A sod seam is the joint where two pieces of new grass meet. The prep that makes those joints hold is soil prep before sod. We stagger the seams the way a brick wall staggers joints, and we pull them tight so the lawn is one surface. A seam left open is a crack the sand shows through. A seam left high can dam water.
Seams are part of the install, not a separate product. You should not see a grid of gaps the day we leave, and you should not see the pieces curling up at the corners a few days later if the water is right. Curling with dry soil under the corner means that spot needs water. A seam sitting in a puddle means it needs less, and the grade may be holding water there.
What it looks like here
On this sand a seam opens fast when the first weeks are skipped. The piece shrinks, the joint widens, and the lawn looks tiled. People blame the grass variety. Floratam, Bahia, and Zoysia all do this if the roots never leave the sod. The joint is only the place you notice it. Kids, pets, and a mower in week one pull the same joints apart. Stay off for the first week. Regular foot traffic waits about two weeks.
A seam is not edging. Edging is the line that keeps a bed off the lawn. A seam is inside the lawn. Do not fill an open joint with sand and hope, and do not pack mulch into it. If the pieces have died back from the joint, that area may need to be re-laid after the water and the grade are honest. A living seam that is only slightly open often knits if the water starts.
What we do about it
Before a seam exists we pull the old mat, grade water away from the house, and till in topsoil only where the sand will not hold moisture. Then the sod goes down tight. That order is the whole point of prep. Sod laid over weeds, a root mat, or a low spot against the slab will open no matter how neat the joints looked at noon. We do not install French drains. Surface grade is what we set.
The installed range is $0.85–$1.75 per square foot, and the seam work is inside that, not a line item. A range, not a bid. If your new lawn is gapping, send a photo of the joint and of the soil under a lifted corner. Dry soil is a watering problem. A curled piece on wet, low ground is a grade problem. Both are explained from soil prep before sod.