Screeds on precast planks.

Four things separate this from screeding a ground floor.

• Getting material to the deck. A ground floor screed is a pump and a hose. An upper floor screed is a placing problem before it is a screed problem. If plant has to be craned up, the price, the programme and the crane schedule all change.
• Camber and deflection. Precast planks are manufactured with camber, and they deflect under load. A screed laid to a nominal depth across a cambered deck will be thinner at midspan and thicker at the ends, or it will not be level. The deck has to be surveyed and the depth set from the survey, not from the drawing.
• Weight on the structure. Every millimetre of extra screed depth is load on a frame that has been designed to a number. Depth is not a free variable on an upper floor, which is one reason pumped screeds laid thin are preferred over thick traditional screeds.
• Joints and edges. Plank joints, structural movement joints and the perimeter detail all have to be handled. Grout and seal the plank joints before the pour or the screed will find its way through them, which is a memorable way to discover the problem.

Bonded, unbonded or floating? On a precast deck all three are used, and each has a different minimum depth and a different consequence for the deck survey. Floating over insulation is normal where underfloor heating or acoustic separation is required.

Which system, flowing or self-compacting? Flowing screed covers fast and levels itself. Self-compacting also removes the late power floating step, which on an upper deck means no plant and no crew returning to that floor in the evening. Site hours usually decide it. See flowing screed and self-compacting screed.

Which binder? Calcium sulphate flows well and is widely used but is moisture-sensitive and normally needs its laitance removing before adhered coverings. Cementitious avoids both and costs more. The finishes schedule should decide.

Has the deck been surveyed? This is the item that causes most of the arguments. Camber, level and any out-of-tolerance planks change the volume, and volume changes the price and the load. Survey first.

Is acoustic performance specified? Separating floors in residential and hotel work often carry acoustic requirements, which drive the build-up, the isolation layer and the perimeter detail.

Survey and set out

We survey the deck, confirm datums and falls, and plan the pour and boom positions around the building and the programme. The survey tells us the real depth range and therefore the real volume, which is what the price should be built on.

Plank joints are checked and the perimeter detail agreed before anything is pumped. Where insulation or an isolation layer is specified, we confirm it is laid, continuous and properly taped at the perimeter.

Place and finish

Screed is pumped and placed to level, with the system chosen to suit the finish, the loading and the time available. Dapple bars work the material to help it level and release air. With self-compacting systems that is the end of it, with no return visit to power float.

Levels are checked as we go rather than at the end, because a deck at height is not a place to discover a problem after the material has gone off.

We run two track-mounted placing booms, which are two of only three in the country, plus our own concrete line pumps. All operated by our own people.

Boom reach is what lets us screed upper decks without craning plant onto the floor. That saves crane time, saves a lift plan, saves the deck loading that comes with parking plant on it, and removes a dependency on somebody else's programme. On a constrained city site with one crane and a queue for it, that dependency is often the thing that delays the screed.

Where a boom is not the right answer, our line pumps are. Placing is planned as part of the works from the start, with pump positions, hose runs, outrigger loadings and wash-out agreed before we mobilise. See concrete pumping.

Screed surface regularity is normally specified to BS 8204, measured as the maximum deviation under a 2m straightedge:

SR2 is the usual default. Screed materials are classified by binder type under BS EN 13813.

On a precast deck the achievable class depends on the deck as much as the screed, which is why the survey comes first. A screed laid on a deck outside tolerance will either use more material or fail to hit the class, and both outcomes are better priced than discovered.

We survey the finished screed against the class specified and issue the results.

Pumped screed covers a deck quickly. What governs the day is access: how many boom or pump set-ups the building allows, how the material gets to the pump, and how much of the deck is genuinely ready.

We programme from the ready date backwards. On multi-storey work that usually means fitting into a floor-by-floor cycle, and we will tell you at tender whether the cycle you have is realistic for the screed.

• Upper floors. Reached with booms rather than cranes.
• Live and occupied buildings. Clean, contained, pumped methods, phased around use.
• Restricted hours. Self-compacting systems remove the late power floating that forces night shifts.
• Constrained city sites. Limited set-up space, shared crane, tight delivery windows, restricted wash-out. All agreed in the method statement rather than improvised. Teaching blocks and student accommodation are a common example, see education flooring.
• Floor-by-floor programmes. Screed slotted into the frame cycle, in sections, around other trades.

• Deck survey and confirmed datums before the pour
• Surface regularity survey against the specified BS 8204 class
• Delivery tickets and batch records
• Pour records: area, depth, date and conditions
• Manufacturer's declaration of performance for the system installed
• RAMS issued and agreed before mobilisation, including boom set-up and outrigger loadings
• Sign-off pack at handover

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