Due to insufficient ventilation under the roof or to excess humidity in the underlying premises, condensation on the underside of metal roofs can cause damage similar to that produced by water infiltration: humidification of the insulating panels , deformation or even collapse of false ceilings.

The structures concerned by this sheet are, for the most part, roofs made with ribbed plates made from galvanized steel sheets. This roof usually comprises five elements: the ceiling, the vapor barrier, the thermal insulation, the possible air gap, the metal plate (with or without condensation regulator).

Due to the high thermal conductivity and low porosity of the material, steel pan covers are naturally exposed to the phenomenon of condensation. The extent of this phenomenon can be reduced with an adequate design of a vapor barrier system plus roof insulation and taking into account the humidity class and the ventilation of the underlying premises.

The ventilation of the cover

The air present in the plenum on the underside of the cover is charged with the water vapor contained in the air coming from the house or the underlying room, despite the presence of the vapor barrier intended to limit this penetration. (vapor barrier injury ...).

In addition, the roofing material can have a temperature lower than that of the plenum, up to a point called dew point below which condensation occurs. If the roofing material is porous enough, the condensation droplets are retained for some time. With metals, which are not very porous by nature, these condensates fall more easily on the insulation and the false ceiling.

Proper ventilation of the plenum eliminates much of this moisture.

Excess humidity in the underlying premises

The choice of the insulation system depends on the classification of the premises according to the quantity of water vapor produced (low or medium humidity, see DTU 40.35). Compliance with hygrometric conditions (quantity of water vapor produced, renewal of air in the room) prevents most disorders.

The excess humidity can also be due to occasional or perennial ventilation faults in the premises (ventilation of the ventilation, ventilation failure, new air intake, etc.).

The quality of design and implementation

In the event of a discontinuity in the vapor barrier, water vapor migrates towards the possible air space and towards the roofing material. There is then a significant risk of condensation in cold weather.

Good practices

Insulation

Adapt the precautions to be taken to avoid losses according to the position of the insulation, under purlins (cold roof), between or on purlins (warm roof).

Unlike a cold roof (free passage of outside air on the underside of the roof), a warm roof prevents the penetration of outside air towards the underside of the metal.

If the insulation is under purlins, a condensation regulator must also be provided on the underside of the tank, or thin insulation comprising a vapor barrier placed on purlins of minimum permeance benefiting from a Technical Assessment for this job. Then check the good ventilation of the whole of the cover, in particular the bottom of rampant.

If the insulation is between purlins, the blanket must be warm with an unventilated air gap or without an air gap (the insulation is against the blanket). The vapor barrier is under the insulation. This system is used in rooms with low humidity only (see DTU 40.35). Check the absence of air inlets at the periphery (closures) and in line with singular points, penetrations, roofing accessories and junctions with the lighting plates.

Insulating polystyrene panels supporting blankets (insulation on purlins): its products are under ATec. With regard to the specific risks, check the list of observations made by the C2P (Commission for the Prevention of Products used) of the AQC for any recommendations.

Define and take into account the humidity of the room by considering:

the occupancy rate;

the quantity of water vapor produced inside the room per hour;

the heating level.

The care taken in the junctions and the continuity of the vapor barrier can avoid disasters.

Identify the class of the room (depending on the quantity of water vapor produced in the room and the renewal of air).

Design the system (hot or cold roof, positioning of the insulation) taking this class into account.

Take special care when installing at sensitive points (junctions and continuity of the vapor barrier, for example).

Effectively ventilate the underlying premises.

To consult

NF P30-101: Coverage - Terminology.

NF P34-401: Roofs - Ribbed galvanized steel plates, pre-painted or not.

P 34-310: Continuous hot-dip galvanized construction steel sheets and strips for construction.

NF DTU 58.1: Building work - Suspended ceilings.

DTU 40.35: Roofing in ribbed plates made from coated steel sheets.

List of observations from the C2P (Commission for the Prevention of Products used) of the AQC: check any recommendations.

Sloping roofs for industrial and commercial buildings are most often made using self-supporting ribbed metal trays, with or without attached waterproofing.

With a span of about 3 m, these bins are attached to metal, wood or concrete frame purlins.

The propensity of such a structure to flex is particularly important and aggravates the climatic overloads that it is likely to withstand.

Any abnormal and unforeseen stagnation of water inevitably initiates an iterative and irreversible phenomenon which leads to the ruin of the cover: the creation of a puddle increases the applied load, which necessarily generates an increase in the deformation and therefore an aggravation of the puddle. .

The update of DTU 43.3 in 1995 has since made it possible to sensitize professionals to this major problem by specifying more explicitly the numbers and positions of the downspouts to be implemented.

