I need to make a sound effect for a building slowly collapsing on screen. The initial explosions are easy enough to do, however I'm having some trouble thinking of something I can record that would mimic the sound of a building collapsing? The whole building takes 10-20 seconds to fully collapse from one side to the other, any ideas?
This is going to be a process of layering. Glass breaking, stone smashing, wood cracking, metal being deformed. All of these sounds can be found in a good library. Layer everything together and then pitch and time stretch different elements so that they match the mass of the building that you are representing. After that worldise the layers as a single group so that the destruction sounds like it is bouncing off the surrounding buildings.
What kind of building are you looking to simulate. Is it a skyscraper of metal and glass, a wooden house, a multi-story brick school? It's going to take a number of layers, and you're going to need to think of the different materials involved and how they'll fail to build character into the design. How does the collapse affect the environment around it, is that even something you should consider? There are a lot of angles you can approach from.
The first Sound Design Challenge I ran was a video of a controlled explosive building demolition. Check out some of the entries to see how people approached it, and maybe it will give you some ideas. All of the entries can be found here.
I had a similar project and found a great place for source material was to go to a demolition site of a house. Now not every demo company will let you on site, but call around until you find one of the more liberal ones. Take your own safety precautions: steel toed boots, visibility vest, hard hat and goggles may be required. Use long cables to keep yourself out of harms way and only put a mic in a dangerous position if you're willing to risk losing it.
The sounds of machines tear down walls is beautiful, but those machines are also pretty loud and spoiled most attempts at getting a clean recording of the destruction. The best stuff I got was when the workers went on lunch or finished for the day, leaving me with a Lego toybox of wreckage to play with. Take the opportunity to get a variety of materials, shifting piles of wreckage and large amounts of rubble. Bring a shovel to help with this. Consider what materials your building in shot is made of and what we're seeing and get plenty of variations of those materials.
You can also bring a sledge hammer with cloth wrapped around the top to get a cleaner sound minus that metal impact sound. Ask if you can break the windows before they tear down the walls. There are concerns of road and ambient noise to account for as always, so you may need to do some finer debris work in studio. Take buckets of stuff home with you if you need to.
In the mix I have found the sound of trees falling down can work well and gunshots that are pitched and/or slowed down are perfect for adding a big whump to a structure collapsing. A good bang and convincing trail of debris will help make it sound dangerous. If you're needing to add more a descending rumble as things collapse, I've tried recording under a big metal bridge as a train went over which was OK. An avalanche might be a good base if you can find a suitable recording - otherwise you might have to build your own from the above and other sources.