Soser (demolishing)--one of the thirty-nine forbidden melachos of Shabbos--is only forbidden min haTorah when one is demolishing for the sake of rebuilding. Demolishing for a destructive purpose is forbidden mid’rabonon. A minority opinion among the Achronim maintains that demolishing for a destructive purpose is actually forbidden min haTorah, but is not subject to punishment like other melachos. A similar concept is found with regard to eating less than a k’zayis of forbidden food. Although it is forbidden (min haTorah) to eat any amount of it, only eating a k’zayis is punishable.

Dislodging wood from the walls or roof of a shack (for firewood) on Yom Tov is a form of soser and is forbidden. If the wood is merely leaning against the shack, its removal would be permitted. If the wood had been placed there to add thickness to the shack’s walls, it may not be moved. The poskim disagree about whether it is permissible to remove wood that was placed with the intent of supporting the wall from falling.

If a shack collapses on Yom Tov, it would still be forbidden to use its wood for firewood. This is because at the onset of Yom Tov, the wood was not allowed to be used because of soser (see above). As such, it was classified as muktzeh machmas issur (muktzeh due to a prohibition) -- and it remains in that category throughout Yom Tov, even if the circumstances changed. The Rishonim disagree about whether one may stipulate before Yom Tov that an item should be permitted for use if circumstances change in a way that would otherwise allow it to be used.