Substrates & Preparation
Camber, Jointing and Acoustic Prep for Hollowcore Plank Floors
Hollowcore floors need camber surveyed, joints grouted, and an acoustic separating layer confirmed before liquid screed brings the floor to a level finish.
Hollowcore concrete planks turn up mainly in commercial and multi-storey residential projects, spanning between beams or load-bearing walls with hollow cores running through their length. They're structurally efficient but arrive from the factory with a natural camber, and their long joints between planks need attention before a screed will sit level and stay put.
Because hollowcore is usually a suspended floor rather than ground-bearing, sound insulation between floors and fire-stopping at penetrations often factor into the preparation sequence alongside the levelling itself.
Precast hollowcore is manufactured off site under factory conditions, with pre-tensioning strand running the length of each plank giving it both its structural capacity and its characteristic upward camber. That efficiency is exactly why it's specified on multi-storey commercial and residential schemes, but it also means the finishing trades inherit a floor with a predictable, if variable, starting geometry rather than a flat plane.
Living with plank camber
Because each plank cambers slightly under its own precast tensioning, a floor made up of several planks side by side rarely presents a flat surface even once joints are grouted. Rather than treating this as a defect, it's usually accounted for in the screed design by surveying levels across the floor plate and setting a datum that allows for a variable screed thickness within the manufacturer's minimum and maximum limits.
Acoustic separation on suspended floors
Where hollowcore forms a floor between occupied storeys, building regulations for sound insulation usually require some form of resilient layer — acoustic quilt, isolation board, or a floating floor system — between the structural plank and the screed. This needs specifying before the screed programme is set, as it changes both the build-up depth and how the screed behaves as a floating slab rather than a bonded one.
Surveying before ordering insulation and screed
Because camber varies from plank to plank and can reduce once the floor is loaded, the practical approach is to survey levels reasonably close to the screed pour date rather than immediately after the structural frame goes up. This gives a more accurate picture of the datum the screed will actually be poured to, and avoids ordering insulation to a level that no longer matches the loaded structure.
On larger floor plates spanning several bays, it's worth surveying a grid of points rather than just a handful of spot checks, since camber differences between adjacent planks can be more noticeable than the overall camber of a single plank in isolation.
Coordinating fire and acoustic requirements
Hollowcore's hollow cores are sometimes used as a convenient route for services, but any penetration through the plank for cabling or small-bore pipework needs fire-stopping detailed and agreed before it's covered by insulation and screed, since compliant fire-stopping products and methods vary depending on the size and type of penetration.
Acoustic separating layers are specified to a performance standard for the whole floor assembly, so the insulation, any resilient layer and the screed together need to be considered as one system when checking compliance, rather than assuming any generic acoustic quilt will automatically meet the requirement.
Hollowcore Planks — the points that decide it
- 1Precast planks have a factory camber that varies plank to plank, so the finished soffit is rarely dead flat before screeding
- 2Longitudinal joints between planks are typically grout-filled on site as part of the structural works, before any screed preparation begins
- 3A separating or acoustic layer is common under the screed on suspended hollowcore floors to manage impact sound between storeys
- 4Structural topping is sometimes cast over hollowcore for composite action — where present, the screed sits above this, not directly on the planks
- 5Camber variation across a floor plate may need a levelling strategy that treats screed depth as variable rather than assuming a flat start point
- 6Service penetrations through hollowcore cores need agreeing and coring before insulation and screed installation, as coring afterwards is far more disruptive
- 7Plank camber generally reduces somewhat once dead and live loads are applied, so levels are sometimes checked again closer to the screed date rather than immediately after erection
- 8Fire-stopping at service penetrations through hollowcore cores needs to be agreed with the fire engineer before coring and before insulation covers the detail
- 9Impact sound insulation performance is usually assessed as part of the whole floor build-up under Approved Document E rather than judged on the screed alone
Working with hollowcore on a multi-storey scheme?
We'll survey camber, confirm acoustic build-up and plan the pour around your structural and fire engineer's requirements.
Or call our landline on 01392 237700
Hollowcore Planks — your questions
In short
Hollowcore floors need camber surveyed, joints grouted, and an acoustic separating layer confirmed before liquid screed brings the floor to a level finish.
Working with hollowcore on a multi-storey scheme?
We'll survey camber, confirm acoustic build-up and plan the pour around your structural and fire engineer's requirements.