Large droplets of condensation affect the modular fiber cement panels of the false ceilings, separated from the glass wool insulation by an air gap. There is impropriety at the destination of the dyeing workshop, as a result of these runoffs damaging the fabrics.

The dyeing workshop is a room classified with very high humidity (W / n> 7.5g / m3) and the concept of warm roof with insulation on purlin is not suitable for such a room.

There are also workmanship faults such as the absence of foam closures under the steel sewer trays as well as of closoirs against the ridge profile. The ribs are only imperfectly sealed by the compression of the insulating wool and parasitic air inlets are possible.

After exceeding the dew point, this air and the vapors, generated by the specific activity of the dyeing plant, are brought to condense under the steel pan subjected to a lower temperature on the exterior face.

Comply with the fields of application of DTU 40.35 (only valid for buildings classified as low or medium humidity - i.e. W / n <5g / m3). Independently of the execution problems, it would therefore have been necessary to treat the hygrometry of the workshop beforehand or to consider a completely different type of roofing.

Water damage inside the building following the perforation of a metal gutter.

Diagnostic

As part of an industrial building rehabilitation operation, a sheet steel gutter was installed directly on an existing masonry wall located on the property line.

Eight years after receipt, the gutter exhibits a general state of corrosion over its entire length with localized perforations in the lower part.

The state of degradation of the gutter is the consequence of a corrosion problem originating from a chemical incompatibility between the constituent metal of the gutter and its support.

Several years after the completion of the cover, infiltration occurs in the attic. During the investigations, it was noted that the zinc sheets were perforated by significant generalized corrosion on the underside.

On the one hand, insulating panels are chemically incompatible with zinc. It is a cause of corrosion.

On the other hand, since no ventilation has been provided between the insulation and the zinc, the water vapor coming from the housing condenses under the zinc. By stagnating there it also causes its corrosion.

It was necessary:

Or make the zinc roofing in accordance with standard NF P 34-211-1 (DTU 40.1) Building work, roofing by elements in sheets and long sheets of zinc. This text only provides for wooden supports; and ventilation on the underside of the zinc.

Either use insulating panels and a non-ventilated zinc installation process on the underside covered by a Technical Assessment, not observed by C2P.

The disorder was manifested by the appearance of drips of rust along the metal frame.

After soundings and removal of a certain number of panels, the disorder turned out to be generalized, and concerned a very advanced corrosion of the lag bolts fixing the sandwich panels in the metal frame elements, going as far as their virtual destruction. complete.

This disorder affected the holding of the blanket.

The client is worried about the significant bending of the plates and, in rainy weather, notices water infiltration through the overlaps between the plates.

Three causes are cumulative:

1- The center distance of the purlins is excessive. He is 1.90m while he should not exceed 1.10m. This is the cause of plate bending.

2- The slope of the roof is insufficient. It is only 6% whereas it would take at least 25%. It is a cause of infiltration.

3- The overlap of one plate on the other in the direction of the slope is excessive. It is 34cm instead of the 20cm required. Due to the bending of the plates, this covering, located on the purlins, opens even more widely and promotes water infiltration under the effect of the wind.

As regards non-traditional cover plates but subject to a Technical Assessment, this one had to be respected:

- Limit the purlins' center distances to 1.10m

- Give a minimum slope of 25%

- Limit the overlaps between plates, in the direction of the slope to 20cm

The industrial framework comprises 11.50m span trusses made in 2 parts (the trapezoid and the hat) assembled on site. Only a few months after delivery, significant creaking occurs in the frame. The owner notices that some pieces of wood are unraveling.

The assembly of the cap on the trapeze must guarantee the transmission of forces. It is therefore necessary to mold or connect the parts to each other, not only at the level of the superimposed entanglements but also of the crossbowmen.

The assembly of the entries is here carried out only in the middle of the span by two vertical uprights fixed by spikes in each entry. The assembly of the crossbowmen is carried out by a piece of wood pointed on one side.

Under these conditions, the transmission of forces cannot be done correctly. The inlet assembly is opening.

Compliance with the execution plans provided by the frame manufacturer would have made it possible to achieve satisfactory assemblies capable of withstanding the forces applied.

The superimposed entrances would have been assembled by spikes or connecting plates in several places or over a large area.

The crossbowmen would have been assembled by moisage or by connectors on both sides. The month would have been about 1m in length.

The steel tank cover with light strips made of polyester plates has emergences on the roof:

- a longitudinal lifeline near the ridge,

- a metal gangway with transverse access.

These elements are supported by the metal frame of the building thanks to metal posts which pass through the roof.

Many infiltrations occur at the right of these penetrations.

