Good evening. This is my first posting to this esteemed newsgroup.
I'm in the process of renovating a chalet-type bungalow, in the course
of which I need to fit some 7 x 2 floor joists(OK, they're 170 x 50,
but I prefer to think in one system at a time). Half of my first floor
has proper 7 x 2 joists; the other half, for some reason, only has 4 x
2 ceiling joists. Something to do with the history of the building,
which had rooms added upstairs after it was built. I hadn't noticed
this feature until I removed the chipboard floor, as the chappie had
battened over them with 3 x 2s to make the floor levels match.
Anyway, I digress. Half of my first floor is joisted with 4 x 2 at 400
(ish) mm centres. There is a ground floor brick internal wall dividing
the space, and the span from this to the wall plates on either side of
it is about 3.5 m.
When installing my 50 x 170s, is there any significant advantage in
running them alongside the existing 4 x 2s and bolting them to them,
rather than running them independently between the 4 x 2s, and bolting
them together where they meet over the brick internal wall? It might
seem a no-brainer, except that the joists and the rafters of the
sloping roof both come down to the wall plate, and don't always meet,
which would in any case prevent all the new joists from touching the
So I'm thinking of simply running the new (7 x 2) joists in the middle
of the gaps between the old (4 x 2) joists, nailing or bolting the
ends of the 7 x 2s together where they meet over the brick internal
wall, and perhaps nogging them with 3 x 2 which I would then nail down
into the old 4 x 2s where the noggins crossed them. I should add that
the existing 4 x 2s have a tie-beam nailed above them, half way along
their span, on either side of the internal brick wall.
A long-winded way of asking a simple question, sorry. I've not done
this before and it is pushing the envelope of my DIY ;-) If anyojne
has any general comments or advice, I'd be pleasd to hear them.
from geraldthehamster contains these words:
Who is forcing you to put in new joists?
Provided the 3 x 2s are securely fastened to the 4 x 2s the result could
be just as strong as a 7 x 2. Joists are sized to limit deflection and
you might have some bowing now built in if the additional 'battens'
weren't well and truly attached to the original ceiling joists but if
you haven't had a problem up to now with flexing downstairs ceilings why
go to the extra trouble of inserting additional joists?
It wouldnt make much difference either way, as the stiffness of 7x2
is much greater than that of 4x2. I would have though glue&screwing
3x2 on top of the 4x2 would be easier though.
Are the 4x2s proving too noisy?
Nobody is forcing me - I'm inserting stronger joists because the floor
flexes. 4 x 2 might have been adequate for children's bedrooms but I'm
planning on some heavy furniture up there. I want a solid floor to my
master bedroom, that doesn't move about ;-)
The 3 x 2s were laid crossways over the 4 x 2s, and just screwed here
and there, so I don't think they added anything much, other than extra
Ah, well they would have been good to prevent sideways bowing.
I you are set on teh 7x2's those are rather deep and not that wide and
its worthwhile using herringbone - or noggins if you like - to stop
sideways movement. If you use decent quality chip and screw and glue it
-T & G is best - that will stabilize the tops. I would be inclined for
practical reason to nail them alongside what you have. Makes cross
braces easier to insert.
the new joists up a little (i.e. on a spacers on the wall plate)
from the existing means that there is no coupling between new floor and
old ceiling (apart from at the noggings replacing the tie beams). It
also means you don't have to contend with what will likely be a
significant sag in the old floor / ceiling.
Unfortunately the cross-battens were installed some time after the
ceiling was created, and some sideways bowing hadalready occurred -
another reason why not all my new joists could bolt directly to the
old in any case.
My main reason for 7 x 2 (which the table tells me are adequate for
the span) is so that the floor level in that half of the house matches
the floor level in the other half, with which it is continuous.
So would it be OK to nog them with 3 x 2, nailed or screwed down where
they cross the old 4 x 2 joists? I guess I need to replace the
function of the tie-beam for the ceiling joists, as well as preventing
the new joists from bowing.
