Pergola building - I need wood

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Man, I always get stuck when choosing materials for projects. To the point that I rarely start. But this current project is under way and I'm determined to see it through. We've moved 4 tons of lawn and soil from a corner of our garden, levelled it and then put a sub base in which is compacted ready to receive some slate chippings to finish it.

We've dug out 5 holes to receive 9 foot (2700mm) posts to create a corner pergola - that's one post at the back corner, and two on each of the other points of the triangle. The idea there is that 3 of the posts will be square to each other, so that beams can be attached flush from the back corner out to the points. And then the other two posts will be at 45 degrees to take another beam/beams across the front. Then rafters will tie that all together.

The resulting triangle will be 2500 x 2500 x 3700 (there's some overhang for style reasons). My intention is to use a metal bar instead of a centre rafter, which will be used as a pull up bar so it doubles up as an outdoor gym. That will be about 1700mm long, hitting the front beam dead centre of a 3500mm span (approximately).

And this is where I get stuck. I want to know if that span will support the weight of an average human hanging on it dead centre. Rather, I want to know what wood to use and what dimensions so that it is strong enough not to sag a lot.

I was going to use 2x6 (50 x 150) beams and rafters. But I could swap the weight bearing beam for a 5x5 (125 x 125) beam.

I was spec'ing this with Green Oak, since it will look good and shouldn't need treatment to last many years. I'm not fixing the posts into the ground, rather I'm going to back fill them with sub base gravel which will be compacted. That'll help with drainage, will be better for the environment, and should reduce rotting. Though, these holes are currently full of water as our garden is clay and poor drainage.

I can save money with soft wood, but it'll need treating and I suspect won't be as strong.

So, who knows their wood and its strength in construction?
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Comments

  • underdogunderdog Frets: 8334
    I'd say oak would be your best bet for lasting, though all wood that isn't a living tree will rot in the end. I'd definitely out some postcrete in the holes to be honest it will just make it easier and with a layer of gravel in the hole first help protect the wood a bit better. 

    As for size 6x2 (or if you're really worried 8x2) will be plenty. I've used 8x2 as the main floor joists in a log cabin who's floor is 10-15 foot above ground level and it's solid as can be.
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  • Cheers @underdog ;
    I'm assuming that your 8x2 joists aren't 3500mm apart though, so they will be sharing the load.
    I look at these dimensions and think "that looks suitably over-engineered to do that job", but then my fears creep in.

    I'm also thinking that I'm not going to be doing pull-ups directly on the end of the bar, I'll be about half way along it. The other end will be supported by one of the posts in the ground, so that will be absolutely solid. The cost of oak has got me looking at how to reduce the spec and still get something strong and attractive.
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  • underdogunderdog Frets: 8334
    edited October 2020
    Cheers @underdog ;;;;
    I'm assuming that your 8x2 joists aren't 3500mm apart though, so they will be sharing the load.
    I look at these dimensions and think "that looks suitably over-engineered to do that job", but then my fears creep in.

    I'm also thinking that I'm not going to be doing pull-ups directly on the end of the bar, I'll be about half way along it. The other end will be supported by one of the posts in the ground, so that will be absolutely solid. The cost of oak has got me looking at how to reduce the spec and still get something strong and attractive.

    No the joists are closer but your joists are not bearing much of a load at all, there's no weight of a floor on them. The only weight outside of their own weight you need them to support is yours (from what I can gather?) I'd be confident that a 4x2 either end of a metal pole would hold the weight of even a big guy forever and a day to be honest. So anything bigger would be 100% fine to me.

    The trick on the farm for posts go in to the ground is stand them in a tub of creosote for 24 hours before they go in, seems to make them last longer.

    You can get posts with pre fitted "bands" that protect them in the ground but for the life of me I can't recall their name.

    I will be honest if I was building the same thing I'd be using bog standard treated timber and treating any cut ends, it would save a lot of money.
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  • MusicwolfMusicwolf Frets: 3627
    underdog said:

    I will be honest if I was building the same thing I'd be using bog standard treated timber and treating any cut ends, it would save a lot of money.
    That's what I did and, twenty years later, it's still standing.
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  • Bog standard treated timber isn't available in the dimensions I want for aesthetics. If also imagine that the strength and stiffness of softwood would be significantly poorer than that of oak.
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  • FuengiFuengi Frets: 2849
    Google "timber spans" and you should get a link somewhere to timber specs - C16 C24 etc - and their spans. It might be a building control document.

