To purchase this Sefer, click here
12. Placing an item to support the beam of a roof that is caving inwards:
Using a vessel as a support: A beam from the roof of a house which has broken [and has begun to cave into the house] is allowed to be supported by a bench or on the long beams of a bed, [being] that they are a vessel. [However, they may] not [be placed there] in order to lift up [the beam back into the roof] as this is [considered] building, rather [it may only be placed] in order to prevent the [beam] from falling down any further.
Using a non-vessel as support: However, it is forbidden to use an item which is not a vessel to support the beam from under, even if it had been prepared to be moved [from before Shabbos, and is thus not Muktzah], because [doing so] is similar to building. This is in contrast to [supporting the beam with] a vessel which [is not similar to building being that] it is not usual to nullify it to a building.
Placing the vessel in loosely: The vessel needs to be slightly loose under the beam in a way that it may be removed whenever one wishes to do so. However, if it is inserted [under the beam] strongly in a way that one cannot remove it from there anymore, then it is forbidden [to be done] because [doing so] nullifies the vessel from what it was prepared to do [beforehand].
May only be done if:
A. One uses a vessel to give the support, such as a bench or table as opposed to a mere plank of wood [even if not Muktzah].
B. It is placed in a way that it simply prevents the roof beam from caving in any further and does not lift it back up into its roofing position.
C. The vessel of support is placed loosely, in a way that it can easily be removed from there.
What is one to do if the roof of his Sukkah is caving in?
Based on the above, one can place a bench loosely under the Sechach to prevent it from falling any further down.
 Admur 313:23
 Tosafos Yom Tov on Shabbos chapter 23 Mishneh 5