Cracking the Cryptic: Why Stone Tile can Look Wet When Dry

By Foster Lyons

Why does stone sometimes look wet when it’s dry?

This problem is almost exclusive to stone tile flooring in wet areas, such as a shower floor, and marble. And once it happens, the only fixes are expensive.

Some say don’t blame the stone. But it’s kind of the stone’s fault. Stone is porous and slightly translucent. Those properties, together with water and modern materials and assemblies, are the cause of the wet stain problem.

Pores in stone are sort of like drinking straws. They suck. That is, if dipped into water, they will pull water in because of capillarity in the same way water in a straw will climb upward. If the pores are large, water can enter and leave rapidly, same as drinking straws. Tiny pores in stone are like cocktail straws. They fill and drain slowly. Opposite of flow volume, large pores don’t hold water with much force, while small pores hold water tenaciously. That’s why water climbs higher in a thin straw than a wide straw.

But why aren’t all marble floors (with all that tremendous sucking force) totally saturated with water and looking wet?

Because those pores are so small (about one-tenth of a micrometre) that they can’t move liquid in and out with any speed. The force is enormous but the fluid movement into those pores is very slow because of their minute size. If water is poured on translucent marble and then wiped off, it will suck up almost nothing. There just isn’t enough time for the water to wet the interior of the stone. However, if that marble is submerged in a puddle of water and left for some time, it will eventually suck that water into its pores. It will look wet because it is wet in its pores.

Strangely, this staining rarely happened in the past. That’s because tile setters set the stone in a thick, very porous setting bed — sand or a sand-lime cement mud bed, never a non-porous material — and they allowed it to drain out easily. This is called the full bed method. Basically, all six sides of the stone are dry most of the time and never in contact with water long enough to allow any significant uptake in the pores. And when not being wet from the top, they’re drying out. The stone stays dry inside and out.

This full bed assembly is still used today in many wet floor applications. It’s a well-documented, industry standard installation method. Pitch the sub-floor (quarter-inch per linear foot at a minimum), waterproof that pitch (no wrinkles, puddles or mini dams), ensure lots of sand and not much white cement in the mud bed, and employ two-stage drainage with weep holes to protect from clogging.

This type of assembly allows water that gets into the grout between the stones, or the mud bed below the stones, to fall to the waterproofing and drain out. The force of gravity that pulls the water down through the porous grout and mud setting bed moves the greater volume of water away from the porous stone faster than the stone can pull that same liquid into itself. That imbalance of fluid volume flow rates keeps the stone dry and without wet stains.

So, why do so many tiles look wet these days?

Quite simply, installers have forgotten the fundamentals. They have failed to remember that if marble sits in a puddle, it has plenty of time to suck up water and when it does, it looks wet since it is wet. And because those pores hold water with great tenacity, it takes a long time for the marble to dry out and return to its original colour. Worse, it may never come back to its original colour if that water reacts with various minerals in the stone or moves salts around within the pores.

It’s not unusual for manufacturers to modify their products over time for cost reduction and marketing advantage. Usually, they keep doing this until the product is no longer fit for the intended purpose. For the wet stain stone problem, thinset mortar was that modification that broke the camel’s back.

Introduced in the 1950s, thinset mortars have allowed tile and stone to be set in a thin bed of mortar rather than a thick bed of mortar. This saves space, time and money. But thinset mortar has its drawbacks. One is it is not as porous as a sand-cement setting bed. In fact, sometimes it’s nearly waterproof. So, when there’s a little imperfection or crack between the edge of a stone tile and the surrounding grout, water will drop down past the exposed surface of the stone and then just sit there until it eventually gets pulled into its pores.

If an installer is especially careful and explicitly follows every industry guideline and manufacturer’s instruction, there’s a chance for success because no water will get past the exposed surface of the stone. But good luck with that, as there’s always a defect somewhere.

What should an installer do to minimize problems brought about by inevitable defects?

One-hundred per cent solids epoxy thinset mortars and solids epoxy grout can be helpful in avoiding the wet staining problem, if they’re compatible with the stone (meaning, they don’t stain the stone). These materials stick to the stone better and are more flexible. Because of this, there will be less (maybe even no) cracks at the stone edges and the materials. Additionally, they are non-porous, so they don’t hold any water.

Adding a drainage mat below the thinset adhesive and draining it into the lower weep holes of a two-stage drain can also help. Basically, with these materials, there won’t be any puddles for the stone to sit in. There will only be water on the exposed surface of the stone and that water just doesn’t have enough time to get pulled into the stone in any volume to cause it to look wet-stained. In short, the stone remains dry.

Foster Lyons is the founding principal of Foster Lyons Building Science LLC, a full-service building science and construction consulting firm that serves the architecture, engineering and construction industry. Foster’s work includes providing advice to architects regarding building enclosure design, building assembly design and material selections with particular emphasis on water and moisture management issues.

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