[youtube]http://www.youtube.com/watch?v=MxC2gZq7pFw[/youtube]
Sewer caps coming off during a backup is not that common, but rarely it does happen. This video shows a heavy metal cap being blown up due to a backup in the city storm sewer. Luckily, there were not any injuries.
The sewer caps on your main sewer trap are also vulnerable. The caps have to be put on securely and no water should be coming out of them when you run water in your house. If your caps are loose or old or do not sit in the trap tightly or have lost their threads, it means it is time to buy new caps. During back ups, caps hold the sewer water back and in many cases prevent a mess by keeping the sewer water in the sewer pipe. There are different types of sewer caps. Sewer caps are also called fit-alls. The most common types of sewer caps are:
- Lead Caps
- Brass Caps
- Rubber Caps
If you never had your sewer cleaned and you are the original owner of the house, then more than likely , your sewer cap is a brass cap. All caps have their own pros and cons. The cost of all caps is pretty cheap but the main thing is you have to have caps on your main trap. Having caps can prevent many other issues other than just holding back sewer water. Caps also help in preventing smell from coming into your house. Caps can also help in avoiding sewer clogs, how? By blocking anything from falling into the sewer trap.
Caps can also prevent sewer rats from coming into your house (sewer rats coming through the sewer pipe into your house is extremely rare). A sewer pipe without caps will not not look good either. 
You have to make sure you have proper caps installed to avoid many sewer related problems.
