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Joist spans

Alan T
in Building Regulations

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I'm just building an extension with 50x150 joists with a flat roof on top. My architect told me to double up on the two longer spans which are 3.2m long. The building inspector has said they are too long for the span tables which shows a max of 3.1m. I can't find any reference in the span tables for a double joist (ie 100x150 in total). Shouldn't doubling the thicknes allow for a bit extra length?  Can anyone point me to a reference for this?

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