    If you get stuck ping me a PM and I'll photo and send you a sheet. 
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  • ESBlondeESBlonde Frets: 3576
    edited October 2020
    The thing that kills wood is not water so much as the microbes and fungi. They need water to flourish, but it's thier presence that eats the wood fibres. Contact with 'soil' means there are just so many more available to attack the timber, this is why treatment is required to keep them out. Most of the really effective treatments are now banned for DIY use (and lots of professional uses too) and require specialist handling. but you can still get good old fashioned creosote if you look hard, it should be kept well away from any edible plants and it stinks for weeks after application (so the wife will complain).

    Some wood has a natural resistence to rot, oak is quite good but ceder is very good and was used in fencing for hundreds of years.


    Edit. Another rot resistent timber is chesnut which is often available direct from a mill at reasonable cost.

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  • SnapSnap Frets: 6256
    underdog said:

    The trick on the farm for posts go in to the ground is stand them in a tub of creosote for 24 hours before they go in, seems to make them last longer.


    That's cracking advice there mate.
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  • I've done an order. Custom cut fresh sawn oak from a local timber mill - only 10 days lead time which is great.
    I've measured, calculated, adjusted, designed my joins, adjusted again, calculated again, and repeat.....
    I sure hope I got it close enough, otherwise that's an expensive mistake!
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  • DominicDominic Frets: 16004
    The Engineering works by calculation of the "moment " in a leverage equation calculation which takes into account not just the span but also the "point load" and calculus of deflection .Like steels,there is also the factor for consideration of the bearer deflecting over span under it's own weight . It's not the same as an even distribution of weight on a floor where the floor surface acts as a "spreader" .
     The integrity of structural timber is graded in proper timber yards from C12 to C24 based on the mass,grain ,natural faulting and knots,shakes etc .
    I don't know how much you weigh but I would say that you are under-engineered without going into the calcs .
    2 slimmer but taller timbers bolted together with M10 bolts at 70cm centers will increase the torsional strength exponentially .
    Marine Ply Beams formed from laminated ply bolted together ON EDGE can be as strong as mild steel .
     I work with structural engineers all the time .
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  • ToneControlToneControl Frets: 11788
    Dominic said:
    The Engineering works by calculation of the "moment " in a leverage equation calculation which takes into account not just the span but also the "point load" and calculus of deflection .Like steels,there is also the factor for consideration of the bearer deflecting over span under it's own weight . It's not the same as an even distribution of weight on a floor where the floor surface acts as a "spreader" .
     The integrity of structural timber is graded in proper timber yards from C12 to C24 based on the mass,grain ,natural faulting and knots,shakes etc .
    I don't know how much you weigh but I would say that you are under-engineered without going into the calcs .
    2 slimmer but taller timbers bolted together with M10 bolts at 70cm centers will increase the torsional strength exponentially .
    Marine Ply Beams formed from laminated ply bolted together ON EDGE can be as strong as mild steel .
     I work with structural engineers all the time .
    I saw a  massive ply beam being put in as a ridge beam for a roof once in some American building programme, very impressive

    My summerhouse bothers me a bit, it's been there 6 years, but I suspect the suppliers were not careful enough, the 5.5m purlins have visible bending, and I wonder how much is acceptable
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  • the_jaffathe_jaffa Frets: 1777
    edited October 2020
    Dominic said:
    The Engineering works by calculation of the "moment " in a leverage equation calculation which takes into account not just the span but also the "point load" and calculus of deflection .Like steels,there is also the factor for consideration of the bearer deflecting over span under it's own weight . It's not the same as an even distribution of weight on a floor where the floor surface acts as a "spreader" .
     The integrity of structural timber is graded in proper timber yards from C12 to C24 based on the mass,grain ,natural faulting and knots,shakes etc .
    I don't know how much you weigh but I would say that you are under-engineered without going into the calcs .
    2 slimmer but taller timbers bolted together with M10 bolts at 70cm centers will increase the torsional strength exponentially .
    Marine Ply Beams formed from laminated ply bolted together ON EDGE can be as strong as mild steel .
     I work with structural engineers all the time .
    This is almost right but there are some errors.

