Thread: Edge jointing long beams (for outdoor use)

The glue line needs accurately jointed surfaces to work. And the kind of glue is also critical. I think of ways to save money often, but that wouldn’t be one of them. Structural requirements of a beam are what they are for a reason.

Having said that, I have made curved laminated beams before, but they were accurately milled and the glue expensive and I used lots and lots of clamps.

Many more educated than I on this forum will have your answer.
however, you may be able to calculate your beams a different way

nailed together to be double thickness at 150mm
decreased spacing compared to your 250mm ie current spacing 1200 new spacing with 150mm @ 600
blocklayer.com has a roof rafter calculator

no doubt you may need to provide more info but that’s my 2 cents
heers

Yoboseyo

Much depends on whether you require council approval. If this is a completely new structure and council approval is required you will need an engineer's certificate to join your timbers in the way you describe and the engineer may deem more than you have envisioned. It will turn out much more expensive than if you had just bought the large dimension timber.

If council approval is not required, because for instance you are replacing or repairing an existing pergola, but you still wish it to be structurally sound you could consider gluing and screwing (actually, bolting would be better) the components together incorporating a slight camber in the beams so the curvature is at the top. This woud be easy to do if you were doing multiple layers that were , say 40mm to 50mm deep, over a 4m length. You will not be able to bend two beams 150mm deep. I was contemplating a while back using a similar technique with 25mm timbers (about eight layers) also with a mild steel strap 30mm x 5mm on the underside of the timber bolted through at regular intervals. This is for an existing shed that has seen better days and is starting to sag in the roofing timbers. I have not done this as yet so I can't speak from experience as to the success.

This method to which I am referring (without the bolting/screwing and steel strap) is sometimes called Glulam, although that may be a trade name: I am not sure.

Regards
Paul

Thanks for the thoughts, all

The structure itself doesn't require approval, as it's not attached to a building/short enough/far away enough from boundary, so I doubt the method of construction will call for one.

I was thinking of adding some other reinforcement like screws or some other kind of alignment. As it's not going to jointed down the middle, there will be a thin part and a thick part, I was thinking the thin part can be the "non-structural" (I didn't think about splitting the thin part into pieces, but that's an option too).

I was kind of fancying my chances of getting a cleanly mated surface, given my experience in truing up shorter pieces with hand tools. Just playing it out in my head - I won't know until I actually start doing it though. For one, there won't be a reference face to be square against

Originally Posted by yoboseyo

I was wondering whether this is feasible. I need 250-300mm × 4m beams for a pergola and they are more than double the price of 150mm. I can save a lot of money by jointing and gluing up 150mm beams.

Is it feasible for these lengths? I was thinking that while its hard to get it true, it might not be necessary as the beam will flex a lot across these lengths. I can spot plane it true until the gaps are gone. The cosmetics won't matter too much.

Thoughts?

Use these? I-Beams | WoodSolutions There are span tables on that site as well.

How are you going to prop them accurately, that is likely the biggest hurdle.
I'd guess a laser would be the bees knees here.
And what's the longest RIGID straight edge you've got?
Could get a graphite stick or dark crayon to paint onto the straight edge and keep finding high spots with that until you have full length shavings.

Tom

Originally Posted by Tom trees

How are you going to prop them accurately, that is likely the biggest hurdle.
I'd guess a laser would be the bees knees here.
And what's the longest RIGID straight edge you've got?
Could get a graphite stick or dark crayon to paint onto the straight edge and keep finding high spots with that until you have full length shavings.

Tom

Im thinking a 4m aluminium angle from the supplier

Then I can edge joint parallel beams together so the squareness across the thickness cancels out. Then keep going to I can get full length shavings as you say. May not even need to be full length if I just keep spot planing and checking

If it were me... I'm assuming these beams will be side to side, not top to bottom. Top to bottom is an other process altogether but more than doable. And, this is a pergola out in the weather, trying for furniture grade joints is futile and a waste of time. If it's not a problem to see the joint, I'd batton screw them together every 300mm, alternating 50mm from top and bottom, then fill the screw holes with bog or epoxy and sawdust mix (in truth you could easily get away with spacing of 600mm). If I didn't want to see the joint I'd use polyeurothan glue. That will fill the worst gaps and it's an external glue so will last as long as the pergola lasts. One side lightly sprayed with water, don't soak it. Then roll the glue on the other face and slap together, then batton screw probably every 600mm, alternating as above. Screw on the side you will see the least. Once the glue is set off, run a power planer over it to clean it up. Then, most important part of the job, have a beer or three... Then repeat for the next beam.

If you're attaching top of one to the bottom of the other, i.e. increase it's height. Most important is make sure you joint the surfaces to be mated square. Or so that they add up to square. Over the whole length, I wouldn't worry about being out by 6mm, you can easily pull that together with clamps and glue over a 4m length. However, that isn't the case if you have the gap at the end and it only goes about, say, 600mm back. The force needed to pull the two beams together is such that they will pop later or you simply can't pull them together at all. This is where the polyeurothane glue works well, in that you don't need to worry too much about the gaps. The glue will easily fill a 6mm gap so dont' get too hung up on tight joints - remember this is a pergola not a dining table. You can clamp and drill and drive some long batton screws in or simply clamp and let the glue flash off. If you go the glue only, don't crank the clamps down too much, introducing stress is a bad thing. Remember to hydrate - by this time you should have had at least 4 beers...

Oh ya. As for needing a straight edge - na! Eye ball it, or string line it. Square, or adding up to square is a bit more important. Like I said, you're building a pergola, such exactness isn't necessary

I am going to try to source 200mm wide beams, but I haven't ruled this out if in a pinch. I'm only contemplating this due to the scarcity of such wide beams in a hardwood like blackbutt (my preference for this is aesthetic - not span considerations).

Disclaimer, not a structural engineer.
If i had/wanted to do something like this, id do something like edge joint 100+200, making a wide beam, then repeat, and laminate the 2 'opposite' so the joints are not aligned.
---x--------
---------x---
Does that make sense? Each individual beam could be thinner but not half eg if your design called for 300x50, then use 38mm boards, 100x38 and 200x38

But probably shallower beams closer together would be easier...

You can save yourself a lot of time and money now and into the future by using RHS or C or Z purlins as they won't twist warp bend or rot like timber is going to do.

If you are doing it for visual reasons use an structural timber or steel section and clad it in your preferred timber with expressed joints.
If it is out in the elements the joints are going to open up anyway