Creeping plasterboard ceilings under roofs are damaged by water infiltration around the roof frames. Thermal insulation gets wet and loses its insulating capacity. Water seeps in long, heavy rains.

The roof frames were added over the existing ribbed trays after cutting them. The assemblies are therefore not made by overlapping the upstream sheets on special base pieces for connection to the roof frames.

The ribs of the sheets come into contact with the frames, creating a succession of small basins without an outlet. These basins are charged with each rain.

And the waterproofing was carried out by gluing bituminous sheets. This arrangement is not reliable because the slightest lack of adhesion of the bituminous sheets allows water to infiltrate.

Special base pieces had to be used for connection to the roof frames or skylights. These parts create a corridor free of any obstacle all around the roof frames, and the various recoveries are made in the direction of the water flow: upstream on downstream.

The work had to be carried out in accordance with standard NF P 34-205-1 (DTU 40.35) Roofing in ribbed plates made from steel sheets.

The roof of the 2nd floor of this building is made of ribbed steel sheets with a waterproofing coating arranged on insulating panels.

More than 2 years after acceptance of the works, water retention is observed on a span of this cover with a deformation of the latter.

During a strong gust of wind, the roof and the load-bearing frame partially flew off, revealing half of the house

The purlins supporting the steel tanks were simply placed on top of the masonry with a simple matting with mortar.

The roof had large roof overhangs.

The continuity of the purlin elements was achieved by simple soft irons.

No ventilation of the attic made it possible to attenuate the phenomena of pressure / depression.

This roof had been redone in the precipitation after the storm of December 1999. The initial fiber cement roof had been replaced by steel trough on this occasion. The greater rigidity of the new roof, combined with all the defects observed, caused the roof / frame assembly to take off.

The installation of a serious fixing system of purlins (new and old) on the masonry was essential. The roof overhangs should have been reduced as much as possible and the attic should have included upper and lower ventilation.

Infiltration very occasionally occurs in a room, during certain winds with rain, at the foot of the recovery partition under cover. Next door is the bathroom, lit by a masonry skylight.

No sign of water damage in the adjoining bathroom. No defect in the gutter waterproofing statement receiving the EP from the curved cover.

Against the skylight play, the flashing strip of the roof presents a localized discontinuity due to the smallness of this play. The plaster of the plaster cannot be stopped properly and the water is passing behind the cover reading.

Achieve perfect continuity of the flashing door strip before rendering the plaster on the skylight, perhaps by locally reducing the height of the roofing upstand.

This localized defect, visible during construction, should have been reported in good time to the negligent roofer by the project manager and by the plasterer.

We observe in picking the ceiling of a living room traces of humidity affecting the painting of the ceiling. Stains resulting from infiltration are also visible on the wall, baseboards and floor of the living room.

The housing wall has on its exterior face an EP descent with water box. There is no overflow on the cover part located above the housing.

Loading of the gutter (which has a counter slope) in very rainy weather is possible with water penetration under the zinc cover.

In addition, we note the presence of wanes on the zinc cover, testifying to counter slopes. The slopes measured in roofing are less than 5% (of the order of 3%).

The implementation of the zinc roofing must respect the minimum slopes of 5% prescribed in DTU 40.41 - §3.2.

Concerning the absence of overflow, these are non-conformities with DTU 60.11 - §4 and DTU 40.5 - §5.4. The latter, dealing with EP evacuations, clearly specifies that any gutter must imperatively include an overflow orifice so that water does not enter through the banks in the event of total or partial obstruction of the orifice. evacuation. In addition, the diameter of the overflow must be at least equal to that of the downspout.

Infiltrations appeared on the right of the roof frames shortly after the work was carried out. The ceilings are damaged. The paints are stained. The thermal insulation is soaked and no longer performs its function properly.

The connections of the roof frames with the existing zinc roofing were poorly made. In fact, the sealing was carried out by applying elastomeric mastic and not by overlaps and mechanical assemblies between metal elements.

All metals vary in size with changes in temperature. The assemblies between metallic elements must therefore allow their free movement while guaranteeing tightness.

The mastic, such as it is used here, is constantly stressed in tension then in compression and cannot resist durably. In the long run it comes off or breaks.

Roofing frame installation work is the responsibility of roofers and not carpenters. They must be carried out in accordance with standard NF P 34-211-1 (DTU 40.41). Coverings by metallic elements in sheets and long zinc sheets. The assemblies allow free movement of the metal elements while guaranteeing tightness by their shape.

The polycarbonate sheets are dislocated from the aluminum profiles which assemble them by simple clamping. The occupants feel cold air currents and water infiltration occasionally occurs depending on the direction of the rain-laden winds.

The cold air passages generate significant heat loss in winter.