I will be using resawn 5 1/4 pitch pine boards for the finished floor.
Ah. I see where you are coming from. Hm. Strictly you SHOULD use
..especially as you wont be getting a lot of additional strength from
What will stop the bases of the new timbers from moving sideways?
Obviously te noggins will tie the tops together well..
If you made up diagonal noggins from the bases of the 7x2's to the top
of the 4x2s and vice versa..I think you might get a very good increse in
rigidity. I was dubious about this myself but the BCO insisted, and it
was a noticeably better floor rigidity when we had done it. Did not take
that long either. Or you could run a circular saw across the existing
beams and knock out notches and put the herringbones through them..lets
face it, they wont be doing much once you are finished! The braces need
be no more than 2x2 or so.
As you have surmised correctly the 100 x 50 (Probably an old ceiling?
if there is a binder across it as you say) is insufficent for the span
The concept of "attaching" a 3 x 2 to a 4 x 2 one above the other is
only feasible if you effect a perfect connection to form a couple
between the tension and compresion zones of the beam. In practical
site terms this is not a starter. To do so you would have to use a UF
or PF glue on two perfectly planed surfaces and even then I would be
However the idea of beefing up the tension zone by fixing the 7 x 2 to
the 4 x 2 is theoretically and practically feasible. It would require
double sided toothplate connectors (timber dogs) at about 600
centres- a right angle drill attachement - bolts - 2" plate washers -
in other words a properly engineered job. Personally I think that is a
complete waste of time and I would if I were to do this simply get
what benefit (limited) I could by using 4" nails at appropriate (6"?)
centres. Far better is to ignore this and put in decent 7 inch joists
of a stiff wood (get some machine stress graded timber - say old
fashioned SC4 (I have forgotten the new notation) or SC5 or up the
thickness to 3". or double up the 2" timbers to get your stiffness.
A further thought is to ensure your joists and rafter existing
connections are sound and if not use framing anchors to nail a
connection - use square twisted nails.
Where they meet over the middle brick wall I would use a decent
connection. multiple nail or even a double sided tooth plate and bolt
as this is also a tie for your roof to stop spread of the A frame.
Nogging is always good idea and as others have said - herring bone
strutting. IF nogging do it at about 1/4 span each end and in the
If you use sheet flooring (Yuk) rather than floor boards then you can
glue and pin this to the joists to form a T beam structure which will
stiffen up the floor a lot.
I wouldnt nail to a ceiling, screwing is stronger and wont break it
Its totally up to the OP, but gluing and screwing (every 6") 3" wood
onto the existing joists does work. Its not necessary to try to match
the strength of a one piece 7" beam.
If you use new full length 7" beams, keeping them independant from the
eixsting ceiling will help greatly with noise insulation.
Re noggings its all up to you how much extra stiffness you want.
Why not put the main timbers in then test it, and add noggings if
inadequate. 7" over 3.5m without noggings should be more than
Noggings don't add much extra strength, but they are a handy way to
replace the lateral restraint function of a tie beam if you need to
remove one. Herringbone (either wooden, or the modern metal stuff) will
add stiffness, but that is not so easy to fix to the old ceiling joists
as well as the new ones for lateral restraint of the old joists.
Much depends on what you want to achieve...
from geraldthehamster contains these words:
That configuration wouldn't add very much strength at all. I was
assuming that the original constructor had at least half a clue and had
added the 3 x 2s to the 4 x 2s to make ersatz 7 x 2s.
Thanks everyone for your helpful and informative replies. I can
certainly take enough from this to do the job.
Could someone describe "herringbones" to me, in this context? I can't
immediately visualise what is meant.
A row of X shaped struts arranged between adjacent joists. Used on long
spans typically at the 1/3rd and 2/3rd positions (or just once in the
middle on shorter ones), they couple the joists together and add extra
stiffness. Makes the floor less bouncy.
Looking end on you would see:
Where # is joist and x is herringbone.
Herringbone struts were traditionally shaped from timber - typically
something like 2x1" and nailed to the joist sides. Modern ones are metal: