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ave you ever been asked to provide requirements. In applications that require sig- an underlayment recommendation nificant thickness to be filled, an alternative Practical to level, resurface, or fill space over lightweight underlayment might be considered a structural element? This article as an addition to the floor system. One such Hcan provide a framework for developing appro- option is Expanded Polystyrene Foam (EPS), Solutions priate options. a highly cost-effective, rigid sheet of foam that By definition, underlayments are not structural. is well suited as a deep-fill solution in spaces The term underlayment is often used in reference with a consistent depth to be filled. Another solutions for the practicing to several types of products including poured option is a deep-fill screed, a gypsum or cement structural engineer toppings, either cement or gypsum, and sheet underlayment that has been pre-blended with materials. This article focuses on underlayments a lightweight aggregate, such as EPS beads, defined as the layer between a structural subfloor which is mixed with water on the job site and and a finished floor that facilitates leveling and installed as a liquid. This lightweight deep-fill adhesion. Underlayments commonly enter the screed can weigh as little as 30 pounds per cubic conversation when there is a specified floor height foot (pcf) and provides a suitable replacement or when a correction of the structural floor ele- for EPS foam sheets® when density is of the ment is required. Although both cement and utmost concern. There are a few scenarios when gypsum topping products present self-leveling a deep-fill screed may be more advantageous characteristics, many factors drive the decision than EPS foam. of which product type best suits the project at • Significant levelness issues: for example, severe hand. Armed with answers undulations in the subfloor take less time to toCopyright a few project-related correct with a cementitious, lightweight prod- questions, narrowing that uct than with a foam board. Self-Leveling Self-Help scope is a relatively straight- • Requirement of a fully bonded floor system forward process. throughout the entire floor assembly: EPS foam Key Differences and What are you trying to accomplish? is loosely laid over the subfloor while a cementi- tious, lightweight underlayment will bond to Practical Applications To identify whether your project requires a the subfloor. for Cement and Gypsum gypsum or cement underlayment, the most • Depth of space excludes the use of EPS: EPS important question to ask is, “What problem foam with holes or EPS foam topped with Underlayments or need am I addressing?” Figure 1 provides reinforcement requires a minimum 1-inch top- several common floor issues mitigated by each ping over the EPS. EPS without holes or not type of product.magazinetopped with reinforcement requires a mini- By Beth Lee and Josh Jonsson S TWhen Rasked to Urecommend C a lightweight T Umum 1½-inchR toppingE over the EPS. A deep-fill solution to meet a specific floor height, it is screed requires only a ¾-inch topping. important to consider the depth of space to • If conduit or cable needs to be encapsulated be filled. As a structural engineer, you might within the buildup, then a cementitious, light- be asked to strike a balance between structural weight topping will be less time consuming, as needs and weight considerations in a project foam board would need to be cut and placed where a specific thickness has been included in around the conduit or cable. Beth Lee ([email protected]) the project plans. Gypsum and cement under- It is important to note that both EPS foam and is the Business Development and layments possess lower densities than regular deep-fill screeds require a topping with a weight Project Manager for Maxxon concrete while also achieving a flat surface ready that is equal to or greater than 110 pcf before floor- Corporation in Hamel, MN. for floor coverings. This makes them viable ing installation (Figure 2). The gypsum or cement options for filling space and meeting thickness underlayment topping provides the density and Josh Jonsson is the Vice President of Sales for Maxxon Corporation in Hamel, MN and can be reached at [email protected].

Figure 1. Common uses for gypsum and cement underlayments. 14 January 2018 Given the typical end-uses of light wood elements during the construction process. frame construction, a high compressive This makes it ideal for time-sensitive proj- strength is not often needed. However, if the ects by significantly shortening the waiting underlayment will be exposed to construc- period before the floor covering installa- tion traffic for an extended period, or its end tion. These characteristics also allow it to be use requires exceptional strength, a high-end installed earlier in the construction process gypsum underlayment can achieve a compres- if needed. sive strength in the neighborhood of 4500 Typically, gypsum underlayments are pounds per square inch (psi) and offers the installed by mixing the gypsum binder, potential for better crack resistance than its locally sourced sand, and water into a cement counterpart. pump. The liquid screed is sent through a While wood frame projects using an under- hose into the building where a crew installs layment will typically necessitate the use of a the material to a specified depth. The crew gypsum underlayment, concrete floor slabs runs periodic quality checks of the material, require further consideration to determine ensuring consistency throughout the proj- which type of underlayment provides the ect scope. Due® to the size of most projects, optimum solution. If the subfloor is con- gypsum underlayments are sourced directly Figure 2. In this project, a lightweight deep-fill crete, several additional questions will assist from the manufacturer and often shipped screed was installed and allowed to dry overnight. in selecting the best solution for the project. directly to the job site. This requires that The next day, the hardened screed was topped with jobs be scheduled in advance to allow time a gypsum underlayment to provide the density and What is the available time for repair? for crew mobilization and shipping of the compressive strength needed for point load dispersion Often the flooringCopyright installer discovers level- material. It should be noted that a limited and floor bonding compatibility. ness or surface quality issues on a job site. selection of bagged, pre-sanded gypsum compressive strength required for adequate Typically, they will also be the ones to cor- underlayments are available through retail point load dispersion, as well as meeting the rect the issue. Such a discovery may require outlets to meet the needs of uncommon flatness and bond compatibility requirements that the problem is remedied quickly. specialty projects. for floor coverings. Cement underlayments pre-mix the binder, What is the budget? What is the structural building material? aggregate, and additives in the bag, creating a highly homogeneous mixture which is Cement underlayments can be quite expensive Because of wood’s flexural properties, underlay- combined with water at the job site. Due due to the cost of the binder and additives. ments used over wood subfloors must possess to the product’s inherent levelness-seeking This is offset to some degree by the inclusion the ability to resist cracking caused by move- properties and the characteristic compact- of silica sand in the bag; however, the cost ment. Cement underlayments can achieve nessmagazine of job scope, installation of a cement remains prohibitive for large spaces unless high, concrete-like compressiveS strengths. T With Rself-leveler U is not complicatedC Tonce properU other R factors demandE their use. Large jobs proper floor preparation, they achieve excellent technique is learned. These products are requiring a cement self-leveler will typically bonding to concrete subfloors. (Many cement readily available to flooring installers at be outsourced to a company that specializes underlayments require shot blasting of the concrete home improvement and specialty stores. in the pumped application of underlayments. to a Concrete Surface Profile of 3-5. Check with Additionally, several cement underlay- In large projects, a gypsum underlayment the underlayment manufacturer for proper floor ments include calcium aluminate as part can provide a suitable, cost-effective alterna- preparation procedures.) Although this makes of the composition. Calcium aluminate tive. High-end gypsum underlayments offer them ideal for many concrete applications, the will give a self-leveler rapid strength forma- a quality compressive strength and the ability stiffness associated with cement binders make tion, provide self-drying characteristics, and to be poured extremely thin (Figure 3 and them susceptible to cracking. This phenomenon include a robust performance ability when Figure 4). If absolute flatness is required, two is exacerbated by the elasticity of the subfloor. exposed to occasional moisture or exterior options exist:

