Why do terrazzo floors have divider strips?

Finish + Form
4 days ago
9 min read

On Spec #5 — published by Finish + Form

Author's note: This article interprets published industry guidance, technical standards, and manufacturer documentation on terrazzo systems, movement accommodation, and seamless installations. Its purpose is to provide context for architectural specification, not to prescribe a particular solution. The discussion that follows is based on the following sources:

Primary Sources: National Terrazzo and Mosaic Association (NTMA) Terrazzo Specifications and Design Guide; NTMA Technical Bulletins S-6 and S-8; NTMA Crack Detailing and Joint Treatments for Epoxy Terrazzo (T-24 series).
System Documentation: Duracryl International Section 09 66 23.16 specification; Durabella Installation Manual V1.0; Duraforce Specification (03/2024); Durascreed Installation Manual V1.0.
Comparative Systems: Cementitious terrazzo, epoxy terrazzo, and biopolymer terrazzo, interpreted using published industry guidance and manufacturer documentation under comparable design conditions.

Walk through almost any airport, courthouse, university, or civic building and look down. Thin lines of brass, zinc, or aluminum run through the terrazzo, dividing it into panels, tracing changes in color, occasionally forming letters or logos. They're easy to read as decoration because decoration is what remains visible. But it's important to remember: divider strips are not simply decorative details. They're engineering devices used to manage movement and stress within a flooring system, and understanding why they exist is the first step to understanding why some systems can reduce or eliminate them.

Divider strips were introduced to solve practical problems: depending on the terrazzo system, they can separate pours, help manage curing stresses, accommodate movement, help control cracking, and create clean transitions between materials and colors. These strips have also become a defining aesthetic of terrazzo, but its imporant to remember that their visual role has always been secondary to the primary engineering functions.

Today, architects increasingly want large, uninterrupted floor fields, and while today's material systems can now deliver them, a seamless floor is not simply a floor with the strips left out. As the strips solved an engineering problem, we must resolve the same engineering questions by different means. So, before a design team eliminates divider strips, it should first understand why they were introduced in the first place, which problems newer chemistry has retired, and which problem no chemistry retires.

Divider strips are not simply decorative details. They're engineering devices used to manage movement and stress within a flooring system, and understanding why they exist is the first step to understanding why some systems can reduce or eliminate them.

The design challenge

A terrazzo floor is a thin, rigid, bonded layer riding on a much thicker concrete structure. Unfortunately, these two layers don't always want to be the same size. Cementitious toppings shrink as they cure. Concrete slabs crack, deflect under load, and move at their joints. Buildings themselves expand, contract, and settle. Every one of those movements arrives at the flooring layer as stress and a material that cannot accommodate stress relieves it the only way physics allows: by cracking wherever it chooses.

The divider strip is the traditional answer. By dividing a large field into small panels, strips give shrinkage somewhere orderly to happen, give pours somewhere clean to stop, and give substrate movement a controlled line to express itself along instead of a random crack through the finished surface.

The standard

The governing guidance for terrazzo installation in the United States is published by the National Terrazzo and Mosaic Association (NTMA), whose Terrazzo Specifications and Design Guide and technical bulletin series establish national practice for every major terrazzo system. These NTMA documents answer three questions:

How far apart must the strips go;
When must a strip become a true movement joint, and;
What happens at the boundary between the terrazzo and the structure.

For traditional sand cushion cement terrazzo, the numbers are specific. NTMA guidance calls for single divider strips set into the underbed at five-foot centers or less to control the anticipated shrinkage of the cementitious topping, with panels of four feet by four feet described as ideal and rectangles held to no more than 25 square feet. Where additional control joints are required, Technical Bulletin S-6 directs the use of double divider strips placed back to back, which are designed to handle significant slab movement. Where a metal expansion plate has been incorporated into the floor slab, its presence indicates that the designer expects considerable movement in that location and the terrazzo system must be detailed to manage it.

