This post gives a detailed description of how to mix up and properly install a shower base “mud bed” — the layer of sloped low-density concrete between the waterproofing liner “pan” of a shower and the shower floor tile.
This is a very traditional way to build a watertight shower base that’s still done today on many many many renos and complete new builds. And, it does work well when each part of the waterproofing actually works:
One crucial bit is this “secondary mortar bed” concrete layer that’s also called (depending on which contractor you’re talking to) a “mud bed”, “mortar bed”, “deck mud layer”, “drypack concrete layer”, or sometimes (erroneously) just “the pan”. If it’s made from the wrong sort of concrete mix or installed with an improper slope, then this layer won’t do it’s job of shedding water immediately and letting absorbed moisture evaporate back out eventually.
So, whether you’re thinking of doing your own shower base or just wanting to understand everything a tile contractor should be doing at this step of a custom tile shower install, this post should help.
STEP ZERO: The Waterproofing Liner
The most important part of any shower base is the waterproofing. On my own custom shower installs, I use Kerdi fabric to put a waterproofing layer directly beneath the shower floor, curb, and wall tile. This creates the driest, most guaranteed mold-free shower install possible.
But, sometimes I moonlight for other shower installers who have particularly challenging custom shower floor slopes to create. Most traditional custom shower install contractors use a very traditional method of shower construction that puts the waterproofing liner “pan” (either pieces of flexible PVC plastic or a fabricated-in-place literal fiberglass pan) beneath a concrete mortar bed that’s intended to both absorb and release the moisture that passes through the shower floor tile and grout.
It’s a bit of a Rube Goldberg approach to waterproofing a shower that was necessary back in the 1960s, 70s, and 80s before there were waterproofing materials that you could install tile directly to. But, lots of contractors still use this approach. And, many of ’em have trouble finding skilled installers who can perfectly slope a truly custom concrete mortar bed. Hence the moonlighting.
Be CERTAIN that the liner is actually Waterproof!
Whether PVC or fiberglass, a shower base waterproofing liner must be waterproof. Testing for this is simple. Once the liner is in place and all the glue for the seams, nail holes, etc is dried and cured, simply block the drain and fill it with water.
Then, let the water sit for at least 20min (roughly double the average time that the average American spends taking an average-length shower). Ideally, let the water sit overnight. If the water level doesn’t change by morning, then you’re good. If it does, then yell at the installer to fix the leak.
Be CERTAIN that the liner has a Preslope!
The whole strategy behind traditional old-school shower base construction is an attempt to design around three basic facts:
- Tile and grout aren’t waterproof
- Tile can’t be installed directly on top of PVC plastic or fiberglass
- Water obeys gravity
So, a layer of porous concrete between the shower tile floor and the PVC/fiberglass waterproof liner below takes care of the first two issues. But the third? Better make sure that liner is pitched towards the drain!
Because, moisture absorbed by the mortar bed needs to find a way out. Hopefully, most of this moisture can simply evaporate back out through the shower floor grout. But if you are building a 3’x5′ or larger shower then think about the amount of water that’ll be absorbed by a 1″-3″ layer of concrete that’s 15ft2 or more in area. You really don’t want moisture underneath the floor tile sitting constantly all along the perimeter of a shower.
That’s where gravity can assist. Traditional shower drain assemblies have little “weep holes” drilled through the part of the drain that you don’t see — precisely the part of the drain that’s buried in the concrete “mortar bed” layer between the waterproofing and the shower floor tile. If the waterproofing layer is pitched towards the drain, then any moisture trapped between the mortar bed and the waterproofing layer will eventually wick downhill and drip out the weepholes:
But this can’t happen if the waterproofing layer is dead flat or (even worse) haphazardly installed with literal dead slope patches in the corners.
STEP ONE: Mark Your Levels
So, assuming that the PVC or fiberglass waterproofing liner has been correctly installed with effective seam taping/gluing and has an effective preslope (and has been water tested to confirm this), it’s time to mix, pack, and slope the “secondary mortar bed” layer.
Planning is key for this. Generally, you want to minimize the thickness of the concrete “secondary mortar bed” layer because this will in turn minimize the amount of moisture that your shower retains between uses. But, you need to make sure that the mortar bed is at least 1/2″ thick at the drain for install on a concrete slab and at least 3/4″ thick at the drain for installations on a wood subfloor. And, the “pitch” (the slope angle you’re aiming for so that water will quickly run downhill to the drain from all parts of the shower floor) needs to be at least 1/8″ per linear foot of run (or, to really ensure no drainage dead spots, 1/4″ per foot of run).
