I am renovating my attic. Because the house is about 150y old there is no underlay. I installed an underlay from the inside by stapling an underlay to the bars which hold the roof. The underlay should guarantee that the water can drain to the outside. Now the problem is that I used the printed side facing inside, whereas in most videos I have watched so far the printed side always faces outside. Will the underlay still be waterproof?
I am using this product
Underlay for roof
EDIT: The technical documentation states: (translated)
3-layered, functional layer protected by PP fleece on both sides
Best Answer
If I understand your question correctly, you have attempted to install a roofing underlayment sheet material from the inside/underside of the wood roof membrane in an attempt to prevent rain from getting in the attic.
That type of sheet material is designed to be applied to the exterior side of the wood roof membrane, under whatever final roof treatment (shingle, mineral asphalt sheet, edpm, etc.) is installed. See the picture below, which is from the website for the product in the link you supplied.
What you are trying to do will not work and will likely just redirect any water that does get through to a different undesirable area.