Every joint is a leak with a date on it

Sectional gutter comes in lengths, usually ten feet, and is joined together on the house. Seamless gutter arrives as a coil of flat aluminium on a truck, is run through a forming machine on site, and comes out the other end as one continuous piece cut to the exact length of your run. A forty two foot run is a single forty two foot piece of metal.

What a joint actually is

It is a lap: two pieces of metal overlapped, usually with a slip connector, and bedded in sealant. Structurally it is fine. The sealant is the part with a clock on it.

Consider what that bead has to survive on the south side of a St. Louis house. It gets to well over a hundred degrees in the sun in July and below freezing in January, which means the aluminium either side of it is expanding and contracting all year and dragging the bead with it. It sits in standing water, then dries out, then sits in water again. It gets ultraviolet light all day. It has grit and biofilm sitting on it. Sealant is a consumable in that environment, and consumables get consumed.

When it goes, it rarely announces itself. It weeps. A slow weep at a joint does not show up as a dripping gutter, it shows up as a dark stripe on the fascia below, then peeling paint, then soft wood, then a section of fascia that needs cutting out. That damage happens on a timeline of years, which is exactly long enough for nobody to connect it to the joint that caused it.

What seamless actually fixes

It removes the joints in the straight run. That is it, and it is a lot. On a forty foot elevation with sectional gutter you may have three or four laps. On seamless you have none in that stretch.

What seamless does not do is remove joints from the system:

• Corners still have to be joined, inside and out. These are still sealed joints.
• End caps are still crimped and sealed.
• Outlets are cut into the trough and are still a detail that can leak if it is done badly.
• Very long elevations may still need a joint, because a coil forming machine and a residential street have physical limits.

So the honest claim is not that seamless gutter cannot leak. It is that the number of things that can leak drops from many to few, and the few that remain are at corners and outlets where you can see them, reach them and maintain them.

Material, and the thing about thickness

Residential seamless gutter here is almost always aluminium coil, and coil comes in different thicknesses. Heavier coil dents less when a ladder leans on it and holds its shape better across a long span. It is not a dramatic difference, and it is not worth pretending it is, but it is a real one and it is a fair question to ask whoever is quoting you: what gauge is the coil.

Colour is baked on at the mill, which is why seamless gutter generally holds its finish well and why you should not expect to repaint it successfully later. Pick the colour you can live with.

When replacing is not the answer

If your existing gutter is sound aluminium, correctly sized, hung straight, and simply weeping at two joints, sealing those joints is a legitimate repair and it is cheaper by an order of magnitude. It will need doing again eventually. That is a reasonable trade if you are not planning to be in the house for another twenty years, and any company that will not tell you that is not being straight with you.

Replacement earns its money when the run is also the wrong size, or sagging, or hung on spikes that have worked loose, or fastened to fascia that is already going soft. At that point you are correcting several things at once and starting from a coil makes sense.