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The under-roof screen is a good way to prevent the entry of dust and snow into the attic. Here, in 5 questions and answers, the essentials of what you need to know to install one and any problems that may occur over time.

Can we put an under-roof screen without removing the tiles?

In an old house, the roof is made of tiles, which are directly placed on the frame without underlayment film. To insulate the attic, how to proceed? Should an under-roof screen be installed to ensure watertightness? If so, is it possible to do it without removing the tiles?

A screen or film of under-roofing must be implemented above the rafters, which implies, in renovation, to deposit the cover.
If you have a house for several seasons and you have been able to check that the cover was waterproof, including vis-à-vis the powder snow lift, theunder-roofing screen is useless.
If there is concern that coverage will deteriorate over time, one solution is to isolate with expanded polystyrene panels or polyurethane foam, insensitive to any infiltration of water.
If the cover allows significant amounts of water to pass after each heavy rain, it means that it needs to be reworked, and thus to deposit or even replace tiles. In this case, both take the opportunity to install a roof underlay.

Underlay screen: how to be compensated for infiltration?

The roof of a house completed since 4 years has a semi-flexible bituminous roofing screen, open to the ridge to ventilate the air that enters through doors. The problem is that powdery snow has infiltrated the ridge (in good condition, without cracks or breaks). This snow melted and caused damage. Which insurer must take care of the damage? Homeowner's multi-risk homeowner's insurance or business insurance, which probably did not respect the DTU?

Multi-risk home insurance can cover consequences of infiltrationbut not the work concerning the cover itself.
When a sub-roofing screen is installed, an air space of at least 20 mm, necessary for ventilation on the inside, must be respected. While the surface ventilation of the screen will be ensured by the installation of a counter-lathing of at least 20 mm, powered by the doors, the interruption must be only a few centimeters (from 2 to 5 cm) at the ridge line.

If infiltrations result from a laying fault, the ten-year guarantee the builder should cover the repairs. The best thing then to make a declaration of loss and play insurance damage-book.

Is the installation of an under-roof screen mandatory?

In the case of the construction of a house of 109 m2 located in Deux-Sèvres, the installation of a roof underlay screen to prevent water and snow infiltration is optional. It would come back to 4100 euros. The structure of the house will be brick, with farmhouses and Provençal tiles, the ground is flat. There will be no possibility of laying under-roofing film afterwards because there will be no access to the attic. Is there an obligation to put a roof underlay? Is this kind of film likely to tear with time?

Underlayment screens have six essential functions:

  • protect against powdery snow, which infiltrates much more easily than rain, and evacuate the meltwater;
  • limit the risks of uprising tiles in high winds;
  • preserve the woods framing;
  • allow access to slopes minimum coverage when the DTUs (40.1 and 40.2) provide for it according to the zone and site conditions;
  • collect any water infiltrations;
  • protect against dust and soot.

It is not obligatory that in some cases very specific and in certain areas at risk, especially very windy or in the mountains, which does not seem to be the case here. This is a recommended solution but not required.

As for the price, it seems high, and therefore debatable, but everything depends on the quality of the under-roofing film (a price of 10 to 20 € of the m2 great maximum seems classic). Finally, it must be known that a well-installed film has no reason to tear. Care must be taken to use only products highly permeable to water vapor (HPV).

Warning: even with access to the attic, we can not install a screen after laying tiles or slates!

Does an under-roof screen deteriorate as you get older?

Is the laying of a protective film of under-roofing obligatory during the construction of a house? A manufacturer warns against the risk of this material, porous at first, clogging due to dust, which will prevent proper aeration and may result in condensation. Should we opt for a product with very high energy performance (THPE) with laying of rockwool (R = 6)?

The highly vapor permeable (HPV) underlays are completely watertight but vapor permeable, they allow the evacuation of moisture produced by residents to the roof. This type of screen is standardized by the Scientific and Technical Center for Building (CSTB).
This manufacturer also has a surprising position: the quality screens do not get clogged because of the dust!
The solution adopted here (product THPE and rockwool) will provide good thermal resistance (R = 6).

Does an under-roofing film prevent snow infiltration?

After two episodes of snow, infiltration by the tiled roof was observed in a house of 80 m² built 6 months ago. Would the installation of a roof underlay and / or a vault prevent this phenomenon? Is the presence of an under-roof screen mandatory in a new home? Can an individual home builder refuse to install one?

Underlays are not always made mandatory by the rules of construction, but only depending on the exposure (normal, protected, exposed - annex of DTU 40.24) and altitude (from 600 to 800 m). The builder must say if he has respected this "rule of art" according to the location of the house.

Underlayment screens are nonetheless important elements, whatever the situation, and on several levels: ventilation, insulation, protection of the underlying premises against the penetration of powder snow (which is the case here), waterproofing in case of deterioration of the cover, reduction of the risks of animals entering the attic, etc. They can be a solution to the problem of snow infiltration through the roof. An individual house builder must perform what is in the contract: if nothing is stipulated, he has no obligation, except to fall under the umbrella of the famous DTU.

Anyway, it is not normal that the snow has entered this attic. We must play the game guarantee of perfect completion (since we are in the first year of construction).

Finally, if the DTU does not have a mandatory application, it will support the removal and the rests of the tiles to perform the operation.

On the same topic

  • Questions answers
    • Is it mandatory to install a roof underlay?
    • Can we put a reflective film as a roof underlay?
    • Can we keep bituminous shingles under a new roof?
    • Fiber cement roof and underlay screen
    • Insulation and under-roof screen
    • Can we put an insulation in direct contact with a roof underlay?
    • Is an under-roof screen mandatory?
  • DIY tips
    • System of peripheral connections of the roof windows

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