The introduction of epoxy terrazzo changed which of these problems needed to be solved by the material itself. NTMA's crack detailing guidance for epoxy terrazzo explains that thin-set epoxy resists cracking for two reasons: 1) the binder has extremely high tensile strength, and 2) the system contains no excess water, eliminating the shrinkage cracking associated with cementitious curing. One of the strip's original jobs, which was managing curing shrinkage, was effectively retired by chemistry. Now, what remains is the substrate. The same guidance requires that divider strips precisely follow every concrete joint, even a crooked one, notes that strips are not flattening or leveling devices, and establishes flexible epoxy membranes, installed at a nominal 40 mils and engineered for high elongation, as the industry-standard treatment for crack detailing. It also states plainly that the architect or engineer must specify movement joints and show them on the drawings.

The evidence

Read side by side, the industry guidance and the manufacturer documentation show us that each generation of terrazzo chemistry has reduced the engineering roles that divider strips must perform, while preserving the one role no flooring system can eliminate: accommodating structural movement.

In cement systems, strips do three jobs at once. They absorb curing shrinkage, they separate pours, and they provide controlled lines for substrate movement. The five-foot spacing exists because the cementitious topping itself demands it. In epoxy systems, becasue the shrinkage job is gone, the strips persist mainly where the substrate demands them, directly over concrete joints and on membranes that handle reflective cracking. This means that the strip count may fall but the movement joints stay.

The biopolymer system documented here extends the same trajectory one step further. Duracryl's three-part specification for Durabella states its divider strip position in six words: strip materials are only required for construction joints. The installation manual describes a system not limited in area size that installs without joints, attributing its crack bridging behavior to the flexibility of the material. The mechanical basis for that flexibility is consistent with independent laboratory testing examined earlier in this series, which measured the biopolymer's substantially lower stiffness and greater elongation before peak load compared with epoxy terrazzo in both bending and tension [refs 9, 10]. The manufacturer's own figures point the same direction: the binder's elongation at break is reported at approximately 145 percent per ISO 527, and the Duraforce membrane binder at approximately 165 percent. The supporting layers beneath the finish are also documented as seamless and flexible, indicating that the engineering approach extends beyond the finish layer itself and into the overall flooring system [ref 11].

For substrate cracking, the system takes the targeted approach rather than the grid. Crack suppression membrane is not a standard requirement, but the specification requires locating all cracks and joints in the substrate before installation, directs the contractor to make written membrane recommendations wherever substrate cracks could transmit through, and carries membrane coverage for up to five percent of the floor area in the base bid. Duraforce itself, the biopolymer with an embedded fiberglass mesh, installs at 1.0 millimeter, almost exactly the nominal 40-mil thickness the epoxy industry standardized for the same job. They may have a different chemistry, but they provide the same engineering answer at the same thickness. As for the building movement, the documentation is as plain as NTMA's: existing movement dilatations must be respected and followed in the Durabella.

What the Evidence Means

The guidance establishes that divider strips were never one thing. They are three engineering devices sharing a profile: a shrinkage control, a pour separator, and a movement accommodation. Understanding which of those jobs a given system still needs is the entire specification question.

What the documentation shows is that a seamless installation is achieved by reassigning the strip's jobs, not by ignoring them. Shrinkage control moves into the binder chemistry. Crack control moves into flexibility and targeted membranes. Pour separation becomes an installation logistics question; the Durabella manual recommends single-phase installation, not for stress reasons but to prevent variation in finish, which is an honest reminder that pour boundaries never fully disappear, they just stop needing brass.

What the documentation does not do is eliminate movement. Both the industry guidance and the manufacturer state, independently and in nearly the same words, that joints required by the structure pass through the flooring regardless of what the flooring is made of.

Specification Considerations

Picture a museum where the terrazzo runs from the entrance hall through three galleries without a single line interrupting it. Nothing about that floor happened at the pour. It happened months earlier, when someone mapped every crack and joint in the slab, decided where the building's own movement joints had to surface, specified membrane over the areas that needed it, and chose a material whose chemistry does not shrink as it cures. The uninterrupted floor is the visible outcome of a series of engineering decisions made long before the first bucket was mixed. This shows that seamless flooring is not the absence of engineering, but a result of simply moving engineering out of sight from the finished product.

