Floor joist span question

[R D S]

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.

[meow2222]

If its good & solid, no need for more support. Wood is springy. 2" boards will provide a fair bit of structural strength.

NT

[Roger Mills]

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.

[The Natural Philosopher]

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.

[Roger Mills]

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.

[The Natural Philosopher]

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..

[bblaukopf]

3rd power of height, 1st of thickness.

A 10x1 is 4x stiffer than a 5x2.

[bblaukopf]

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.

[harryagain]

This sounds like quite an old building. There were no standards those days, carpenters did all sorts of odd things. I'm surprised it's so assymetric. You sure there is not a joist missing? Oak was commonly used

Big joists 4 feet apart were common with 10" wide X 2" thick floor boards. I would try to install an extra joist in the wide space. All depends on what the floor loading is.

If it has to be replaced in total. There is a rule of thumb formula for joist sizing. Only works in imperial.

Assuming 2" joists @16" 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.

[Roger Mills]

Sounds about right. I know that 7 x 2 joists are ok for a span of 10 or

11 feet.

[meow2222]

You can use such sizes, though there about twice the minimum depth they need to be. Your call.

NT

[Johny B Good]

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?

[Tony Bryer]

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.

[harryagain]

The sizing is more about preventing deflection rather than loading. If the floor deflects, cracks will appear in the ceiling below.

[harryagain]

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.

[Roger Mills]

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.

[meow2222]

That's about the minimum I'm perfectly happy with. The Victorians did it, and its perfectly safe. At that size you get very slight shake when people jump up & down. One can go up from there to any extent desired - unless BC are involved of course.

Yup. Reduces noise transmission, though there are cheaper ways to do that.

NT

[Johny B Good]

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.

[meow2222]

yes, if it moves too far. But why people pull their hair out over the odd hairline crack I'll never understand. They're part of life in old houses.

NT

[The Natural Philosopher]

I think they would be good with herringbone struts in.

BUT wood is cheaper than chippies ..