    C graded timber is for softwoods rather than hardwoods like oak. Oak will have a D grade.

    The torsional strength is not really a concern either as the beam will be loaded vertically rather than with a lever arm.

    Taking me as an example; I weigh about 120kg which is about 1.2kN. Hanging that weight off the centre span of a 3.5m beam will create a bending moment of (1.2x3.5)/4 = 1.05kNm

    For a D30 grade (lowest grade for hardwood) Oak beam then you would need a Z value of 116.6 x10^3 mm^3. A 150x50mm beam gives 187.5x10^3 mm^3 therefore is well in. And that's with me being pretty heavy.

    Upshot is, even if you've gone with a 150x50mm Oak beam then it will be well in and not be a problem.

    I'd probably go for a bigger section that that though purely for aesthetics
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  • DominicDominic Frets: 16004
    ^ sorry ,I thought he was using softwood 
    Pedants you engineers !
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  • the_jaffa said:
    Dominic said:
    The Engineering works by calculation of the "moment " in a leverage equation calculation which takes into account not just the span but also the "point load" and calculus of deflection .Like steels,there is also the factor for consideration of the bearer deflecting over span under it's own weight . It's not the same as an even distribution of weight on a floor where the floor surface acts as a "spreader" .
     The integrity of structural timber is graded in proper timber yards from C12 to C24 based on the mass,grain ,natural faulting and knots,shakes etc .
    I don't know how much you weigh but I would say that you are under-engineered without going into the calcs .
    2 slimmer but taller timbers bolted together with M10 bolts at 70cm centers will increase the torsional strength exponentially .
    Marine Ply Beams formed from laminated ply bolted together ON EDGE can be as strong as mild steel .
     I work with structural engineers all the time .
    This is almost right but there are some errors.

    C graded timber is for softwoods rather than hardwoods like oak. Oak will have a D grade.

    The torsional strength is not really a concern either as the beam will be loaded vertically rather than with a lever arm.

    Taking me as an example; I weigh about 120kg which is about 1.2kN. Hanging that weight off the centre span of a 3.5m beam will create a bending moment of (1.2x3.5)/4 = 1.05kNm

    For a D30 grade (lowest grade for hardwood) Oak beam then you would need a Z value of 116.6 x10^3 mm^3. A 150x50mm beam gives 187.5x10^3 mm^3 therefore is well in. And that's with me being pretty heavy.

    Upshot is, even if you've gone with a 150x50mm Oak beam then it will be well in and not be a problem.

    I'd probably go for a bigger section that that though purely for aesthetics
    Thanks dude.
    I also realised that my weight isn't going to be 100% on the beam, since it will be supported at one end by a vertical post. So, it'll only be ~50% of the weight/force.

    That's what made me go ahead and order. I'm absolutely certain this will be fine. If not, I think I can do things to strengthen it that won't compromise the aesthetics and function.
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  • Update....

    So, after the early indecision, my wife and I have been using every spare moment to progress this project. Today we fixed the pull up bar in place so it's usable. I have to finish off the aesthetics which I'll do this week if it's dry.

    We're so proud of the fact it went together well, and the pull up bar is rock solid.


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  • HAL9000HAL9000 Frets: 9550
    As an aside...

    i remember listening to something on Four Counties Radio years ago where listeners were asked what a pergola was. First caller claimed it was ‘someone who breaks into your house and nicks your things’.
    I play guitar because I enjoy it rather than because I’m any good at it
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  • This weekend we did the joinery for the top beams, and then cut the aesthetic finish on everything. I also mounted an infra-red heater (seen at the bottom left in the photo before mounting) and then sat down and chilled for a bit. It's great. Very therapeutic.



    I've already started using the pull-up bar, and with the addition of some gymnastic rings I can do a variety of exercises to build strength.

    We'll enjoy it for a few weeks, starting with New Year's Eve when we plan to attach some lights, light a fire and sit with red wine and some nibbles.

    Next up is a hot tub to sit next to this, built from a reclaimed IBC container and initially heated with a Lay-Z-Spa pump. After that we intend to build a pizza oven from an oil drum, which might also be a water heater for the tub. And, I intend to build some benches with integrated cool boxes under the seats.

    Lockdowns have been tough so these projects, the gym, and the peaceful space outdoors is going to get me through the next few months.
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