Joists 3" square !!??

[Arthur]

I've just lifted one of the middle boards on a suspended (ground) floor. The joists are 3" square that are resting on a 'wall' that consists of a couple of layers of bricks. The bricks are mortared and this 'wall' runs across close to the board I lifted The 'wall' seems to sitting on a footing.

I was expecting to find the joists that support the upper floors which are

6" x 3" buggers. is this common method? Or does it look like the builders I bought the house from, bodged it together.

Thanks for any advice.

Arthur

[BigWallop]

Is the whole joist 3'' square ? Or is it set 3'' into the brickwork and you only see 3'' sticking above the supporting wall ? If the joists are only three inches square, then get the builder back in to replace them all with something a lot more robust than that.

[Set Square]

3 x 3 sounds unusual - 4 x 2 is more usual - as opposed to (typically) 7 x 2 used on upper floors.

The upper floor joists have to span the *whole* width of the room. The ground floor joists have intermediate supports, and thus don't need to have such a large section.

Have a look at

explains it all.

[Peter Taylor]

BigWallop wrote

The section size needed for the joists depends upon the load they support, what timber they are, their span and how far apart they are. If you can post this information I can tell you if they are adequate or not. Is the floor firm and solid or does it deflect badly when you jump on it? Is it level or sagging?

Peter

[Arthur]

Not much chance there. I bought the house 12 years ago. The joists are sitting on wooden pads on the wall.

[Arthur]

They are on 15" centres, There is a little flexing which is why I lifted the board. The floor looks flat enough.

Arthur

[Arthur]

The page you showed me has these figures

Spacing (distance apart) of joists.

400mm 450mm 600mm Size of joist ; ; ; ; 38 x 147 mm 2.85 m 2.71 m 2.33 m ; ; ; ; The 38 x 147 has a cross sectional area of 5586 sq mm while the 75 x 75 is 5625 sq mm.

At its widest point into the alcove, the room is 3.2m wide although the length of the joist in the centre of the room are only 2.3m because of the concrete base provided for a hearth.

[BigWallop]

So it's a Barrat Homes property ? They have a clause somewhere in the small print on the deeds. It says something like "If you have any more than X amount of people in your property at the same time, they do not take responsability for the soundness of the flooring structure". So get your title deeds out and have a read through them.

[N. Thornton]

a sleeper wall?

You didnt tell us how old the house is, or when these timbers went in, or how long these 3x3s are, or what spacing they have between them. If its a new build, its not OK. Even if its an oldie, its borderline, and you'd need to watch your loading. More info.

Regards, NT

[Set Square]

Yes, but are you sure that - in the widest part - the joists go from side to side in a *single* span? With downstairs floors, it's quite usual to have one or two little support walls under the floor, supporting the centre section(s) of the joists.

Incidentally, the bending stiffness isn't determined *just* by cross-sectional area - the actual shape matters. If you think about it, a 6 x 2 joist will provide a lot more bending stiffness in a vertical plane when its 6" dimension is vertical than it will if you lie it on its side. A 3 x 3 won't be as stiff as a 6 x 1.5 even though the cross-sectional area is the same.

[Bob Minchin]

The stiffness or resistance to deflection of a beam is proportional to the thickness and proportional to the cube of the depth.

Regards

Bob

[Arthur]

Sorry. Should have mentioned it is a very old mid-terraced...circa pre-1910. Unless you are being flippant at the expense of Messuir barret :)

Arthur

responsability for

[Arthur]

Yep. Old mid-terrace, circa pre-1910. The 75 by 75's look very old like the they have been there a long time; not like they were installed by the slag arse builders I bought it from 12 years ago. The bricks used are like the bricks the house is built from. And they have 15" (400mm) centres.

At its widest point into the alcove, the room is 3.2m wide although the length of the 4 joists in the centre of the room are only 2.3m because of the concrete base provided for a hearth.

Arthur

[N. Thornton]

yes, but the 3x3 also has a fraction of the strength. 3x3 is not equivalent to 6 x 1.5 at all. Try the sagulator to see the difference.

3"x3" on 15" or 16" centres is nowhere near the build regs requirements IIRC.

Having said that, BRs are far above whats needed for the thing to stay up: the penalties of 3" joists are sagging and bending, noise transmission, plaster cracking, and the risk of collapse if huge loads are applied, such as a house full of people, waterbed, etc.

Regards, NT

[Lurch]

On 22 May 2004 14:52:27 -0700, [e-mail] (N. Thornton) strung together this:

Although, the OP has said that they have been in for 12 years, and they're downstairs. Not that I'm disagreeing with you though, just a 'in this instance' type observation.

[Set Square]

Don't forget, this is a *downstairs* floor - so there won't be a plaster ceiling under it (assuming there's no cellar!).

The OP has not given an explicit answer to my earlier questions about intermediate sleeper walls. In my view, there's a high probablility that these joists don't span the whole room width - but are supported in the middle.

[BigWallop]

No. Not at all flipant. The Barrat Homes deal used to have a clause in the deeds that says they don't accept responsibilty for the weight you put on the floors in the house they sold you.

I was shown this by a lawyer years ago. I couldn't believe my eyes.

[Arthur]

The 'sleeper wall' is what I mentioned at the top although I didn't know it was called a SW a the time. To describe it again diagrammatically.

A---------+ +--------------B | | | | | E----------------+ | | | | | X=============================Y | | | | C----------F--------------------------------D

A to B = 4m A to C = 3.2m E to F = 2.3m Joists at 400mm centres All joists 75mm square X-Y is 2 courses of bricks and the joists are sitting on wooden pad blocks.

The brickwork looks tidy and has gaps in the lower course, for drainage I presume.

Hope this helps.

Arthur.

[Capitol]

Is there another sleeper wall either side of the hearth? If so, then the joists would meet a "deemed to satisfy" interpretation of the building regulations. If not, then I'd add one to stiffen the floor up a bit.

Regards Capitol

[Coherers]

The max clear (unsupported) span is A - X in your diagram - this is the one to be looking at in the tables. Assuming your diagram is to scale, you are talking 2 - 2.2 meters for this (??)

For downstairs floors where there is no ceiling below to support, you are likely better using the "not more than 0.25KN" table in the regs, which gives you slightly longer permitted spans e.g.

400mm 450mm 600mm Size of joist ; ; ; ; 38 x 147 mm 2.98 m 2.87 m 2.51 m ; ; ; ;

As others have said, attempting direct conversions from the figures in the tables is problematic, but looking at the figures for the 75mm sizes and the fact you have centres at less than 400 mm, I'd really not be concerned. Probably been there for nearly 100 years already, so.....