course above rather than laid on top of it. Self-adhering ice-and-water membrane belongs across the tie-in itself and up any new valley, because that is where water concentrates and where movement is greatest.

The existing shingles also have to be cut back and the new courses laced into them, so the courses continue across the joint. Butting new material against old leaves a seam running across the slope, which is both a visible line and a path for water.

Even the identical product, from the same manufacturer, in the same color, will not match a roof that has been weathering for ten years — sunlight shifts the granule blend and no new shingle comes out of the bundle looking aged. You have three honest options: accept a visible difference where the addition ties in, re-roof the entire affected plane so the change lands at a natural break like a ridge or valley, or re-roof the whole house and get a uniform result plus a new system warranty. Which one is right depends on how visible the elevation is and how much life the existing roof has left.

An addition rarely stops at the roof. Gutters and downspouts usually need re-routing so the new roof does not dump water onto the old one or against the foundation. Siding has to be matched and its weather barrier lapped into the new flashings. Windows and doors on the addition should match the house in operation and profile, not just in size. Exterior paint on a new elevation almost always means painting adjacent elevations to avoid a hard color line. And a deck built off an addition needs a properly flashed ledger, because a ledger fastened through siding without flashing is both a rot problem and a safety one.

CMAC is veteran-owned, GAF Master Elite® certified, a GAF President’s Club award winner, an NRCA member, and rated A+ by the Better Business Bureau, working across Texas, Oklahoma, Arkansas, Tennessee, and Georgia.