Building regs

I'm replacing a hearth and Building Regs Part J specify that the constructional hearth has to have a depth of 500mm from the fireplace. Unfortunately there is a wooden joist 460mm from the fireplace. If I was to comply with the regs I would have to remove this part of the joist. It all seems a bit much for 40mm. What are the consquences of breaching the regs?
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Depends on the inspector. Some will be a bit generous in how they hold the measuring tape knowing what the problem is, others will just not approve it and they don't really care what you have to do to fix it.
Is the project going to be inspected? If not, I'd just go ahead, but that is me, I'm not condoning cheating. If it is going to be inspected, I'd contact the local inspector, mention that you are fixing up an existing hearth that was in before the regulations and ask how you can comply. It may be as simple as putting a piece of sheet metal over the wood for added protection. Or, since it is a repair, be allowed as is under the rules.
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rule is there so a big fire doesnt burn down your home!!!
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The rules are under WETT and the Canadian Building Code. If you breach them, and fire was to ensue at any place for any reasons, you would be without Insurance coverage even if the inspector approved/passed/turnt a blind eye. Is it worth the risk??? Just had a WETT inspection of our cottage. The flue collar was too close by 1/4". Was forced to tear out and redo with margin of error on the safe side. If ever you have to file a claim, you want it denied?

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Your claims of a denied claim is dubious.
The OP is re-doing an existing fireplace, not new construction. It may be grandfathered under the rules. Safer to have it rebuilt, even if 40mm to close by present rules, than to have something that was drastically wrong by today's codes. But, as I stated, there may be an acceptable work around. Or there may not. If you don't ask, you will never know.
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Does "Constructional hearth" mean something different than: "the fireproof surface in front of the fireplace opening? If not, why do you have to mess with the joist? can't you just run over the top of it?
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If only it was that easy! The regs say that where the constructional hearth is place on top of combustible material then it has to be 250mm deep! It would put a big concrete platform in the middle of my lounge. Good for karaoke but not nice to look at otherwise.
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