What is a slab soffit?

A slab soffit is the underside (bottom face) of a concrete slab. In multi-storey buildings and car parks, the soffit is the ceiling surface you see when looking up at the floor above. Soffits are particularly vulnerable to water damage, carbonation, and spalling because moisture tracks downward through cracks and construction joints. When rebar corrodes in the soffit zone, loose concrete can fall — making soffit inspections and repairs a safety priority. MPS regularly carries out soffit repairs using Sika-approved patch repair mortars applied overhead. In more detail: the word soffit applies to the underside of any horizontal or sloping element, so you will also hear of beam soffits, stair soffits, balcony soffits and bridge-deck soffits. On drawings, a soffit level (or 'underside of slab' level) gives the height of the slab's underside above datum; together with the top-of-slab level it defines the slab thickness, and it sets the headroom and the space available for services below. On site, soffit formwork is the horizontal decking the slab is cast on, and because soffits are often left exposed the finish it leaves (fair-faced, as-struck, or ready for plaster) is specified separately. The soffit of a flat slab is plain; ribbed and coffered (waffle) slabs have downstand ribs on the soffit; and slabs cast on metal decking have a steel soffit. The signs of a soffit in trouble are rust staining, cracks running along the line of the bars, hollow-sounding areas when tapped, drips, efflorescence or stalactites at cracks and joints, and fragments of concrete on the floor below. Repair starts with making the area safe (debris netting or an exclusion zone) wherever material is already falling. Loose concrete is then broken out to sound material, the exposed steel is cleaned and primed, and the section is rebuilt with a lightweight, non-sag repair mortar applied overhead in layers, or by form-and-pour micro-concrete for deeper repairs, often followed by an anti-carbonation coating to slow further deterioration.