Joist sizes

[Suz]

We want to specify heavier than normal joists in the extension. What size should we ask for?

Suzanne

[Ian Stirling]

Why? What's the span between supports, and what's the load you want to put on it?

Doubling the width, or doubling the frequency of joists doubles the stiffness of the floor. Doubling the height of the joists quadruples the stiffness.

[Guy King]

The message from "Suz" contains these words:

Someone else who wants a shagswing! (insert smiley of choice here).

Explain to whoever's designing it what the purpose of the extra strength is and they'll make the calculations accordingly.

[Tony Bryer]

Quadruples the strength, increases the stiffness by 8. Stiffness is proportional to depth^3 , so just going from 150 to 175 high joists increases the stiffness by nearly 60%

[Ian Stirling]

Err - yes.

[Phil L]

Which joists? ceiling or floor? what for?

[Autolycus]

If you tell him, don't believe his answer.

Maximum deflection is inversely proportional to the Moment of Inertia, I. For a beam of rectangular cross section, I =b*d^3/12. So double the width, double the stiffness, like the man said, but double the depth of the beam, and it's 8 times as stiff.

[Suz]

Oh why is nothing ever simple?

The idea is to have real strudy floor to minimise any bend and noise transference. The longest one will be in the roofspace and as the kids will be there I want to reduce the elephantitis noise from above.

[Ian Stirling]

Because it's real life :/

The other problem.

8" joists - compared to 6" will in fact moderately reduce noise.

What will reduce it a lot more is to completely float the floor. You basically put in joists like:

______________ # # # # # # # # # # # # # # # # # # ____________

Where there are two sets of joists, interspersed. The bottom ones are maybe slightly smaller, as they only support the plasterboard, the top ones support the floor above, and they only are connected at the side walls, being completely free to move in relation to each other. Adding rockwool in the space in between helps too.

[Phil L]

You'd be wise to space the joists at 300mm instead of 400mm, rather than go for more height on the joists...if you change the joist heights it might (probably will) interfere with existing joist heights...you'll need sound insulation between floors too, which is now required anyway.

[Dave H.]

"Suz" wrote

Hi, If you're really after soundproofing rather than strength per se, a better bet would be a normal (or slightly stiffer than usual) floor, with a separate set of ceiling joists spaced between the floor joists and a few inches lower (so that there's no mechanical coupling between the two) also supported on the presumably-brick-and-block-walls, high-density rockwool infill between 'em and two or more layers of plasterboard - this is how a lot of home-studio builders do it to contain the noise - oops, music - as much as possible, and what I'm aiming to do once SWMBC and I have the rest of the place sorted...

Hope this helps, Dave H. (The engineer formerly known as Homeless)

[dom]

Part E of the Building Regulations

detailed guidance on soundproofing between floors.

[Suz]

The idea is to have real sturdy floor to minimise any bend and noise transference. The longest one will be in the roofspace and as the kids will be there I want to reduce the elephantitis noise from above.

I do know that insulation is needed too, but just want to use this opportunity to make a really good addition to the house, not bare basics.

[Guy King]

The message from "Suz" contains these words:

Oh, none of the strenghening will do that. In fact a stiffer floor may well transmit sound and vibration better than a more compliant floor.

What you want is a /soundproof/ floor - that's a different thing altogether.

[meow2222]

New build joist sizes are already on the excessive side. Suggest instead a floating ceiling below and not attached to the floor. This means a separate set of 2x3 joists just to support the ceiling. Much more effective.

The other thing is the floor above. A floating wood on underlay on wood floor will help further.

Finally sealing up of all gaps, however small, makes a real difference. But not nostrils.

NT

[John Rumm]

As the others said, the best way to do this is to de-couple your ceiling from their floor. Insulation between helps, as does choosing a really good carpet underlay for the new floor.

Have a look at what I did for my loft:

last couple of piccies show it best. I still had a small amount of coupling between floor and ceiling since I needed to provide the lateral bracing that used to be archived with tie beams. But even with this there is noticeably less noise transmitted between 2nd and 1st floors than there is between 1st and ground floors for example.

(and in spite of first-born in the room above being small and looking rather dainty, seems to move with the acoustic aplomb of a small heard of buffalo... nice thing is you can hardly hear it through the ceiling)

Another way to increase floor stiffness is with herringbone straps across the joists either in the mid span or at 1/3 and 2/3 span. These can either be cut from timber and done in the traditional way, or using modern galvanised ones. Either way they just nail in place.

[meow2222]

Maybe you have a spalling chucker probe lamb.

I wonder how long it'll take the BR people to realise not being able to hear that thud is a safety risk, and decree all this soundproofing be removed.

NT

[Hugo Nebula]

On 9 Dec 2006 02:42:48 -0800, a particular chimpanzee named [e-mail] randomly hit the keyboard and produced:

What! Remove a Requirement! Are you MAD!!!

No, the proper way to go is to require that all surfaces in dwellings up to 1100mm high are not climbable and have a hardness rating of

0.45[units] or less, as measured using an extremely complex formula reproduced in secondary documentation to accompany Approved Document R, published six months after the introduction of the Regulation.

If you are buying new furniture, you will have to submit a Building Notice to the local authority, who will inspect it and require you to have independent verification of its hardness rating (the Ikea catalogue will not be sufficient), or else you can employ a competent furniture installer who is a member of SOFASA (Sofa self-assessment scheme) who will issue a certificate of compliance to the local authority.

[John Rumm]

You know you should not make suggestions like this in a public forum, you never know who might be reading! Especially when it makes about as much sense as part P.

[John Rumm]

I had to read that three times before I spotted it.... ;-)

The Mozilla spelling checker is not that good at my brand of goof it seems - sometimes we get to an impasse - I know its wrong, it says its wrong, but it can't suggest an alternative no matter how many attempts of subtly different but still wrong variations I try!

(although in the case above I obviously did not read what it suggested to carefully!)