Maximum floor joist spans

[dave1975]

Hi - trying to work out maximum floor spans for 6x3 floor joists to blend in with existing floors. Looking at NHBC and other ‘free’ calculators based on BS 6399-1 the spans for 400 centres seem really small (using NHBC recommended dead load assumption of 0.25 to 0.5 kN/m2) at 2.6m in C16 or 2.9m in C24. We have spans up to 4m so if we used wooden struts ( which we presume are just noggins) can we extend the maximum span of a 6x3 floor joist? Can anyone offer any advice or other sources used by building control (we are Trafford) many thanks

 

[johnny2007]

In the old days if you used c24 you could do the max span times 2 +1 to obtain the acceptable depth or +2 for c16.
So in your case of max 4 metre span it would be 4x2 +1 = 9 inches for c24 and 10 inches for c16.
Nowadays you need structural engineer calculations to get things accepted by building control.
I seriously doubt that 6x3 would be acceptable on a 4 metre span.

 

[^woody^]

You either reference a known accepted set of span tables (which are basically all the same), or you get the joists calculated.

The tables are all very conservative, and timber does have more capacity, but you'll need to prove it if not using a table.

 

[Dereekoo]

Noggins make no difference whatsoever to acceptable span distance

 

[Mr Chibs]

75x145mm, C24 @ 450c is 3.65m span, for domestic floor joists.

 

[Bouy]

If they are actually sized 75 x 150 (c24) they will span 4.1m if your floor finish is restricted to 0.25kn/m2 (22mm chipboard = 0.15kN/m2). Deflection would be on its limit of 12.3mm and structural capacity would be 65% utilised. Foregoing based on BS 5268 - Structural Use of Timber

This only indicates 47 x 170 but it shows how close you are
Free UK Span Table for Domestic Floor Joists to BS 5268-7.1 (C24, 1.5 kN/m² load) - Timber Beam Calculator

 

[jeds]

Having it calculated will gain you some span but you won't get to 4m with 75x150s. 2nr 50x150s would be closer.

 

[jacko555]

Surely decreasing the centers (using more joists) can achieve an acceptable span and deflection using 6x3?

Sister the joists? Add dwarf walls?

You can get there, just at what cost (more wood and set of calculations to prove it for BC), vs, using larger joists set a bit lower so the new floor levels blend existing ones.

 

[Bouy]

This only indicates 47 x 170 but it shows how close you are
Free UK Span Table for Domestic Floor Joists to BS 5268-7.1 (C24, 1.5 kN/m² load) - Timber Beam Calculator

Meant 72 x 145

 

[catlad]

I would happily build a floor in my house out of 6x3 for a 13' span at 16" centres.

 

[jacko555]

Ground floor of my 1920's house is 4x2 at 16" centres and strategically placed dwarf walls.

Upstairs is 7x2

 

[cdbe]

A steel mid span if you're that bothered, otherwise just lower the ceiling in the joining room.

 

[tony1851]

6 x3 C16 @ 300 centres would be fine.

The published tables are conservative - your dead load (exc.mass of joist) will be around 0.3kN/m2.

Bending stress would be less than 4.5 N/mm2 (maximum allowable bending stress would be 6.8N/mm2)
Deflection would be just over 10mm (maximum allowable deflection 12mm).

Trafford Building Control? ....Commiserations.

 

[Bouy]

6 x3 C16 @ 300 centres would be fine.

The published tables are conservative - your dead load (exc.mass of joist) will be around 0.3kN/m2.

Bending stress would be less than 4.5 N/mm2 (maximum allowable bending stress would be 6.8N/mm2)
Deflection would be just over 10mm (maximum allowable deflection 12mm).

Trafford Building Control? ....Commiserations.

My bending stress in BS5268 shows 5.3N/mm which factors up to 6.1 with load sharing?

 

[^woody^]

Existing floors will presumably have oversite to build a sleeper wall off or prop a support timber.

Or why can't deeper sections be used?

Basically, why are 6x3 joists so important when there are several other options for conducting a floor?