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The large volume under the roof allows here to develop two levels, thanks to a "light" and thin floor, which combines ribbed galvanized steel tanks to a compression slab reinforced concrete.

install a light floor

Necessary material

  • tools of bricklayer, carpenter, electrician

Difficulty: 4/4
Cost: € 6,000 in supplies and € 3,500 in technical studies (excluding implementation)
Time: 10 days to 3 people

In renovation, the creation of an intermediate floor is accompanied by strong constraints: load transfer, fixed dimensions of the envelope, lifting difficulties, etc. Yet, the attic of this apartment on the top floor deserved to be recovered to increase its living space.

Release the volume

remove the floor from the attic

To do this, the floor of the attic is entirely deposited. Metal gantries replace the bulky traditional farms and rest on the outer load-bearing walls. Once the space is free, the volume has a sub-ridge height of 4.70 m and 3.50 m at the gutter. What remains low, considering that the ceiling height of the low level is only 2.10 m.

A thin floor

A wooden floor did not lend itself to this project (the joist would have been too thick).
A prefabricated floor would have required lifting tools and the pouring of a concrete floor imposed load shifts via the shoring more important on the internal structure of the building.
The collaborating floor in steel tanks offered the best solution. This process decreases the weight of the steel load-bearing structures and reduces the height of the floors. Finally, this process uses manuportable materials and requires only a reduced team.
The main precaution is to use a structural design office, the only one able to guarantee this type of structure, to calculate the dimensions, the nature and the position of the elements.

Install a light floor to gain the surface: floor

The new floor consists of a horizontal part mounted between three walls (A), raised on one side (B).
To achieve this form, the edge beam is composed of reconstructed and welded sections (PRS) distributing their loads on the facade walls.
The mezzanine (C) rests on columns whose load is distributed via sleepers incorporated in the floor.

Info for DIYers

When the available volume is constrained in height, the choice of a thin structure is essential. The Lewis profiles used in this project are thin ribbed steel bins. They form a lost formwork constituting the frame of a thin concrete slab. A wooden or steel structure can support a floor in steel tray. The advantage of a metal structure also lies in its dimensions: for an identical range, an HEB can be up to two times less high than a traditional beam of solid softwood.

Assembly of the metal structure

release the volume under the roof

  • Traditional wooden farms are here replaced by metal frames (black pieces).
  • Their main advantage is to release the volume under the roof.

120 mm PSUs

  • The floor is positioned 60 cm lower than the old one.
  • 120 mm PSUs are chemically sealed (peg + resin) on the bottom wall and then welded together.

PRS edge beam

  • The PRS edge beam (welded composite section) was prepared and welded in the workshop.
  • It is delivered on site in one element.

the beam is mounted on the winch

  • The beam is mounted to the winch, set up and sealed in the facade walls.
  • Its location must be precise: the flatness of the entire floor depends on it.

PEI 120 mm

  • 120 mm PEIs are then mounted in the frame to form the joists on which the collector will be installed.
  • They are spaced 40 cm apart on the entire floor surface.

ipe set on upe

  • The PEIs are attached on one side to the PSUs 120 that have been sealed in the walls.

they rest on the PRS

  • On the other side, they rest on the shore PRS.


  • Fixing is done on supports that have been pre-welded in the workshop.
  • The IPE 120 are bolted and then soldered.

flatness control

  • Flatness is checked regularly throughout the assembly with a 2 m mason ruler and a spirit level.

structure mounted on poles

  • The office floor structure is mounted on poles.
  • It consists of a 100mm PSU frame.
  • In the corners, struts brace the mezzanine.

bolt the elements

  • The elements are bolted together.
  • Then everything is fixed on the edge beam already in place.

bring the floors together

  • The two floors (office space and bedroom) are joined by the landing of the stairs.
  • The whole is bolted then welded.

metal frame

  • IPE 100 are then mounted every 50 cm in the metal frame (here office space).
  • They form the joists that will be used to support the collaborating tank, made from contiguously laid steel plates.

DIY Info

beam definitions

The beams are named according to their shape, in I, U or H.
Their naming also defines the characteristics of their wings: IPE and UPE when they are parallel, IPN and UPN when they are inclined.
H-beams are defined by the thickness of their wings.

Floor creation

collaborating ferry

  • The 16 mm collapsible tray is formed using manuportable sheet metal plates installed over the entire surface.

cutouts with a grinder

  • The cuts are easily done with the grinder.

abut the plates

  • The plates are carefully butted to ensure good continuity and to prevent concrete from flowing between the joints during pouring of the slab.

plate fixation

  • To ensure the stability of the structure, the plates are fixed in the joists using a gun nailing gun.

form the bins

  • The steel bins are then boxed.
  • The top of the boards indicates the upper limit of the rough floor.
  • The boards of the formwork will serve as a guide during pouring.

wire mesh

  • A 10 x 10 cm grid welded wire mesh is cut and positioned on each steel tray.

pouring concrete

  • A concrete dosed at 400 kg / m³ (about 50% cement, 25% sand and 25% gravel) is poured.
  • The trellis is taken off with the trowel so that it is well coated with concrete.

smoothing the concrete screed

  • The screed is smoothed with an aluminum mason ruler of at least 2 m.
  • Leaning on the formwork makes it possible to slide the ruler by a continuous movement.

Get a flat surface

  • We proceed in stages.
  • If holes appear under the ruler, concrete is added and smoothed until the screed is perfectly flat.

Video Instruction: Surface Preparation - Preparing a Concrete Floor