We are installing a Bathroom in an existing bedroom on the first floor of a 3 bed victorian terrace.

We've heard building control have regulations regarding the double joisting or sistering of joists to spread the load when a bath is full of water.

What are the regulations? Is it as simple as double joisting?

What is double joisting? Is it as simple as bolting a new joist to an existing joist? In this scenario the original joist is still taking the full load on each end that is supported by the brick wall.

Or must the new joists be supported by the brick wall (or hung) in their own right?

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    What country are you in? – auujay Dec 27 '10 at 15:12

Different countries, or even more specific, the localities in the US use different standard. Even when they adopt a standard, they may use a specific year of adoption. I suggest more detail, or contact the building officials for details on the code that they use.


Andrew, if you are here in the USA, most floor systems are build 16 inches on center and if built in last 40 years or so usually are sized for 60 pounds per square foot or more. The only time we use extra framing is when a very large load like a hot tub or very large whirlpool bath is being installed. If the plumbing, especially the drain requires a floor joist to be cut, or other wise compromised, then a sister or a header box should be installed. You need to find out if the dead and live load of the tub you have selected exceeds the capacity of the existing floor system. You may need to speak to a qualified contractor or your local building inspector for guidance. Good luck

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    The house is "a 3 bed victorian terrace." so I think it was built more than 40 years ago ;) – ChrisF Dec 28 '10 at 13:56
  • Thanks for the replies. I am in the UK. I guess the plain and simple answer is that I must do the calcs. When I find out I must double joist I'm still confused as to whether the new joist must be self supporting. If the doubled joist must be bolted to the first joist then surely I can't use a joist hanger as the trim of the hangar will prevent the two joists from butting up to each other? – Andrew McGregor Dec 28 '10 at 21:41
  • A typical "sister" joist must be supported at both ends on a vertical wall or post and also on any mid way or other supporting members along the way. You should also fasten the sister to the original joist with framing nails or screws/bolts. Nothing says it can't be independent of the other joists as long as it is sized and supported properly for the span. – shirlock homes Dec 29 '10 at 10:44

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