Best way to join two 50X100 (2 by 4) planks?

[crispin.proctor]

Hi folks.

Firstly - my woodworking skills are a bare minimum (Never had much need) however, I am technically competent (He says :)

That said - I am looking for advice.

I am building a pergola which has a span between the posts of 5.4m. It's a fairly long span. (Or maybe not?)

The problem is, the only wood I can get is 2 lengths of 3m. I plan on joining them together to make an almost 3m length.

The join I intend to use is cut into the broad side by 25mm (half way) and 75mm along it's length. Doing this two each end. Then overlapping them (Is it called an overlap joint? :) but putting 4 m8 bolts thought. One in each "corner"

My questions are:

1) Would a generous dose of glue on the join help? 2) Is this the best way to join?

My concern is sagging. It's a decent span and joining it less than excites me.

Having a look around the net, this seems to be the most common way.

Thanks for comments...

Cheers, Crispin

[crispin.proctor]

I should add that there won't be to much of a load on the it. It's a pergola. There are two 6m lengths spaced 1m apart with a light lattice bracing them.

Weight might be an issue with snow buildup although we don't get heavy snow that settles for long in my neck of the woods.

[Jim Behning]

2x6 or 2x8 should be available. 2x4 is often just used for vertical things. For horizontal you usually use thicker lumber. Big generalization there.

[dpb]

Agree--only to add would need more info to judge what would actually be the best-suited, but 2x4 is probably not enough for that span -- it'll sag of its own weight if nothing else and unless braced will be terribly flimsy.

If were to joint as suggested, definitely need to fill the mating surfaces either w/ a waterproof glue or at least a mastic/caulk or the water will get in between the two pieces and keep them damp leading to early failure from rot...all in all, not a very good design (but you knew that :) ).

--

[jo4hn]

I don't think that a 5.4 m span is a good idea. You will get sag even from a single board over that length, let alone one that is joined. For that length something like a laminated beam or a wood I-beam would carry its own weight plus that of whatever cross members and plant life you might have. A post in the middle is best. If you want to try it, DAGS on "scarf joint". Better yet, there is software about that will calculate lumber size for a given load and span. Mights as well get all sciency about this. mahalo, jo4hn

[Joe]

I wouldn't be concerned with any sagging. I would be concerned of the liability risk of having a load bearing overhead structure who's sole purpose is to have people under it.

Get a steel I-beam and box it with a thin facia to make a "fake" beam. You can make this look slightly curved (the facia exterior) or make a much larger beam than the steel inside. Many cosmetic possibilities. It will never sag and that span in steel is trivial.

Or

Purchase an exterior grade glulam. (usually cedar)

Steel I-beams in my area are cheaper than the correct wood would cost. (a redwood 2x10 should span 16' 8" (5.03m) for a rafter with minimal snow load)

Don't worry about sagging, worry about hurting a friend's child who's over for a barbeque when the structure fails. Even though you may have seen poorly built structures that survived a long time, remember that much of the wood today is nowhere near the quality of the old growth wood of the past.

[crispin.proctor]

Thanks all for your answers. Not exactly what I was hoping for. More what I was fearing.... :) I'll see if I can get a thicker / longer piece. I guess I knew all along it was not right.

John - ease up on the paranoia. I might just line it with lead. That way, the people sitting under it will be safe from radiation as well. If I for one second think that it's dodgy - It'll come down and I'll be here asking for help. To make a pergola from two steel I beams - practical yes. Easy on the eye? Hardly...

Thanks again everyone

[Joe]

Read slower, I said wood boxed steel I beam. No steel is visible. They are gorgeous and used in luxury homes showcased in Architectural magazine, vineyard tasting centers, fine hotels, amusement parks, etc.. Around here most large beams used in construction are glulam or wood boxed steel. It's very difficult, much more expensive and structurally less sound to use solid wood, especially when spans are over 30' (10m).

One of my favorite wineries has a beautiful enormous pergola for outdoor wine tasting and dining using boxed I-beams. I would estimate the largest beam (roughly 14"x20") span is about 60' (20m), arched with 4 supports total. Spans between supports around 20' (~7m). It supports a thick overhead lattice with vine plants. Round column supports are also steel with wood surrounds. The majority of visitors would have no idea the beams and columns are "fake".

paranoia, an old guy with personal experience. All too often, replacing failed shoddy construction. I also have close friends who were hurt from a deck collapse. A deck that was not built structurally sound, didn't look dodgy, but collapsed when people gathered for a wedding reception. The homeowner is now deservedly bankrupt from lawsuits since he built an unsafe structure without permits/inspection and therefore was not fully covered by his homeowners insurance.

No offense intended, but you did suggest using spliced 2x4's for a 5m overhead span. That is "dodgy" and would never pass code inspection. Where I live we have to have permits for all overhead outdoor construction.

And my name's not John, but since you didn't bother fully reading my post I can't expect you to read my name either. Cheers.

[Max]

"Joe" wrote

Well said, Cheers.

Max

[Lee Michaels]

"Joe" wrote

Amen Brother.

I have seen similar things and build everything stronger than code. People have complained that I build things "too strong" or "too heavy duty".

I used to build rustic furniture out of 2 X 12 hemlock. You could park a pickup on one of my coffee tables. I have built bookcases thirty years ago that are still solid and strong today.

[crispin.proctor]

Back in my box.

Joe - My apologies. You are right - did not fully read your post. At a glance I saw steel I beam. I immediately thought back to my days as a crane repair guy and pictures of steel girders came to mind - overkill yes.

I will bow to all of you and heed the advice. Anyone want to buy a lengths of 2X4 I have lying around now? :)

I guess I bought first, asked questions later.

I will try find a local supplier for the box steel. If not, an extra support would be called for.

Thanks for you help though folks - much appreciated.

Cheers, Crispin

[Hoosierpopi]

CRISPIN:

If I understand you correctly, you need a longer board than you have at hand and thought to put two boards together end to end (albeit with a lap joint) secured by four bolts or such.

Since it appears that you are from some foriegn country where they don't know about inches and feet, you may not have wood stretchers in your local shops. But I am sure that, should you ask a local carpenter or two, they would be glad to send you 'round to a shop that might have one or two on the shelf.

Failing that, you might offer to trade someone with a longer piece of wood but bereft of a saw, your two shorter pieces.

If you insist on following your technically workable approach, a long scarf joint (sort of a long lap joint absent the right angles) glued up with a quality exterior-rated adhesive and employing a screw or two where the relative thickness allows would do.

Adding a "sister board" spanning the joined area, albeit out of sight (should the design allow), would be an improvement. In any case, spanning 20 feet with anything less than about a 5 x 30 cm board is ridiculous.

Where did you get the design?

Here's one for you:

Googling Pergola Designs will yield a plethora of ides, images and plans.

G.E. Tarballs