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Planning a pergola... and building it

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Planning a pergola... and building it

Postby spb » 19 Apr 2021, 00:45

I've got a commission to build a pergola at some point this summer, currently in early planning. The client for this one is my mother, so there's plenty of leeway for collaborative design where appropriate. She's not fussy about the materials or construction, but I'd like to use green oak and proper framing, which I've never worked with before. I'm hoping for a bit of a sanity check on both our ideas by posting at this stage, with updates of course to follow.

This is where it needs to go, in front of the living room french windows:

Image

The window/door arrangement was inherited with the house, and frankly should never have been built the way it was - in the summer, the thermal expansion makes horrible creaking noises with the occasional bang thrown in, and when the wind gets up in the winter, the whole assembly shakes and rattles. It's just too big for the materials used. Part of the brief, therefore, is to provide (at least once the plants have grown around it) some degree of shade and wind disruption to mitigate those flaws, without preventing the light from getting in through those top windows. Another part of the brief is to preserve the view from the window.

Those requirements lead us to four posts at the corners, horizontal beams running parallel to the wall between them, and 'rafters' notched into the top of those beams, with the roof structure at the level of the divider between the doors and windows above.

She's got the village handyman on retainer for a morning a week to do whatever garden work she can't manage any more, so the current suggestion is that he dig out and concrete in some 100x100 metal post footings that I can place the timber into once it's done. This is attractive to me because he's more familiar with concrete work than I am, lives an awful lot closer, and is in much better physical health than me. If there's preparatory work that can be done in advance to reduce the amount of time I need to take off work and go and stay, that's only an advantage.

Size wise, it needs to be (approximately, plus or minus 300mm) 4.2x3.3m in footprint. That's to (a) span across the windows, which are around 4m wide, and (b) give room for the table and chairs in the photo. For scale, those are roughly 600mm paving slabs, and the outer posts will be set into a bed that's yet to be dug adjoining the patio edge.

Questions in my mind at this point are mainly the following. Obviously many of them are interrelated:

  • Is the plan to have post footings in place beforehand a sensible one?
  • Will 100x100 posts look too spindly in a structure this size? If yes, will it be viable to use larger posts and shape them down to 100x100 at the base to fit into a post footing?
  • What size section will I need to avoid noticeable sag on the cross beams, spanning 4.2m or thereabouts?
  • Assuming diagonal braces between the posts and the cross beams, what degree of bracing will be needed in the other plane (perpendicular to the wall) to prevent racking in that direction, and how is that best arranged when there are only 'rafters' running that direction, no beams?
  • Do I actually need to include larger beams in that direction as well as those making up the roof, in order to brace them?
  • If the 'rafter' pieces are notched into the tops of the beams, what's the traditional method to fix them in place? I can't really see how you'd drawbore a half lap, and this location gets non-trivial wind on a regular basis.

It's all very much an idea at this stage. With any luck, if what we're thinking above is remotely sensible, I'll start looking for timber suppliers and tools and we'll be looking at starting in a month or so.
Last edited by spb on 11 Jul 2021, 21:56, edited 1 time in total.
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Re: Planning a pergola

Postby Mike G » 19 Apr 2021, 07:41

Nice little project, and a good introduction to green oak framing.

Firstly, the plan to set shoes is concrete is fundamentally the best approach, but is fraught with problems.It's obviously absolutely critical that they are put in the right place, and to the right height, and achieving that isn't easy. Ive done it myself taking a great deal of care, and yet found one of the feet 12 or 15mm out of position, and it really makes things awkward. I think there are two ways of getting them right. You either make a temporary frame to which you screw the shoes before setting them in the concrete, or, you make the entire structure, lift it onto some props to hold it in the right place, then fit the shoes and pour the concrete around them. Both approaches take some thinking about.

I think 4 x 4 frames are right size for a pergola of this scale. People do make the mistake of over-sizing oak. I do worry slightly, though, that you are planning it rather low. The height of the top of the doors means you won't be able to grow anything over it, because growth dangles down. I'd want to be a foot or so taller, I think. Mine (below) is 2.1m to the underside of the joists.