If your planned shower has a nearly square footprint, then you’re golden. For example, for a square 3’x3′ or rectangular 3’x5′ shower base with the drain set in the center you can simply set the screw-in drain assembly top to accommodate the proper mortar bed thickness (again, at least 1/2″ deep for concrete slab or 3/4″ deep for wood subfloor) and then measure from the top of the drain to the farthest shower wall. Add 1/8″ to 1/4″ of rise (depending on how skilled you’re feeling about concrete sloping) for every foot of distance, and then mark off a dead-level guideline along the walls at that height.
For example, if you’re installing a custom shower with a 4ft by 4ft square footprint with a centered drain on a wooden subfloor, then set the drain 3/4″ high and mark a level line 1″ high all along the perimeter. You can then use this as a convenient guideline to ensure your mortar bed is nice and even all along the shower base perimeter.
Or, if you’re installing a custom shower with a 3’x6′ rectangular footprint with a centered drain on a concrete foundation, then set the drain 1/2″ high and mark a level line 1″ high all along the perimeter. Although the resulting finished shower floor will then have a slightly steeper slope on two sides of the drain, most folks won’t notice this. Instead, they’ll admire the way the shower floor quickly and evenly drains from even the farthest corners.
STEP ONE-POINT-FIVE: Know When To Call an Expert!
It’s seriously not that hard to pack in an evenly sloped layer of very dry concrete mix when:
- You can reach all parts of the shower floor by leaning over from the curb
- You also have super-clear wall markings as a guide to work from
- You also only have to make a simple even slope to a center-set drain
However, if you have an offset drain or an oddly configured shower floor geometry or a truly super-custom situation like multiple offset drains in a very large shower floor, then at least consider calling in an expert.
Best case, a poorly sloped shower floor results in standing water dead spots. Worst case, this standing water contributes to a permanently musty shower smell, long-term mold intrusion, and eventual shower waterproofing failure. So, if you’re dealing with a tricky install that risks either a not good or not good at all outcome, think about getting someone with experience to do the job.
STEP ONE-POINT-FIVE-ONE: Still Consider Calling an Expert!
Sometimes, even conscientious plumbers who install excellent waterproofing liners with an effective preslope don’t get the preslope slope perfectly perfect around the perimeters of the shower stall. In those cases, it can be pretty difficult to figure out how to mark level guidelines to make the dry pack slope dead-easy to install.
In these cases, even the “AS SEEN ON TV!” -quality supposed quick fixes to ‘easily’ installing effectively sloped drypack without guidelines or even a level will not work. All of those ‘goof-proof’ plastic pitch guides only work when set on a level base. And, if your shower’s waterproofing layer is dead level or (even worse) actually sloped away from the drain in spots, then it’s already designed to fail.
Point is, there are many situations (effectively but not perfectly evenly pitched waterproofing liner preslope, very offset drain location, odd shower floor geometry, etc) where the drypack slope will simply need to be fashioned using nothing more than eyeballs and dead reckoning (checked with a bubble level, of course).
This is no problem for someone with lots of experience, but probably a bigger challenge than you’d want to tackle as your very first project.
STEP TWO: Mix the Mix!
This is where most diy folks go wrong. It will seem really really weird to mix up a concrete “mix” using barely enough water to make a bucket of sand kinda moist. But that’s exactly what you need to do for an effective mortar bed install.
Two things are important to know for this step:
- It takes not much at all water to effectively hydrate a cement mix so long as the water is completely and evenly mixed all through the cement mixture
- A concrete shower base only needs compression strength since tensile strength is irrelevant to a concrete block contained on all four sides that’s just needed for a stable tiling base
Seriously, it takes hardly any water at all to mix up proper deck mud for a shower base. If you add too much water, then it will be very difficult to shape properly. Add the water slowly, and use the small shovel to mix it well to wet all the concrete particles before adding any more.
The best analogy for this is sand castle building. How do you know that the beach sand you’re using has the proper amount of water mixed in? When your castle towers just barely hold together as cylinders. Too little water and the sand just crumbles apart. Too much water and the sand slumps into sad shapeless lumps.
But, just the right amount of water and you get this:
Here is the equivalent for properly workable deck mud:
And, here’s the recipe described really clearly. The only thing I’d add is two practical tips. A small hand shovel is all you need to fully combine the mix (you can invest in lots of hoes, fancy rakes, etc, but not necessary), and you don’t want to mix up more than one bag of topping mix at a time. Why? Just try it sometime, and you’ll then know.
For a condensed recipe/description, just do this:
- Mix one 60lb bag of “sand topping mix” with half a bag of sand in a large 20gal mixing tub.
- Stir the drymix a bit with a small shovel to get the added sand fairly evenly distributed.
