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First floor joist spans

I am building a two storey extension and based on the TRADA Span Tables For upper floors with a 22mm thick chipboard decking and 12.5mm plasterboard ceiling, (on which a dead load of between 0.25kN/m2 and 0.5kN/m2 may be assumed and a span 2600mm between load bearing walls I need 47mm by 147mm joists at 400 centres.
I have two questions.
1. Will building control expect a structural engineer to have done calculations for these joists or will using the TRADA span tables to choose the correct joist size be enough?
2. The proposed joists will be thinner than the existing ones in the house which look to be about 195mm thick. I will make sure the floor is level but then the ceiling below would be out, can I just build the ceiling up with more pasteboard or something so that it is level?

4 Answers from MyBuilder Architectural Designers

2 by 6 isn’t sufficient in my option I’d use 2 by 8 minimal ideally at 400 centres or if just close up the centres to 300/200 especially as your adding more weight on the ceiling then what should normally be there better safe then sorry.

You should ask building control who should work out the calculations for the span but I imagine there be happy with 2 by 8 for a 2.6meter span.


Answered 10th Jan 2020

this is a question you should be asking building control or an engineer. But I would advise you speak with building control first and arrange an inspection, then take it from there.


Answered 10th Jan 2020

My advice would be to stay with the same depth x width and centres of the existing house joists. You have checked that these existing joists are on the top side of strong enough in accordance with acceptable approved tables, so no problems. This way all should line through without having to pack out to line through to existing ceiling. Finally, run this by your building control officer prior to ordering and building. There should be no need for structural calculations as these will have been done to produce approved joist selection tables.


Answered 11th Jan 2020

The TRADA span table calculator is accepted by building control.
If they do query any joist size you can obtain a calculation PDF sheet from the calculator.
All joists that I have submitted to various local authority building control using the TRADA calculator have never been refused.
To obtain a level ceiling just use spacers to level the plasterboard.


Answered 12th Jan 2020

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