Design teams evaluating large uninterrupted terrazzo fields may find it useful to begin with the substrate rather than the finish, since every system examined here, traditional or seamless, ultimately answers to the concrete beneath it. A joint and crack survey of the slab, conducted before the flooring system is selected, tends to reveal how seamless a given floor can honestly be.

Where a seamless system is under consideration, teams may wish to request the manufacturer's installation documentation alongside the marketing material, and read its position on three specific items: construction joints, crack isolation treatment, and existing movement joints.

Durabella is one example of a system engineered for this approach; its documentation requires strips only at construction joints, prescribes targeted crack isolation membrane where the substrate survey demands it, and requires existing movement dilatations to be respected through the finish. A flexible, higher-elongation binder may contribute to a system's ability to span the small substrate imperfections that would telegraph through a stiffer topping, although substrate preparation, membrane placement, and installation quality shape the outcome as much as the material does.

For traditional systems, the NTMA guidance remains the reference for strip spacing and joint treatment, and comparisons between systems are most meaningful when each is evaluated against its own governing documentation rather than against another system's assumptions.

Honest limits

Not every project should eliminate divider strips. Movement joints required by the structure must be honored in any flooring system; both NTMA guidance and Duracryl's own manuals state this without qualification. Where a building's expansion joints, substrate conditions, or structural design require joints, they pass through the terrazzo, seamless or not.

The membrane approach has a stated boundary. Duraforce's documentation limits it to reflective cracking in the horizontal plane; vertical movement, excessive deflection, and disbonded toppings are explicitly outside its scope, which matches what NTMA says about the epoxy industry's membranes. No membrane of any chemistry substitutes for structural joint design.

The evidence here is also a different kind than this series usually presents. Installation guidance and specifications document design intent and required practice; they are not head-to-head laboratory measurements of crack performance, and no such comparative test is cited in this article. The mechanical properties referenced from earlier articles establish that the materials behave differently under load; they do not by themselves predict how any particular floor will perform over any particular slab. A five percent membrane allowance in the base bid is itself an acknowledgment, from the manufacturer, that real substrates come with real cracks.

Finally, seamless systems concentrate responsibility earlier in the process. A strip grid forgives an unexamined slab; a seamless installation does not. The substrate survey, moisture testing, and flatness verification the specification requires are not formalities, they are the joints you didn't pour.

What next?

Divider strips deserve better than the word decorative. For a century they have been quiet, competent engineering, managing shrinkage, movement, and cracking with a strip of metal and some careful spacing. Systems that can reduce or eliminate them do so not by defying that engineering but by relocating it into the binder chemistry, membranes, and pre-pour investigation. Whether a project's answer is a traditional grid, a seamless field, or something between, the specification should be able to say which of the strip's three jobs each element of the system is doing.

If you're intereted in the full source documentation referenced in this article, they're available on request. Remember, the best specifications are not written around assumptions. The best specifications are written around evidence.

SPEC-tacular by design, not by accident.

Finish + Form connects architects and designers to sustainable, high-performance materials with the specification support, samples, and documentation to bring projects to life. To explore Durabella for your next project, schedule a consultation.

Reference

National Terrazzo and Mosaic Association, Terrazzo Specifications and Design Guide.
NTMA Technical Bulletin S-6, Control Joints in Sand Cushion Terrazzo, 8/13.
NTMA Technical Bulletin S-8, Divider Strips for Thin-Set Cement Terrazzo Systems, 8/13.
NTMA, Crack Detailing and Joint Treatments for Epoxy Terrazzo, T-24 detail series.
Duracryl International, three-part specification Section 09 66 23.16, Durabella Biopolymer Terrazzo.
Duracryl International, Installation Manual, Durabella Biopolymer Terrazzo, V1.0, October 7, 2025.
Duracryl International, Duraforce Crack Isolation Membrane specification, version 03/2024.
Duracryl International, Durabella specification document, version 08/2025.
Intertek Building & Construction, Test Report N3216.01-106-31 R0, July 18, 2022 (flexural, ASTM C580; discussed in On Spec Article 2).
Intertek Building & Construction, Test Report P20220434 (tensile, ASTM D3039; discussed in On Spec Article 3).
Duracryl International, Installation Manual, Durascreed Biopolymer Screeding, V1.0, October 7, 2025.