This structure, from your description, will be free-standing (not attached to the house). Therefore, bracing, bracing, bracing. Oak framing generally uses curved ("arched") bracing. Here is a picture of the pergola I am planning for later this summer (the wood for it is in my current order, so I'll have to get on with it).

Pergola.jpg
(87.97 KiB)


You only have 4 posts, not 6, so you'll only have braces at the ends, but they are a critical element of the structure, bracing against racking, but also reducing the effective span of the plates (longitudinal members). Your end cross-members have to be joined in properly to the framing, because, as you note, they, and the braces, are what prevent racking in that direction.

That's a big span. I can see why you don't want a central post, but I think you are going to need to reduce it a little. If you could move each post a foot or more in-board that would really help. You are also going to need your bracing to be big....possibly bigger than that shown in my drawing. Visually, a heavy horizontal looks odd on a pergola. You might bump it up to 5x4, but certainly no bigger. It's not a piece of furniture or a house, and if it sags just a little it isn't a huge problem.

The cross members would have been wedge-pegged ("trunnels") in the old days, but I think there is merit in using a coach screw, and it's what I'll be doing. That way there is no vertical end-grain providing a route for water to track in.
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Re: Planning a pergola

Postby spb » 19 Apr 2021, 13:12

Thanks Mike - that's exactly the sort of insight I was after.

Mike G wrote:Firstly, the plan to set shoes is concrete is fundamentally the best approach, but is fraught with problems.It's obviously absolutely critical that they are put in the right place, and to the right height, and achieving that isn't easy. Ive done it myself taking a great deal of care, and yet found one of the feet 12 or 15mm out of position, and it really makes things awkward. I think there are two ways of getting them right. You either make a temporary frame to which you screw the shoes before setting them in the concrete, or, you make the entire structure, lift it onto some props to hold it in the right place, then fit the shoes and pour the concrete around them. Both approaches take some thinking about.


Good point. My instinct is to prefer the temporary frame approach, which could then possibly be re-used as a layout aid in building the actual structure since it'll (hopefully) exactly match the post locations. I'd been hoping to avoid handling the entire weight all at once by raising it in pieces.

Another question that occurs on this part: how careful do we need to be about the steel post shoe and the oak frame? Will a regular powder coated one be OK given that it's not penetrating the oak, just being pressed into the surface?

I do worry slightly, though, that you are planning it rather low. The height of the top of the doors means you won't be able to grow anything over it, because growth dangles down. I'd want to be a foot or so taller, I think. Mine (below) is 2.1m to the underside of the joists.


From the patio to the top of the door opening is 2.2m, so (in theory) if growth can be kept away from the centre of the doors I'd have thought it should be OK. The bottom of the door opening is around 270 above the patio, so a tad over 1.9m headroom when stepping out.

The client also wants the upper windows to still open, as apparently they're very useful for removing queen wasps from the house. The plan was to achieve that by setting the framing a little away from the wall, in level with the dividing trim, and notching out the back ends of the cross pieces as needed.

This structure, from your description, will be free-standing (not attached to the house). Therefore, bracing, bracing, bracing. Oak framing generally uses curved ("arched") bracing. Here is a picture of the pergola I am planning for later this summer (the wood for it is in my current order, so I'll have to get on with it).

You only have 4 posts, not 6, so you'll only have braces at the ends, but they are a critical element of the structure, bracing against racking, but also reducing the effective span of the plates (longitudinal members). Your end cross-members have to be joined in properly to the framing, because, as you note, they, and the braces, are what prevent racking in that direction.


Yes, free standing was my intention, though I'm not wedded to the idea. If the best way to resist racking is to extend the end cross members to the wall and fix them with stainless brackets, that's also an option.

What does joined in properly mean in this context? Would you extend the tenon through the plate and into the cross member? That'd seem tricky to drawbore given the limited amount of space above the plate, but I'm having trouble seeing what other type of joint would hold it down properly given the arrangement of the three timbers.

