Dry Pack Mortar Calculator

The floor of a shower is a water-moving device: it needs to have a consistant slope to direct water with maximum efficiency via gravity. There cannot be any low areas (puddling places) or bumps (it won’t be smooth). This slope is made by dry pack mortar. Yes, no poured concrete slab. No. A very dense combination of cement and sand are packed in to make a rigid bed for the waterproofing liner to sit on.

When doing this, getting the amount just right is important, don’t stop halfway through the pour or you will have cold joints. But neither do you want too much, then you’re wasting material. The calculator knows how to calculate volume based off the shape of the pan. So, you don’t need to guess what impact the drain location have on the overall mass.

How to Make a Shower Floor

The cement ration adds structure to the bed; it’s what holds things together. It gives the mix its strength. In the industry, that means a five to one sand to cement. You want the mix to be workable but still lock in those grains. A three to one mix are too sticky to shape into a slope easily. A six to one mix won’t hold up under pressure and could crumble during compaction. Toggling these ratios allow you to see how changing them affects your material list. That decision depends on size of the pan and expected traffic load.

The thickness of the mortar bed depend on where the drain goes. If the drain is in the center of a square pan, each point radiates an equal distance and it’s even everywhere. The highest point (thickest) would be at the corners, while the lowest (thinnest) are at the clamp ring around the drain. So the maximum thickness is figured based on the length of the longest run from the drain to the furthest corner.

Volume isn’t simply area x average depth. It’s a pyramidal shape, with variable sides. To describe the shape, you plug-in the coordinates of the drain. From there, the tool figures the compacted volume. It also calculates how much dry sand volume is required to reach that density and how many cement bags must be mix up.

Water is the most dangerous variable in dry pack mortar. Too much water and you’ll have a slurry of a mess instead of a clumpy sand that you can pick up in your hand. Too little water and it won’t bond. And you’ll also see dust come out as days go by. So don’t add too much water. Don’t turn it into slop. Add just enough water to make the sand clump together in your hand. And if you add too much water at any point, it will start to slump and lose the shape you carefuly packed into it. The water amount range comes from the cement weight, provided by the calculator which gives you a target for how many gallons to add. That range is tight, so just add a little water when dry and mix. It is better to leave it a little dry and spray it with water than to ruin the batch by adding too much.

People only start thinking about waste and dry volume once the project stalls. Because dry sand is less dense than compacted mortar, it requires 25% more volume (or at least a dry factor of one point two five). This is because loose sand contain air voids which shrink as you pack it down. Then, add a waste % for mistakes made on the job site, uneven substrate, and other spillage. For a simple pan, ten percent is a reasonable buffer. For larger pans with more complex curb detail, fifteen percent may be required.

Because the mud here is vertical instead of sloped, we calculate the curb separately. When you go to purchase your materials, round-up. Can’t get half a bag of cement? No way to get a fraction of a cubic foot of sand? The tool rounds the bag count UP so that you will have sufficient binder. It would of been better to have a bag left over than to run out halfway up the slope.

Have your numbers, check your substrate for stability & cleanliness, then mix. The math tell you how much to use, but your skill shows in how you pack it. Slope it true, pack it tight, and don’t let the water get in the mix till the last moment. Respect the density & the floor will stand up.