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
Having a look around the net, this seems to be the most common way.
Thanks for comments...
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
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.
On Wed, 26 Mar 2008 06:32:15 -0700 (PDT), email@example.com
2x6 or 2x8 should be available. 2x4 is often just used for vertical
things. For horizontal you usually use thicker lumber. Big
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
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 :) ).
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.
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.
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
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.
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
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".
Not 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
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.
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.
<grunting and flexing in my best gorilla fashion>
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
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.
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
Where did you get the design?
Here's one for you:
And Googling Pergola Designs will yield a plethora of ides, images and
On Mar 26, 9:23 am, firstname.lastname@example.org wrote:
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