Garret - wooden garret

The garret is a horizontal structure placed at each floor of a building, with the task of supporting, in addition to its own weight, that of the floor, internal walls and service loads.

First of all we can say that the geometric characteristic that distinguishes garrets from vaults is that of having a flat intrados surface. This allows for a reduction in the internal volume and to reduce the constraints on the distribution of the rooms.

A garret, compared to a vault, offers several advantages:

• It reduces the weight of the floors. We know that a floor is the set of layers that make up the structure, that is, the actual floor, the subfloor and the floor of the various floors of the building. In short, by floor we mean everything that is in the thickness between the intrados of the structure and the finished floor level.
• It allows for greater simplicity in the execution of the floors, which in this way can rest on a flat and regular underlying. Obviously we can have many types of garrets: wooden lofts, steel profiles and reinforced concrete bricks, corrugated sheet metal and brush lofts for manufacturing systems.

When we talk about garrets, they must comply with some essential performances.

• Resistance to loads
• thermal transmittance
• Linear transmittance
• thermal inertia
• soundproofing power to airborne noise
• insulating power to footfall noise
• reaction to fire
• fire resistance.

In addition, in roof garrets it may also be necessary to control condensation in the mass.

A very important element in the choice of the garret is represented by the span that they must have. With span we mean the distance between the support elements. We can distinguish between net span, which would be the distance between the internal wires of the support elements, and calculation span, that is the length of the beam to be considered in the sizing operation.

Now that we have given a definition of what a garret is and its main characteristics, let's move on to talk specifically about a wooden garret.

WOODEN GARRET

The advantage of wooden garrets is that they are simple and quick to build. In fact, they do not require temporary support works. The elements that constitute it, beams, joists and planking, are able to resist loads as soon as they are placed on their supports. Among other things, they can be left exposed or masked with a false ceiling.

However, this material is now rarely used, in fact it is only used in the restoration of old buildings or when a mezzanine needs to be built.

The most common types of garrets are simple garrets, composite garrets and French garrets.

SIMPLE GARRET

Simple garret are those made up of a series of wooden beams arranged in parallel with a distance of 50:100 cm, on which is superimposed a boarding of 3:6 cm thick boards placed side by side. This solution can be adopted to cover rooms no larger than 3:4 m. If we are dealing with larger garrets, then the composite garretshould be used.

COMPOSITE GARRET

These types of garrets are made by arranging the beams, called main beams, with interaxis less than 4 m, which constitute the support for the joists.

FRENCH GARRET

Also called Venetian, it is made up of beams with a distance of about 25 cm with the planks arranged longitudinally. In this way, the typical inconvenience of old wooden garret is avoided, caused by the fall of dust or sand from the gaps in the boards.

DIMENSIONING OF SECTIONS AND SUPPORT ON WALLS

Section of beams and joists

The resistant section of the beams of the joists is determined with the methods of construction science, considering that these are elements supported at the ends and subjected to simple bending, so that, in the case of uniformly distributed loads, the maximum bending moment occurs in the middle. At one time, a ratio between base and height was calculated. B/H equal to 5/7.

Support on walls

It was created with the interposition of an element called a sleeper, which had the function of distributing the load on a larger surface of the wall. In this case, the beams or joists of the floor could also constitute elements of concatenation of the perimeter walls, especially in the case of stone walls, which were not sufficiently connected by transverse walls. For this reason the ends of the beams were joined by iron brackets connected with an anchoring element called a bolt, embedded in correspondence with the external surface of the wall.

The support of the joists usually occurred on the upper edge of the main beam. Sometimes, the joists could also be embedded in the main beam or connected to it with various devices, for example to contain the thickness of the floor.

Execution details

Usually the wooden garret slab is the base on which the subfloor for the floor will rest, after possible installation of thermal and acoustic insulation. When greater insulation is needed, or even a greater finish, an additional layer can be placed on top of the planking, for example a concrete screed for greater rigidity.

In environments where there are no particular insulation problems, the planking itself can be used directly as a floor. In this case, a small trick is to join the boards with tabs to prevent cracks and warping.

The intrados, as already mentioned, can also be left exposed, in fact in the past in coffered ceilings in prestigious environments, the frame of the floor was left exposed and richly decorated.