Gutters and Summer Heat: Why Tampa Corners Leak in October
Here is a pattern we see every October. The summer storms have ended, the first dry-season front has come through, and someone calls about a corner that started dripping in the last week of September and will not stop. The gutter was resealed two years ago. Nothing hit it. Nothing is clogged. What broke it was four months of afternoons at 130 degrees.
How much a gutter moves
Aluminum expands and contracts with temperature more than most building materials. The roof edge of a west-facing wall in Tampa in July hits 130 to 140 degrees by mid-afternoon, and a storm drops it to 75 in twenty minutes. That is a swing of 60 degrees or more, most days, for four months.
Do the math on a 40-foot run: aluminum moves about 13 millionths of an inch per inch per degree, 40 feet is 480 inches, and a 60-degree swing gives you roughly 0.4 inches. Nearly half an inch of growth and shrink, every day, on every long run on the sunny side of the house. The gutter is fastened at each hanger, so that movement gets taken up somewhere, and the somewhere is the joints.
What it does to the sealant
The miter at each corner and the end cap at each run are sealed with a bead of sealant that bridges two pieces of metal. Every expansion cycle stretches and compresses that bead. Good polyurethane gutter sealant is rated for a lot of cycles. Cheap silicone or the acrylic caulk a handyman had in the truck is not, and it also does not bond to aluminum properly in the first place. After a hundred summer days the bead has either cracked, pulled off one side, or gone hard and lost its stretch.
UV finishes the job. The top of the bead faces the sky and gets baked every day it is not raining. Sealant that is chalky, brittle and cracked on top is done, even if it looks attached.
The corner starts weeping in September because that is when the joint has had the most cycles. It shows up on the west and south walls first, on the longest runs, and at outside corners, which take more movement than inside ones.
Long runs and expansion joints
A seamless run over about 40 feet needs an expansion joint, a slip connection that lets the two halves grow independently. Most Tampa crews do not install them, because most homes do not have a wall that long. But a 60-foot run along the back of a two-story home in Westchase or a long ranch in Plant City does, and if it was hung as one piece with corners at both ends, the corners are absorbing all of it. Those corners fail every two or three years no matter how well they are resealed, and the fix is a joint in the middle, not another tube of sealant.
The other things heat does
- Oil-canning. Thin .027 aluminum on a long run buckles visibly between hangers in the afternoon and flattens out at night. It does not leak on its own, but the ripples work the hangers loose and stress the seams. Thicker .032 stock resists it.
- Hanger creep. Spike hangers back out a hair with every cycle. Hidden hangers with screws hold; the re-hang post covers the difference.
- Downspout joints. The straps hold the pipe still while the pipe grows. Elbows that were pushed together without screws work apart over a summer and spray inside the wall.
- Vinyl gutters. Vinyl moves twice as much as aluminum and goes brittle in the sun. If you have vinyl and it is more than eight years old, this is the summer it started cracking at the joints.
- Sealant on tile-roof installs. Tile roof edges run hotter than shingle. Same failure, sooner.
What a proper reseal looks like
A reseal that lasts is mostly preparation. The old sealant is scraped and wire-brushed off completely, the joint is cleaned with solvent, dried, and a fresh bead of polyurethane or tripolymer gutter sealant is applied inside the gutter, tooled into the seam, and left to cure before water hits it. Sealing over the old bead, or sealing on the outside of the corner where it looks tidy from the ground, lasts one summer. The reseal is $150–$250 per corner and is on the gutter repair list.
On a run that has failed three times, we look for the reason: an expansion joint missing, a miter that has cracked, a corner taking a valley's worth of water. The fix for those is a joint or a new miter, and we say so rather than reselling the same $200 fix every other October.
Why October is the month to do it
The dry season is starting. Sealant needs a dry joint and a few hours to cure, which is hard to get between June and September. From now until May the roof edge stays cooler, the joints are not moving as much, and a bead applied in October has six months to cure before the cycling starts again. The end-of-season check is the walk-around for what else the summer did; the corners are usually the first thing on the list.
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