01Two water paths, drawn
The whole argument fits in one comparison, because everything else follows from it.
Read the middle two rows again. On a welded bowl the grain runs across the joint without a break, so there is no line for soil to follow. On a separate bowl the grain stops at the aperture and starts again on the rim, and that interruption is the thing a health inspector runs a finger along.
02What the gasket is actually doing
A sink gasket is not a waterproofing device. It is a gap filler that works only while it is compressed, and compression is the part that decays.
| Age | Compression | What you notice | What to do |
|---|---|---|---|
| Year 1–3 | Full | Nothing | Wipe and ignore |
| Year 3–6 | Reduced at the corners | Dark line at two corners | Lift, clean, refit or renew |
| Year 6–10 | Set | Bead lifting; movement when loaded | Renew the gasket |
| Beyond 10 | Lost | Water under the rim, staining at the back edge | Renew, or replace the top if the core is wet |
That last row is the one that turns a sink problem into a countertop problem. Water that gets under a failed bead reaches the core of a composite top or the underside of a timber one, and by the time it is visible at the front edge the damage is 300 mm further back than the stain.
03Two millimetres of movement, and why it matters
A steel bowl in a steel top expands at about 16 µm per metre per °C, the same rate as the plate around it, so a welded joint sees almost no differential. A stainless bowl in a composite or timber top sees a real difference, and a hot pot plus a cold rinse is enough to work a bead.
- Steel in steel — no differential worth designing for.
- Steel in composite — allow for roughly 2 mm across a 600 mm aperture between a cold morning and a hot service.
- Steel in timber — the timber moves more than the steel, in the other direction, seasonally.
Undermount clips handle that movement by allowing it. A rigid bead does not, which is why top-mount installations on composite tops are the ones that fail first.
04Choosing on the evidence, not the brochure
Both are legitimate. What is not legitimate is specifying one and maintaining it like the other.
We fabricate both. A welded bowl is built from 1.2 mm sheet in grade 304 or 316L, welded into 5–10 mm plate, dressed and grained across the joint; a separate sink is an aperture cut to ±0.5 mm from the maker's template, the edge broken, and where the top is ours the underside reinforced so the clips have a land to bite. Our thickness range is 5–10 mm and our plate tolerance is stated on the countertop technical data page rather than repeated in a brochure. Same shop, same 15 working days, different drawing.
We also inspect what we send. The aperture position is measured from the datum, the edge tolerance is recorded, and the grain direction is noted, on a sheet that travels with the top. That sheet is what we settle a warranty claim against, which is why we keep it rather than file it.
Either way the delivery includes an inspection sheet recording the aperture position and the edge tolerance, and packing on-edge so the rim is not the part that takes the load. If you are still deciding, the decision sequence for sink joints puts the two questions in order; if the answer is a welded bowl, the next page is fabricating a sink to your drawing.
Integrated sink countertops · Countertop technical data · What the warranty covers · Send a drawing for quotation