Laying underlay membrane - 9 instructions & subsequent solutions

The Content Of The Article:

It is important to think of the underlay membrane when covering pitched roofs.

The drama about the underlay We were on the weekend briefly on the Baltic Sea and have, among other things, the roof of the shed once considered more closely. Damage has caused openings between some roof tiles, here moisture evidently gets through to the small attic room. An acquaintance, who was there during the inspection, was astonished that there is not even a roofing membrane in the roof. That is highly negligent, we should move in immediately, no matter what we intend to do with the roof at all. Of course, I was allowed to explore again what the undercover track is all about.

  1. The underlayment is a film that is inserted below the roof covering in pitched roofs. The roofing is there to drain water, but it can happen that the rain or snow blown by the wind is blown under the roof. This moisture is then to catch the underlayment and drain it downwards / outwards. The underlay is stretched freely between the rafters and is ventilated from both sides with old roof structures.
  1. The Sd value of the underlayment depends on the diffusion behavior of the entire roof structure. In most cases, full openness to diffusion is appropriate, releasing the accumulating moisture back to the outside.
  1. So it is also when an insulating layer is brought close to the web, which is e.g. may be the case with a subsequently injected cellulose insulation. Then you call this vapor permeable web undercover.
  1. The classic underlays are actually used today only in undeveloped attics, they should be slightly sagging, so that accumulating water is dissipated away from the counter battens and down. Each lane overlaps the ones placed in front of it by 10 to 20 cm, you should glue the joints and seams, so the underlayment becomes a closed lower layer, which protects against wind forces.
  1. This underlay is placed under the counter battens on which the lathing for the roof skin sits.
  1. If it is not about the classic underlay, but about the film web, which sits within the overall structure of a roof insulation between the roof and the insulation layer, the exact attachment depends on the entire roof structure, according to the given Sd value and the manufacturer's information.
  1. A sarking membrane can be considered a temporary cover if all connections are secured so that nothing can run behind it and if under the counter battens nail sealing strips were used.
  1. A subsequent contribution of a roofing membrane would be conceivable in an undeveloped attic when it comes only to secure the top roof layer in addition. Then the underlay can be mounted between the rafters, it is attached laterally with strips.
  1. If a film with a certain, calculated Sd value is missing under the roof skin, but within a roof structure with an insulating layer, this would mean that the entire structure of the insulation was not thought through. Here, it is probably neither possible nor useful to introduce a sub-stretch later, in any case, this would either cover the roof or open the structure from the bottom to the layer with the insulating material. Here, the entire roof structure of the assessment should be subjected by a specialist, under certain circumstances, the roof skin has such a tightness and the structure is carried out so correctly up to the inner vapor barrier or vapor barrier that can be dispensed with the underlay completely. If not, only the expert can guess what to do now.

So now we've done the following: Between the rafters, a makeshift underlay was tacked on, that's not at all in accordance with the regulations, but it only has to stop the moisture coming through until next spring. Next spring, the entire roof will be taken apart, in the then planned construction come completely new slides.

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