How to Properly Apply Thinset Mortar for Thin Brick Installations?
- Milan Mrmos
- 3 days ago
- 6 min read
Thinset mortar must be applied with care and precision, not rushed or guessed. A strong bond begins with a surface that is clean, level, and fully ready to support weight. Dust, oil, or debris will block adhesion and ruin the work before it even sets. Every trowel movement must create consistent coverage, with no gaps or air pockets behind the bricks.
Each brick needs to press into the mortar firmly, not loosely placed or floated over uneven ridges. A proper mortar mix has the right thickness—not too stiff, not too runny. Timing also plays a role.
Mortar must stay workable long enough to place and adjust bricks, but it cannot dry out before the setting begins. Errors during application lead to hollow spots, cracks, or full detachment later.
A thin brick wall will only last if every layer has been correctly placed. Mortar is not a background detail—it controls the strength and success of the entire installation. Skilled application does not leave room for sloppiness. Every inch must hold tight.
Choose the Correct Thinset Mortar for Long-Lasting Thin Brick Walls
Using the right thinset mortar is not optional. It determines how well the thin bricks stay in place under weight, pressure, and temperature changes.
Only use a polymer-modified thinset mortar that meets ANSI A118.4 or A118.15. That standard ensures it will bond properly and resist shrinkage or cracking.
What Type of Mortar Works Best
Polymer-modified thinset mortar (ANSI A118.4 or A118.15)
Designed for vertical applications and exterior use
Strong adhesion with controlled flexibility
What to Avoid
Pre-mixed buckets that do not meet ANSI standards
Mortars labeled for floors only
Unmodified or expired mixes
Mortar with added water after initial mix
Comparison
Mortar Type | Meets ANSI Standard | Suitable for Thin Brick |
Polymer-modified (A118.4/15) | Yes | Yes |
Pre-mixed bucket mortar | No | No |
Unmodified dry-set mortar | No | No |
Floor tile mortar | Sometimes | Rarely |
Using the wrong mortar breaks the bond between brick and surface. Always check packaging and match it to your installation location—interior, exterior, or wet areas.
Prepare and Inspect the Surface Before Applying Thinset
Surface preparation is where most failures begin. Any layer between the mortar and the structure—dust, oil, flaking paint—will cause bricks to separate later. Clean, dry, and flat surfaces are essential for the mortar to grip.
Steps to Prepare the Surface
Remove all dust, grease, and coatings
Patch cracks or holes with repair mortar
Sand glossy finishes or sealed paint
Confirm surface is dry, not damp or humid
Approved Surface Materials
Cement backer board (installed with proper screws)
Masonry block or concrete
Scratch coat over metal lath
Clean brick or stucco
Surfaces to Avoid Without Modification
Drywall without backer board
Painted surfaces without sanding and primer
Flexible sheathing or OSB
A strong base keeps the bricks in line and locked into place. Every skipped step here increases the risk of failure. Always inspect before spreading any mortar.
Mix Thinset Mortar to the Correct Consistency for Strong Adhesion
Every part of the installation depends on how well the mortar holds. Start by using a clean bucket, a mixing paddle, and clean water. Pour water in first, then slowly add the dry mortar while mixing with a drill.
Mix for five full minutes—no shortcuts. Let it sit for ten minutes after mixing, then stir again without adding more water. That pause activates the bonding agents.
The finished mortar should feel dense and smooth, not gritty or runny. Think of thick peanut butter. If it drips off the trowel or feels too stiff to spread, the mix is wrong. Always mix small batches you can use within 30 minutes. Once it starts drying in the bucket, throw it out. Never rehydrate it.
Use cold water when working in warm conditions. Heat speeds up set time and ruins workability. Mortar that sets too fast will not grip bricks, and dry mortar leaves gaps behind the surface. Mortar that is too loose will sag or squeeze out too far.
Use only polymer-modified thinset
Follow manufacturer water ratios
Do not remix with added water after slaking
Spread Mortar with a Notched Trowel for Full Brick Contact
Flat mortar will not do the job. It must form ridges that collapse under pressure. Start by pressing a thin skim coat over the surface using the flat side of the trowel. That layer improves grip and seals the base.
