Brother, I just had the same nightmare on a job – every opening’s a different size and the frames are never square!
By the way, what should be the thickness of fire rated doors if you actually want the rating? Most proper FD30s are 44-45mm, FD60s 54mm - anything thinner is usually just a blank hoping the seals do the work.
Gaps over 4mm and you’re screwed too. Definitely read the article mate, breaks down all the certifire rules and why half the doors we fit are technically non-compliant. Saved my arse last week!
