Strengthening joists by adding additional timber, glueing and screwing. Will
the new 3 inch timber be ....significantly.... weakened if first, a three
eighths diameter hole is drilled three quarters down, the screw driven
through into the lower piece, and then the hole filled by a tight fitting
three eighths wooden plug glued in place?
Is this on top or to the side of the existing timbers ?
If to the side then your BCO will (should) insist on proper coachbolts and
The use of glue is quite 'innovative' though. Any experts out there on
whether it will actually work or not ?
=============It would appear that the OP is trying to attach strengthening timbers to the
top of existing joists with screws set in 2" (approx)deep countersinks in
the new 3" timbers. I think that this will probably not leave enough 'meat'
for the screws to give adequate strengthening to the new combined joist even
if it is glued as well. I think the side fixing with coach bolts etc.
suggested will be easier and more reliable.
Try to make main joists taller by adding to the top? Why? That doesn't add any
strength to them unless the new section is banded around with metal strapping
every few inches. Or by the method you say of making the joists literally
doubled up side by side.
If you make the spacing between the joists smaller it also adds strength to the
floor by spreading the loads over more joists of the same strength. Like snow
shoes do on the top of soft snow.
Adding bits to the top won't do anything but make the joists taller, unless you
can ensure that joint between the two is properly solid fixed. Cutting bits out
of the additional timber will only make the joint between less strong.
============I think that the OP has no screws long enough to go through the new 3"
timber into the existing timber - hence the 2" countersinks and plugs. It
doesn't really matter because, as you say, his method appears to be
Hmmm. Obviously the opinions on this group vary from month to month,
considering that in previous longish threads, doubling up seemed to be the
consensus way to go.
Actually, the doubling up advice was given by a Local authority planning
officer, presumably speaking from experience and qualification, (although
the counter sinking was my own query), .....assuming the loft will only be
used for storage. The one bit of reliable advice ever given on this forum is
to consult your local planners.
============Your query was about a particular method of attaching the strengthening
timbers. Your proposed method of screws in deep countersinks and glue would
probably be weak because there would be poor glue contact between the joists
and because there might not be enough strength with screws. Joists are
usually 'sawn' timber which is probably too rough for good gluing.
If it was possible to attach the new timber to the top of the existing joist
by coach bolts going through both new and old timber or (as suggested by
another poster) 'banding' with steel bands then that would be strong .
Either of these methods might not be practicable - possibly because of
access to the underside of the existing joist. In this situation the
alternative suggestion of adding timbers to the side of the existing joist
with coach bolts is a safe and reliable method of strengthening.
Since your 'Local authority planning officer' recommended 'doubling up'
presumably he also suggested some method of attachment. Can you say what
method of attachment he suggested, if any? Perhaps you found his suggestion
unworkable and as a consequence asked here for better advice. I assume that
you haven't asked him about *your* proposed method, but I think that he
would probably say very much the same as people have said here.
Do you think he meant doubling them up as in adding height to the joist as you
propose, or doubling up as in laying new joists alongside the original ones and
doubling the amount?
Laying joists of the same size as the originals will actually double the
strength of the floor by decreasing the spaces in between the joists. As long
as they are the same size and shape as the original joists that is.
No misunderstanding - he even drew diagrams for me! I agree that laying
additional joists would spread the load, but as other threads here have
pointed out, I feel that could be to the detriment of the ceiling. I am
happy with the general idea of increasing the height. Just the final details
to settle. This is not a full scale loft conversion - just to support
chipboard floor and boxes of household junk that SWMBO can't bare to part
Thanks for taking time to reply
Given that the "group" is a collection of individuals, is that so
I think the consensus still is. Adding height to a joist will add
strength to it (more than adding the same quantity of timber in width).
The way you fix the timber on top however will alter how much strength
you add. You get least benefit if the timber is just resting on top, and
most if they are bonded together along their length.
I think it was the counter sinking bit that was causing the main
problem. Why exactly do you want to countersink like this?
The advice you get here is, as always, worth what you paid for it,
perhaps even more.
Don't think that is true is it? Adding another beam on top of an
exisiting one will result in a stronger composite beam even if the two
are not fixed together. Admitedly not as stong as if the two were fixed
such that they were behaving as one.
Its a bit different with dissimilar materials like that. The timber beam
would deflect far more for a given load than the concrete one. So having
concrete under a timber one, does to an extent remove the timber one
from the equation, since it will not be able to deflect far enough to
assert its stiffness and take much of the load. It will help spread
point loads on the concrete though.
The method seems to be a bit odd but the basic idea should work. All
that is needed is for the clamping force to be sufficient to overcome
the shear force along the plane and for the fixings to be close enough
together to prevent any buckling.
We don't know what it is the OP is adding to or even how wide but if he
is adding 3 x 2 on top of 3 x 2 he has at least the potential to get
close to quadrupling the strength of each beam. Laying another side by
side would only double it.
Personally I think 5" screws (without any counterbore) at say 15"
centres would probably be overkill but that is little more than a guess.
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