Figure 3. The floor of this train station needed significant work prior to Figure 4. The same area is pictured following the installation of a high-end repurposing. In this photo, extensive floor height variations can be seen in the gypsum underlayment, which leveled the floors and created a smooth surface locations where tracks were removed. ready for floor coverings. STRUCTURE magazine15 January 2018 blasting or removal of surface contaminants, to ensure the epoxy is absorbed into the top of the concrete slab. Though most floor coverings will only require around 2,000 to 3,000 psi for installation, thin glued flooring materials in commercial proj- Figure 5. Gypsum and cement underlayment average material square foot price. Installed cost can vary ects will require a high compressive strength. widely depending on the scope of a project. Commonly used in hospitals, schools, and 1) A cement underlayment can be used. variables may influence the type of underlay- sports floors, thin resilient floor finishes are Cement underlayments contain excep- ment that best suits the project. more likely to telegraph imperfections in the tionally fine silica sand, which allows Are there unique considerations subfloor. A high-strength underlayment is them to achieve a true featheredge. for the project? more tolerant to the high point load needs 2) A gypsum underlayment can be installed of commercial floors. This type of project will but will need to be buffed once it is dry. Both gypsum and cement underlayments may likely include additional floor preparation on Buffing is required because the aggre- provide airborne sound reduction, which is the front or back-end.® This is because cement gate added to a gypsum underlayment is measured as Sound Transmission Class (STC). self-leveling underlayments that can achieve coarser than silica sand. Airborne sound reduction is a function of mass; a zero edge require the floor to be shot blast Figure 5 is an average cost continuum of an the greater the mass, the more significant the prior to installation of the underlayment, while economical gypsum underlayment, a high- reduction. Given that gypsum underlayments high-end gypsum underlayments will likely end gypsum underlayment, and a cement are installed at a much greater depth than require rework (buffing) of the underlayment underlayment. These numbers are material cement underlayments,Copyright it is reasonable to infer following installation. If a significant portion cost averages; installed cost can vary widely that a gypsum underlayment will give a better of the average floor depth is less than ¼-inch, depending on the scope of a given project. STC performance. In fact, gypsum underlay- it likely makes sense to invest in preparation As you can see, the cost between a standard ments are commonly used in tandem with work and a polymer modified cement-based underlayment and a cement underlayment sound control mats as part of a floating floor/ self-leveling underlayment. can vary significantly. Your project condi- ceiling sound control system. The total system While gypsum and cement underlayments tions will dictate which product is best suited of a sound mat under a gypsum underlayment share many similarities, understanding their for the application but knowing that several is installed to address both types of sound, subtle differences will enable you to con- options exist will pay off either through cost airborne and impact sound waves. Installation fidently recommend the best solution for or time savings. of cement underlayment over a sound con- any project. The rationale behind product What is the depth of the levelness trol mat is not recommended. The stiffness usage often comes from different parties on correction that is needed? of themagazine cement underlayment, combined with a project. The owner of the project has a S T Rthe thinness U at which Cit is installed, T can causeU timeline R and costE in mind, the general con- Cement underlayments are designed with widespread cracking. tractor is mindful of budget related to floor the primary purpose of self-leveling. The Gypsum and cement underlayments possess corrections, the structural engineer is alert composition of these underlayments include similar moisture vapor transmission charac- to weight considerations, and the flooring additives that make them more viscous. This teristics. Neither type of underlayment will installer is concerned with floor covering compels the material to level out on its own, block the passage of moisture vapor; however, considerations. The key to making a well- which makes them exceedingly effective in both can be installed over moisture mitigation informed underlayment decision is to ensure thin applications. Therefore, any application systems, such as an epoxy vapor barrier. Like all the details – assembly, end-use, timeline, that requires a very thin layer of material most cement underlayments, an epoxy vapor and unique variables – are well understood (featheredge to < ¼ inch) will often justify a barrier requires floor preparation, such as shot and addressed.▪ cement underlayment (Figure 6). Floors with areas that are significantly out- of-level will almost certainly benefit from a gypsum underlayment. They provide an economical solution, are available from 2000 psi to 4500 psi, and can be installed thick or thin. For depths greater than 1½ inches, the installer may recommend the underlayment be installed in two lifts to facilitate drying. If the project is a renovation of a gypsum sub- floor, resurfacing should always be performed using a gypsum underlayment or gypsum com- patible patch to ensure a strong bond. Unique project variables may come up during an underlayment discussion. These might include acoustical requirements, the presence of excessive moisture vapor in the concrete slab, or a project with strict subfloor requirements. Figure 6. Installation of a cement underlayment in which the thinness of the application can be observed. Prior Some, though not all, uncommon project to installation, the concrete subfloor had to be shot blast to ensure adequate bonding of the underlayment. STRUCTURE magazine16 January 2018