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?
Just wondering..
ZD
What?
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 so on. 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.
Cic.
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 seriously flawed.
Cic.
That's why I wondered if the OP was relying on the glue to make this joint. My guess is that ResinW isn't quite up to this.
:-) LOL !!! But Uni-Bond says it's "stronger than the actual wood". LOL !!! :-)
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.
The message from "BigWallop" contains these words:
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.
Hmm. I put a re-inforced concrete lintel under an existing wooden lintel and the BCO insisted I prove the concrete lintel could take the load itself.
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.
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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.
Cic.
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.
Using PVA glue to BS 4051 with screws driven through at 6 inch distance - for storage purposes only remember. We are not talking a full scale loft conversion here.
Not at all - strikes me as reasonable, because although I am not qualified - he is. And reasonable for a competent diyer.
"Better?" Alternative maybe.
I asked here about the countersinking -a query, not a proposed method - the main method is his, and one which, incidentally has been employed by at least 2 other people on this group without problems.
Obviously not.
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 with.
Thanks for taking time to reply
Given that the "group" is a collection of individuals, is that so surprising? ;-)
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.
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