Roofs and steel sheets
Will felt under steel sheet solve the dripping problem in an uninsulated building?
Key points in brief
Felt under steel sheet can temporarily retain condensate and reduce dripping. However, it does not remove moisture from the building. To work, it must dry out afterwards. It makes most sense as part of a well-considered solution for an uninsulated building. If the roof leaks or the interior is constantly very humid, the fleece alone will not remove the cause. Moist air in contact with a sufficiently cool surface can release some of its vapour as water. A steel roof responds quickly to ambient conditions. In an uninsulated building, its underside is often a place where condensation forms. Drops can also form when it is not raining outside.
Where does water on the underside of steel sheet come from?
Moisture does not come only from people. It enters a storage building with wet equipment, vehicles and the ground. In the example of a seasonal storage building in Wągrowiec, these could be tools after garden work. This is a hypothetical situation. It shows that the way the interior is used is part of identifying the problem.
How does an anti-condensation layer work?
The fleece on the underside of the sheet has a structure capable of retaining some of the water that appears. As a result, condensate does not have to form falling drops immediately. When conditions change, the moisture should evaporate. Adequate airflow is needed for this.
The water retention capacity is limited and depends on the product and the operating conditions. It is not worth transferring values from an arbitrary sample to the whole roof. Prolonged dampness can exceed the capabilities of the layer. The fleece is an auxiliary element, not an inexhaustible reservoir of moisture.
Where can such a solution be useful?
It may be considered in unheated storage buildings, shelters and facilities with provision for ventilation. What matters is whether drops from the roof surface could damage stored items. At the same time, the environmental conditions and the intended use of the specific fleece must be checked.
In an open structure, air exchange is often easier, but its correctness should not be assumed without assessing the arrangement. Enclosing the sides can change the situation. A solution that worked before extension does not have to remain effective after all walls are closed and tight gates are installed.
What can felt not replace?
It does not provide thermal insulation comparable to an insulation layer. Nor is it a universal vapour barrier. Its presence does not mean that all layers under the covering can be omitted. The roof of a heated room requires a separately designed thermal and moisture regime.
The fleece does not repair a leaking lap or damaged chimney flashing. If water enters from outside, the cause of the leak must be removed. Retaining it by the fleece can only delay the manifestation of the symptom. Therefore, the source of moisture is identified first.
Why are eaves and overlaps important?
In places exposed to rainwater ingress, its undesirable capillary rise through the fleece must be prevented. Solutions provide for appropriate zones and the method of preparing the material. This applies, among other things, to the ends at the eaves and transverse joints.
We do not specify a single width of such a zone for every metal sheet. It depends on the product and the detail. Nor should the preparation method be improvised. Before ordering, establish whether the layer is to be appropriately finished at the factory or requires operations provided for during installation.
During the inspection, also look for places that are permanently wet. Recurring moisture at one point may require checking for a leak. Uniform dampness over a larger area leads to other questions about internal conditions.
How do you compare the extra cost with the benefit?
An assumed surcharge of €1.85 before VAT on each of 126 m² of sheet adds €233.10 before VAT. Check whether the supplier charges on the overall sheet area or the area covered. Both may be described simply as a rate per square metre.
Assess the cost together with the conditions of use. If water-sensitive items are stored under the roof, limiting dripping matters. However, if the building requires insulation and a ventilation redesign, the surcharge for the fleece will not replace these works. The choice should not consist in seeking the cheapest way to cover up the problem.
What should be checked after the first season?
Observe when moisture appears and whether the layer has the opportunity to dry out. Pay attention to places at laps, supports and the eaves. Also check whether ventilation openings have been covered. A change in storage method may increase the amount of water introduced into the room.
If the fleece remains permanently wet, the conditions must be reassessed. Adding more absorbent material alone does not solve the moisture balance. When planning to heat the building, return to the design of the entire building envelope. A former uninsulated roof may require a fundamental change of the system.
Short answers
Does felt remove moisture from the air?
It does not work like a dehumidifier. It retains some condensate on the surface and must later release it through evaporation.
Is steel sheet with felt always better?
Not for every application. The choice depends on the interior conditions, ventilation and roof construction. In some buildings a different layer arrangement is needed.
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