Gravel in a softener is not filtration. It is a graded support layer at the bottom of the tank, beneath the resin, and it exists to protect the lower distributor and to spread flow evenly across the base of the bed. Whether your unit needs one depends on its distributor design, and getting it wrong shows up as resin in the taps.
What the underbed does
Two jobs. It supports the resin above it so the bed sits evenly rather than settling into the base and packing around the distributor. And it distributes flow: water entering or leaving through a small distributor at the bottom of a tank would otherwise channel through the nearest resin, so a graded gravel layer spreads it across the full cross section. That matters in service, where even flow means the whole bed exchanges, and in backwash, where even lift means the bed fluidizes properly rather than in patches.
When it is needed and when it is not
It depends on the distributor. Systems with a fine slotted or screened lower distributor are often designed to run without an underbed, because the screen itself retains the resin and spreads flow. Systems with an open or coarse distributor need gravel to do both jobs. The manufacturer's specification decides it, and adding gravel to a tank designed without it reduces resin capacity for no benefit, while omitting it where it is required lets resin pass into the plumbing.
Loading it without breaking anything
The lower distributor is fragile and central to how the tank works, so gravel goes in gently rather than being poured from a bag at height, which cracks it. Tape the top of the riser tube closed before anything goes in so nothing falls down it, hold the riser upright and centred, and add the gravel in stages, coarsest first where the specification is graded. Then the resin on top, through a wide funnel. Remove the tape before fitting the valve, or the system will not draw.
The symptoms of getting it wrong
Resin beads in the taps, in aerators or in appliance screens mean the resin is escaping past the distributor, which is either a cracked distributor, a missing underbed where one was needed, or a riser that was disturbed during loading. Poor softening with a full salt tank can mean channelling through a bed that is not being supported evenly. Both mean opening the tank. Our softener installation guide covers the media change and what to check while the tank is open.
Questions people ask about water softener gravel
Does every softener need a gravel underbed?
No. It depends on the lower distributor: fine screened distributors often run without one, coarse or open ones need it.
What does the gravel actually do?
Supports the resin bed and spreads flow across the distributor so water does not channel through one part of the bed.
Can I add gravel to a tank that did not have any?
Not usefully. It reduces resin capacity for no benefit if the distributor was designed to work without it.
There are resin beads in my taps. What happened?
Resin is escaping past the distributor: a cracked distributor, a missing underbed where one was required, or a disturbed riser.