http://www.attorneygeneral.jus.gov.on.ca/english/courts/guides/Guide_to_Replying_to_a_Claim_EN.pdf
What if I think someone else is responsible for the plaintiff’s loss?
You may believe that you have a claim to make against someone else who, in your opinion,
should be responsible for paying the claim. If you wish to make a claim against another party,
indicate that in your defence form and complete and file a Defendant’s Claim [Form 10A]. A
defendant’s claim is like a plaintiff’s claim, except that it is made by the defendant against the
plaintiff or someone else.
When you fill out your defendant’s claim, you are the plaintiff since you are now the one making a
claim. This is referred to as being “a plaintiff by defendant’s claim.” In your defendant’s claim, it is
important to correctly identify who you are suing. Refer to the “Guide to Making A Claim” for more
information about completing a defendant’s claim.
Example 4
The roofer in Example 1 might feel that if the roof does
leak, it is because the shingles were faulty. In that
case, the roofer might file a defendant’s claim against
another party, the manufacturer of the shingles. The
roofer can say, "I do not agree that there is anything
wrong with the roofing job but if there is, then it is the
shingle manufacturer who should pay you, not me."
You must file your defendant’s claim within 20 days after you file your defence, unless you make
a motion and ask the court to allow you to file it later. Refer to the “Guide to Motions and Clerk’s
Orders” for more information about how to make a motion.