Now switch to the notched side, held at a 45-degree angle. Pull the trowel across the surface to form even ridges. Work in small sections—no more than a couple square feet at a time. Do not spread mortar across the entire wall. It will skin over before the bricks touch it, and no bond will form.
The right trowel size depends on the brick thickness and surface. Most thin brick installs use a 1/4 x 1/4 inch square-notch trowel. For heavy or irregular bricks, a 3/8 inch notch provides deeper ridges. Use only clean tools. Dried mortar stuck to a trowel ruins consistency.
Gaps between ridges or bald spots mean the brick will not fully contact the mortar. That leaves hollow spaces and weak spots. You should see mortar squeeze out just slightly around the brick edges. That shows good coverage.
Check for full contact:
Press each brick and listen for a hollow tap
Pull a test brick off—there should be mortar on nearly the whole back
If only the ridges touched, rework the technique
Press Bricks Firmly with Full Mortar Contact
Each brick must press into place with enough force to flatten the ridges beneath it. Do not drop or slide the brick across the surface. That will smear the mortar and trap air behind the unit. Position the brick carefully, push it into the bed with steady pressure, and give a slight twist to improve the bond.
Begin at the bottom of the wall and work upward in rows. That gives each brick a stable base below it. Use a level often to keep rows straight. Check every few bricks with a straightedge to avoid slow drift across large areas.
Do not try to fix alignment after the mortar begins to set. Pull the brick out and reset it instead.
Leave room between bricks for the mortar joints. A 3/8 inch gap is common. You can use spacers or mark your own guides. The spacing must stay consistent, or the joints will look sloppy after grouting.
Let the Mortar Cure Without Pressure, Stress, or Shortcuts
Once the last brick sets in place, do not touch the wall. No trimming, no tapping, no brushing. Walk away. Mortar cannot bond under stress. It hardens through time, not through guesswork or rushing.
Leave it alone for at least 24 hours. In colder or damp conditions, wait longer. Surface bonding depends on uninterrupted curing. People make the mistake of trying to grout too soon. That destroys the strength of the mortar bed beneath every brick. Let air, time, and stillness do the work.
Early movement leads to hollow spots, cracked joints, or full brick detachment.
How to Fill Thin Brick Joints Without Smearing Mortar on the Surface
Thin brick joints need precision. These joints are shallow, visible, and tightly spaced. You cannot treat them like tile. Never smear grout across the face. That ruins the finish and stains the surface.
Use a grout bag to fill each joint directly, one by one. Aim to slightly overfill the gap, then shape it clean before it begins to dry.
Tooling must happen fast. Use a tuck pointer or steel jointer to press and shape the joints. That gives strength and a smooth finish. Wait too long, and the mortar hardens unevenly.
Overfilling causes mortar to overflow onto the face—wipe immediately with a damp sponge.
Apply mortar in small sections, not across the full wall. Keep the joint depth consistent. Uneven joints will weaken the bond and ruin the final appearance.
Never grout across the brick surface
Always tool joints while mortar is still soft
Keep a damp sponge ready to clean accidental smears
Clean Thin Brick Faces Without Damaging the Surface Finish
Cleaning must begin before the mortar dries. Thin brick is porous and textured. Any mortar left on the surface will harden inside the brick face and cause permanent discoloration. You cannot rely on acid or scraping to fix it.
After finishing each section of joints, inspect the bricks closely. Use a stiff nylon brush and a damp sponge. Wipe carefully and avoid saturating the wall with water. Too much moisture weakens the joints and washes out color.
The Bottom Line
Thin brick installation does not reward speed or guesswork. Every step serves a purpose, and skipping one guarantees problems later. The wall should not shift, crack, or stain. It should hold tight, look clean, and stay that way.
If the surface was prepared correctly, the thinset mortar mixed and applied with care, the bricks pressed with full contact, and the joints packed and tooled before the mortar set—then the work is done. No part of the wall should feel loose, uneven, or unfinished.
A proper finish does not call attention to itself. It holds up without excuses. That is when the job is over.