- Slowly add just enough water to make the whole mix just slightly moist. Spritz on some water, mix it around with the shovel, and then repeat (SLOWLY!) until all the drypack concrete mix is just barely wetted.
Once you’ve got the hang of it, mixing subsequent bags will go a lot quicker. Go slow for the first couple tries though. If you add too much water, then you’ll have a really really hard time successfully doing the next step.
STEP THREE: Dump it in, Pack it in!
Haul the drypack mix to the shower in 5gal buckets, and dump it in! Work from the back corners of the shower stall area to the front, and pack the mix down with either a wooden masonry float or a hard rubber grout float. Since it’ll go in loose, you need to really smack the mix together to make sure there aren’t any void spaces or air pockets.
Getting the slope uniform throughout the entire shower base is usually the most anxiety-inducing part for newbies.
Here’s a good solution for anxiety — take a couple deep breaths and keep in mind that just a simple bubble level, some straight pieces of wood cut to size, and a bit of patience is all you need to make a perfect slope.
Think about it. Did Egyptians 5,000+ years ago or Greeks ~3,000 years ago or Romans ~2,000 years ago need fancy gyroscopic laser levels or “as seen on TV” gimmicks to get perfect angles for the Great Pyramid of Giza, the Acropolis, or massive aqueducts (where pitch was actually really most important)? Nope. Just some straight wood, simple water levels, and string. And thousands of masons, of course, but fortunately you’re just building one simple shower base.
STEP THREE POINT FIVE: Packing tips
Start from the back (or, for an oddly shaped shower, most inaccessible) parts of the shower floor and work your way towards the shower curb/entryway. Dump the drypack and smack it down with a float. Work in pie-shaped sections from the walls (ideally marked with level guidelines, or just eyeballing for an approximate initial pitch) to the drain.
The process looks like this:
Once a section of concrete mix is well packed, use your float and level to get a near-perfect (as best you can at this point — don’t stress about it yet) straight pitch from the wall to the drain. Then, move on to the next section.
STEP FOUR: Final leveling
Once all the concrete drypack is packed tight and roughly leveled, you can do the final finish by scraping off or smacking down any high points with your trowel. You can also add drypack a handful at a time to fill in any low spots.
If you’re lucky, then you’ll be able to reach all parts of the shower floor by simply kneeling on the shower curb. However, if you’re working on a particularly large shower or one with an odd geometry, you can still get to all parts of the shower floor by gently stepping on the wet drypack. So long as you use a stiff piece of cardboard or a bit of plywood to distribute the weight of your feet and knees, it’ll be fine.
The point is, properly mixed drypack (with an emphasis on DRY and PACKED) is a very forgiving material to work with. You can gently walk on it even when ‘wet’, easily shape it with a trowel or just by rubbing with your hands, and very easily add to it by just smacking in additional handfuls of material.
So, take your time to get the slope just right. The most specialized piece of equipment you need for this step (aside from a trowel and bucket of water for periodically rinsing concrete mix residue from your hands) is a simple $10 bubble level with pitch gradient markers. Like this:
This makes it super-easy to ensure that the slope pitch for your shower floor is 1/8-1/4″ at every part of the floor. And, if you’re worried about any uneven spots, you can simply use a midsize 2-3ft level and a long 4-5ft level (or just cut a few 1×2 wood pieces cut to length) to check the slope from every part of the shower perimeter to the drain.
STEP FIVE: Let it cure!
The concrete will take a good 24hrs to dry and cure. You’ll know it’s ready for tiling when the concrete changes color from dark (ie — wet) to light (ie — dry). Pretty simple.
If you notice some high spots that you missed when packing in the drypack, you can level out any hills by simply scraping off some of the drypack concrete. You might be freaked out at how sandy and seemingly crumbly it is (try tapping with your knuckles — if mixed right, the drypack will actually sound hollow). Do not be alarmed by this.
The concrete deck mud layer is supposed to be very light — the exact opposite of dense sidewalk and building concrete mixes. Since it’s just a layer of concrete sandwiched between tile and subfloor, the deck mud just needs to have good compressive strength. Tensile strength doesn’t matter.
So, if you end up with a light fluffy layer of sandy concrete then congratulate yourself — you just installed a shower floor base correctly!
Last Thought — DON’T PANIC!
If you’re reading this post to simply figure out how to effectively manage a contractor who’s installing a custom tile shower for you, then hopefully this info will make you feel a lot more assured and certain about what to look for to ensure your contractor is doing a competent job.
But, if you’re a diy do-it-yourselfer doing research to prepare for installing your own custom shower, then even this detailed explanation might still seem intimidating. It’s okay. Take a few breaths and repeat “it is just a series of small steps.” Seriously, it isn’t rocket science.
As will your install be!