{"id":14537,"date":"2020-12-14T22:24:07","date_gmt":"2020-12-15T06:24:07","guid":{"rendered":"https:\/\/essential.construction\/academy\/?post_type=glossary&#038;p=14537"},"modified":"2026-11-03T10:09:19","modified_gmt":"2026-11-03T17:09:19","slug":"floor-flatness","status":"publish","type":"glossary","link":"https:\/\/essential.construction\/academy\/glossary\/floor-flatness\/","title":{"rendered":"Floor Flatness [ff]"},"content":{"rendered":"<p><strong>Floor flatness (FF)<\/strong> is a measurement of how flat a concrete floor surface is over short distances, recorded as an F-number after the slab is placed. Higher FF numbers mean a flatter floor. It is measured to ASTM E1155 and is specified together with floor levelness (FL), which measures slope over longer distances.<\/p>\n<h2>FF vs FL<\/h2>\n<ul>\n<li><strong>Flatness (FF)<\/strong> describes bumpiness: how much the surface changes in elevation over short spacings along a line. It is mostly controlled by the finishing operations after screeding.<\/li>\n<li><strong>Levelness (FL)<\/strong> describes how closely the floor follows the design slope (usually level) over longer distances. It is mostly controlled by forms, screeding and, on suspended slabs, deflection.<\/li>\n<\/ul>\n<p>Specs normally give an overall value for the whole floor and a lower minimum local value for any single section. For full tables of typical values by floor use, see our <a href=\"https:\/\/essential.construction\/academy\/tutorials\/concrete-slab-finishes-and-the-f-system-explained\/\">F-number system guide with chart<\/a>.<\/p>\n<h2>How floor flatness is tested<\/h2>\n<p>A tester takes elevation readings along sample lines across the slab with a digital profiler or survey instrument, then calculates FF and FL from them per ASTM E1155. ACI 117 calls for measurement within 72 hours of finishing, and on suspended slabs before shores are removed, so the results reflect the work as placed rather than later curling or deflection.<\/p>\n<h2>Where it matters<\/h2>\n<ul>\n<li>Warehouses with racking and forklifts, where bumps affect safety and wear.<\/li>\n<li>Floors receiving thin finishes such as sheet vinyl, LVT or large-format tile.<\/li>\n<\/ul>\n<p>Two regional notes: flooring manufacturers often specify a straightedge tolerance rather than F-numbers, and in the UK industrial floors are usually specified to Concrete Society TR34 instead. If a slab fails, fixes include grinding, <a href=\"https:\/\/essential.construction\/academy\/glossary\/scarifying\/\">scarifying<\/a> high spots or a self-levelling underlayment.<\/p>\n<h2>Related terms<\/h2>\n<ul>\n<li><a href=\"https:\/\/essential.construction\/academy\/glossary\/scarifying\/\">Scarifying<\/a><\/li>\n<li><a href=\"https:\/\/essential.construction\/academy\/glossary\/shoring\/\">Shoring<\/a><\/li>\n<li><a href=\"https:\/\/essential.construction\/academy\/glossary\/post-tension\/\">Post Tension<\/a><\/li>\n<\/ul>\n<h2>Related templates and guides<\/h2>\n<ul>\n<li><a href=\"https:\/\/essential.construction\/academy\/tutorials\/concrete-slab-finishes-and-the-f-system-explained\/\">FF\/FL Concrete Floor Flatness and Levelness Explained<\/a><\/li>\n<li><a href=\"https:\/\/essential.construction\/files\/information\/white-papers\/cementitious-self-leveling-underlayments\/\">Cementitious Self-Leveling Underlayments<\/a><\/li>\n<li><a href=\"https:\/\/essential.construction\/files\/information\/white-papers\/concrete-slab-surface-defects-causes-prevention-and-repair\/\">Concrete Slab Surface Defects: Causes, Prevention and Repair<\/a><\/li>\n<\/ul>\n<h2>Frequently asked questions<\/h2>\n<h3>What is a good FF\/FL number?<\/h3>\n<p>It depends on the floor&#8217;s use. A slab under carpet can accept much lower numbers than a warehouse floor with high racking. ACI 302.1R gives guidance on typical values for different floor types, and the project spec sets the required overall and minimum local numbers.<\/p>\n<h3>What is the difference between FF and FL?<\/h3>\n<p>FF (flatness) measures short-distance bumpiness of the surface. FL (levelness) measures how closely the floor follows the design slope over longer distances. A floor can be very flat but out of level, or level overall but bumpy, so both are specified.<\/p>\n<h3>When should floor flatness be measured?<\/h3>\n<p>As soon as practical after finishing. ACI 117 calls for F-number testing within 72 hours of slab finishing, and before shores are removed on suspended slabs. Measuring later lets curling, shrinkage and deflection change the results.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Measuring how flat a floor is.<\/p>\n","protected":false},"author":1,"featured_media":14538,"menu_order":0,"comment_status":"open","ping_status":"closed","template":"","meta":{"footnotes":""},"glossary-categories":[466],"glossary-tags":[528,2192,2193,2191],"class_list":["post-14537","glossary","type-glossary","status-publish","has-post-thumbnail","hentry","glossary-categories-division-03-concrete","glossary-tags-concrete","glossary-tags-ff","glossary-tags-fl","glossary-tags-floor"],"_links":{"self":[{"href":"https:\/\/essential.construction\/academy\/wp-json\/wp\/v2\/glossary\/14537","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/essential.construction\/academy\/wp-json\/wp\/v2\/glossary"}],"about":[{"href":"https:\/\/essential.construction\/academy\/wp-json\/wp\/v2\/types\/glossary"}],"author":[{"embeddable":true,"href":"https:\/\/essential.construction\/academy\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/essential.construction\/academy\/wp-json\/wp\/v2\/comments?post=14537"}],"version-history":[{"count":2,"href":"https:\/\/essential.construction\/academy\/wp-json\/wp\/v2\/glossary\/14537\/revisions"}],"predecessor-version":[{"id":20387,"href":"https:\/\/essential.construction\/academy\/wp-json\/wp\/v2\/glossary\/14537\/revisions\/20387"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/essential.construction\/academy\/wp-json\/wp\/v2\/media\/14538"}],"wp:attachment":[{"href":"https:\/\/essential.construction\/academy\/wp-json\/wp\/v2\/media?parent=14537"}],"wp:term":[{"taxonomy":"glossary-categories","embeddable":true,"href":"https:\/\/essential.construction\/academy\/wp-json\/wp\/v2\/glossary-categories?post=14537"},{"taxonomy":"glossary-tags","embeddable":true,"href":"https:\/\/essential.construction\/academy\/wp-json\/wp\/v2\/glossary-tags?post=14537"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}