Hi
I saw someone once suggest one could strengthen existing joists by gluing wood onto the top of them to increase their depth. Can this work? I'm aware that there is no problem with thinning the joists' depth at the ends, so I wonder if this approach is doable.
Regards, NT
This will help if the glue bond is VERY strong. If it ain't, and it fails after a period, you now have an overloaded beam that you thought was twice as strong.
Right. Glue and screw then??
Regards, NT
Plane, glue and screw? AFAIK most joists are unplaned, rough, dirty things which may not be 100% true after a few years up there. Again AFAIK you have to get a good clean woodwood contact for the glue to work effectively. Otherwise you just glue the high spots together. Sounds possible, but potential major hassle.
Cheers Dave R
There are two possible modes of beam extension. Stacking another beam on top, and just sharing the load. The stresses in this case are fairly low, and vertical.
The other is to succeed in laminating another beam on top, so that the beams act as one unit. In this case, if the beams are exactly the same, then the stresses across the joint are similar.
But, the beam behaves totally differently. The top half is in compression, the bottom half in tension. The middle of the beam is not stressed.
If the beams are different (as they will be if they are loaded when you try to do this without compensating on the shape of the top beam) then there will be a lot of shear stress along the joint, trying to pull it apart.
I'd guess that choosing a beam that will end up with the same size of beam, once you shave off some to match half the 'bow' in the other beam, then clamp it together, and glue/screw copiously would be best.
However, you ideally want to match not the section, but the stiffness, which is likely to be subtly different.
Though the second case is much better, as you quadruple the strength of the beam, it's probably going to require a structual engineer to make sure you've got it right.
Why do you need to strengthen the beam?
What about screwing/glueing as suggested, then lining the full length of sides with quarter inch ply thus:
| [] | \ (obviously a 6 inch single depth of ply on each side to join 2 x 3 inch timbers - couldn't draw this) | [] | /
Did this years ago in another house. worked for 25 years until i moved house.
ZD
Screwing and gluing enough. Long screws from unifix-online.co.uk. Long screws here are cheap.
Easy to do. I have done it. Lots of PVA glue and long scews. At various points, especially where joists but onto each other, use joist jointing plates, available from Wickes. Just screw these in using 1.5 to 2" screws. These really tighten up matters. Ensure that the joist on the top goes the full way into the eaves, and is over the supporting wall at each end. The floor realy tightens up and bounce eliminated.
£10.56 for 50 from Scewfix. part No. D35844
Why not just coach bolt another joist to each side of the existing joist. No more height but a lot more strength.
Laminating against on top of another prevents bounce.
IMM wrote..
Rubbish. We have no facts about the existing joists, such as their size, span and condition. We don't know their existing or intended loading conditions or why they need strengthening. We don't even know if they are flat roof, ceiling or floor joists! It's impossible for anyone, even you IMM, to come up with a worthwhile solution, and a DIY bodge such as you are suggesting, not backed up by any thought or expertise, is probably going to result in considerable cost in making the house saleable again.
What engineering principles are behind this idea of gluing and screwing? How much extra load will the joists support once this has been done and can this actually be calculated and/or guaranteed? What about the problem of buckling created by the extra depth? This is a Structural Engineering issue and a considered design solution should be provided by someone with at least a smattering of engineering knowledge.
Peter
A relative of mine suggested putting deeper joists in between the existing ones and strapping these to the purlins (?) at one end while the other end was on top of a supporting inner wall.
Well an interesting thread so far. Why not bolt on the side? Doubling the width of a beam doubles its stiffness, whereas doubling height approximately quadruples its stiffness. Then theres the q of already routed cables etc, and also the q of lathe and plaster ceilings that have slopped over the sides of the joists making it impossible to add equally large ones to their sides.
And finally the difficulty of screwing an unstressed straight beam into the side of a stressed bent one. When screwing on top, the screwing pulls them together automatically. (And possibly pulls the ceiling apart.)
Finally I dont need to do this, but I do want to learn how as I may want to in future. It occurred to me this approach would possibly be less work than replacing the beams altogether, and significantly less expense. Not just less wood, but no crane needed, no replacemnt of ceiling and little or no redecorating.
It also occurred to me that it might even be possible to use some of the assorted scrap wood that passes through and past your average site.
Regards, NT
If I understand that right, s sure recipe for a knackered ceiling. If you have every other wood beam suporting the floor, then every other one flexes and bends down with the loads on it, and the original ones that no longer carry the floor load dont move. So your ceiling gets repeatedly corrugated and uncorrugated, and falls apart.
Regards, NT
Agreed. But it's much easier to bolt joists to the side. I'm not sure how one would even get a good uniform joint on top though perhaps somebody can explain how.
and plaster ceilings that
equally large ones to their sides.
We used joists half and inch lower. Needed to reroute a few cables but wasn't a big deal.
The new joist has to be stressed using props from below before you drill and bolt. If you want to save the ceiling this may be difficult of course.
DIY bodge? A professional suggested this to me. You make the laminate joists as thick as normal joists to take the weight and reduce bounce. I made it thicker in case.
Which precludes you.
I did suggest in an earlier post to consult a structural engineer. The engineer who suggested it to me gave rule of thumb guidelines of span etc. He did say that if the new laminated joist is supported at the eves (on the outer walls) and at intermediate walls across the loft (intermediate support walls) that a total laminate thickness of 8" minimum is needed . I went
10". he did say that were two joists on top of the existing meet, to glues and screw well and have a nail plate well screwed in where all three joists meet, and to have nail plates at intervals across the laminated joists. Screwed in from the top first and then nail platedThat better for you?
On top is easier.
See my other post on this.
So why did you choose this rather than bolting on top?
Bolting on top is easier in that there is no pre-supporting of the existing joists to do: you just drill and add coach screws, along with lots of glue.
Any routing holes in existing joists dont need to be duplicated.
And I would expect theres some chance of saving the existing ceiling, if you put all bolts in part way initially, and tighten them in the right sequence.
I'm not seeing how side placement has any advantages other than saving
2" or 3" of height.Regards, NT
Thicker or deaper? Understand the second term?
The OP wanted to add depth, you concurred about the screwing and gluing but actually you have misunderdstood either the OP's question or your structural engineeers advice which refers to doubling. My structural engineers advice regarding load sharing was to nail every 300mm or so which I think is preferable to screwing. Wouldn't trust the hardening of screws in this application.
Alan
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