Floor joist span question

I have a room, roughly 5 metre x 5 metre square. The floor is 5m by 4m as it falls short of one wall as there is (potential for) access to a basement.
There are only 2 joists, one at the edge of the floor and one approx 1.5m in from this. These joist are 6x3 inch The floor is quite solid, except for near the edge as this joist is quite rotten. The floorboards are full length and about 2 inch thick.
So I could just replace the rotten joist, this would leave me with a nice solid floor, especially as I am putting some stairs in to the currently inaccessilble basement so could implement some sort of upright support.
Would you add extra joists?
And/or... There is only just enough height so if I could replace both joists with more numerous but smaller joists it would be of benefit.
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On Thursday, March 6, 2014 8:47:52 PM UTC, R D S wrote:

If its good & solid, no need for more support. Wood is springy. 2" boards will provide a fair bit of structural strength.
NT
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On 06/03/2014 23:11, snipped-for-privacy@care2.com wrote:

Agreed. If you do decide to rebuild the floor, use 7 x 2 joists and 1" floorboards. That way, the floor level and headroom will remain the same. *Don't* use less tall joists - they have very little bending stiffness.
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On 07/03/14 12:42, Roger Mills wrote:

IIRC deflection goes up as the width times the depth over the sqaure of the span.
But its a long time ago and I cant remember the exact answer.
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On 07/03/2014 12:47, The Natural Philosopher wrote:

Yes, I forget the gory details - but I'm pretty sure that the bending stiffness of a beam depends on the Area Moment of Inertia about the appropriate axis, *not* the cross-sectional area. So a 10 x 1 would be a lot stiffer than an equivalent 5 x 2 even though the material content is the same. Someone will be along with a definitive answer, but I think that the formula involves the second or third power of the height, but only the first power of the thickness.
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On 07/03/14 14:03, Roger Mills wrote:

yes...I could work it out from scratch but I cant be arsed.
Mm. deflection translates to expansion contraction at the top and bottom of the beam, and the amount of that will increase as the depth, likewise the actual FORCE will decrease, so it looks like the square of the depth times the width..
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On Friday, 7 March 2014 14:59:13 UTC, The Natural Philosopher wrote:




It's moment you care about, not force, so it's the cube.
The force itself is counteracted by shear stress (and a 10x1 is exactly as strong as a 5x2 in that regard),. There is also a moment (because the force acts some distance away from a support). The moment is counteracted by a d eflection which sets up opposing tensile and compressive forces.
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On Friday, 7 March 2014 14:03:00 UTC, Roger Mills wrote:

3rd power of height, 1st of thickness.
A 10x1 is 4x stiffer than a 5x2.
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wrote:

I should imagine using 10 x 1 inch joists would require the fitting of anti-flutter boards to stabilise them against twisting forces. Is this always true or does the nailing on of plasterboard on the underside fulfil this purpose?
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wrote:

In days of yore, noggings were used or herring bone struts. You can buy metal pre-made struts nowadays. Very inportant to prevent cracks appearing in the ceiling below.
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On Sat, 8 Mar 2014 10:39:27 -0000, "harryagain"

Thanks for that, Harry.
When I was rewiring the house, just after we'd moved in about 25 years ago, I encountered these 'pesky' boards which made laying some of the lighting cabling a 'little tricky' and I was informed that they were called 'Anti-flutter boards'.
Afaicr, the joists were something like 3 by 7 inches. It struck me as a little odd that there wasn't any mention about 'flutter boards' or the modern equivilent considering how much more wobbly a 1 by 10 inch joist would be.
I suppose it wasn't mentioned here on account it would be taken for granted that such bracing would be a standard part of the structure like nails or screws.
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wrote:

It's common practice to use off-cuts from the joists as noggings. Skew nailed into place Any span more than eight ft should have a row of noggins. Especially if the timber is damp.
Sometimes you will see a row of plasterboard nails in a ceiling where the plaster has popped off the heads. This is caused by the joists twisting as they dry out. Noggings/struts stops this.
BTW. Use joist hangers, don't build into wall, much better for many reasons.
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On 07/03/2014 22:43, Johny B Good wrote:

They would be more likely to twist, certainly, and would need countermeasures such as noggins.
I wasn't seriously suggesting that anyone would use 10 x 1's as floor joists - I was simply making the point about relative bending stiffnesses.
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On 08/03/14 19:46, Roger Mills wrote:

I think they would be good with herringbone struts in.
BUT wood is cheaper than chippies ..
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On 09/03/2014 01:30, The Natural Philosopher wrote:

Yebbut, imagine trying to join two floorboards on a joist only one inch thick.
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On Sunday, March 9, 2014 11:29:45 AM UTC, Roger Mills wrote:

:) nail at an angle?
Out of interest, at what point does the double the depth 8x the stiffness equation break down? If 10x1 is 4x as stiff as 2x5, what about 100 x 0.1? That surely isnt going to be very stiff, because it lacks stability and compressive strength.
NT
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centres. (Normal) The depth needed of the joist in inches = half the width of the room in feet plus 2 . ie 12' wide room = 6+2 inches deep joists. (8") 14' wide room = 7+2 inches deep joists. (9") etc etc
This is for domestic houses.
You will need noggings or struts to prevent movement as they dry out.
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On 07/03/2014 16:59, harryagain wrote:

Sounds about right. I know that 7 x 2 joists are ok for a span of 10 or 11 feet.
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On Friday, March 7, 2014 4:59:31 PM UTC, harry wrote:

You can use such sizes, though there about twice the minimum depth they need to be. Your call.
NT
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On Fri, 7 Mar 2014 11:46:46 -0800 (PST) snipped-for-privacy@care2.com wrote :

You're going to put 4x2s over a 12' span? If joists are oversized you do end up with a stiffer floor which is no bad thing.
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Tony Bryer, Greentram: 'Software to build on',
Melbourne, Australia www.greentram.com
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