That's a big span. I can see why you don't want a central post, but I think you are going to need to reduce it a little. If you could move each post a foot or more in-board that would really help. You are also going to need your bracing to be big....possibly bigger than that shown in my drawing. Visually, a heavy horizontal looks odd on a pergola. You might bump it up to 5x4, but certainly no bigger. It's not a piece of furniture or a house, and if it sags just a little it isn't a huge problem.


Yes, this is the part that feels to me like the key to the whole thing. Unfortunately I'm not sure reducing the span is going to be a popular idea, though I may get away with some bigger bracing. Were you thinking bigger in cross section, or in length, or both?

The cross members would have been wedge-pegged ("trunnels") in the old days, but I think there is merit in using a coach screw, and it's what I'll be doing. That way there is no vertical end-grain providing a route for water to track in.


Having recently discovered Tally Ho and being part way through watching the whole series, trunnels have a certain attraction, though I can see the merit of coach screws, especially as they'll be effectively invisible if driven in from the top.

Presumably if trunnels were used, they'd be in seasoned oak so that the green material tightens around them as it shrinks, similar to the drawbore pegs. Would there be merit in putting anything on top of the end grain to shield it from water, or is that adding complication for little to no gain?
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Re: Planning a pergola

Postby Mike G » 19 Apr 2021, 14:09

spb wrote:........Another question that occurs on this part: how careful do we need to be about the steel post shoe and the oak frame? Will a regular powder coated one be OK given that it's not penetrating the oak, just being pressed into the surface?


Red oxide paint these days is a two-part stuff, and is seriously good. I am perfectly content to have that in direct contact with green oak.

What does joined in properly mean in this context? Would you extend the tenon through the plate and into the cross member? That'd seem tricky to drawbore given the limited amount of space above the plate, but I'm having trouble seeing what other type of joint would hold it down properly given the arrangement of the three timbers.


No, Bolted down onto the plate is perfectly good enough. There isn't a good joint for this situation, really.

I may get away with some bigger bracing. Were you thinking bigger in cross section, or in length, or both?


Length. The cross section isn't really important. I'd make it out of 2" thick stuff, maybe cut out a piece of 225 x 50, or maybe 250x50.

Presumably if trunnels were used, they'd be in seasoned oak so that the green material tightens around them as it shrinks, similar to the drawbore pegs. Would there be merit in putting anything on top of the end grain to shield it from water, or is that adding complication for little to no gain?


I can't see a good way of covering them up, which is, frankly, why I'll be using coach bolts.
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Re: Planning a pergola

Postby spb » 19 Apr 2021, 14:46

Thanks, that all makes sense to me. Those three-way joints do seem to be the place to accept that traditional methods have their limitations, as nice as it'd be to be able to do the whole thing without metal. I think that settles it on the posts and plates being M&Ted together, the cross pieces being notched into the top of the plates and fixed down with coach screws from above, and the braces M&Ted into all three parts. All the long parts being 100x100, and the braces from 50 by whatever width is needed to fit the curve. Pegs at 18mm from spare bits of seasoned oak.

The site is just outside Bury St Edmunds, which I don't think is a million miles from you - any recommendations for suppliers that can do green oak in those sizes and deliver to there? Is self-selection a thing here, or is it likely to be sawn to order and you get what you get?

Also, is there anything I should know about working the green stuff that differs significantly from dried? Especially for preparation and tools - I expect I'll want a bigger chisel (my current largest is 3/4"), but any other suggestions for different tools or methods would be massively helpful.
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Re: Planning a pergola

Postby Mike G » 19 Apr 2021, 16:14

Ridgeons will do green oak, but need a couple of weeks. Ask around locally, because you might get keener prices from a little local sawmill. When you specify it, make sure you state, in writing, that it is to be sapwood-free.

You'll need a 1" and in particular a 1-1/2"chisel, and a proper mallet. Don't worry in the slightest when all your tools turn blue/ black. It cleans off easily when you've finished the job. You will probably need a belt sander, and you most certainly don't need anything finer than 60 grit. 40 grit will be your most used grade. A roofing square helps, because the undulations in the surface of green oak mean a try square is likely to be disturbed by a local bump or hollow. Spade bits are really useful....peg holes, and for hogging out the bulk of the waste in mortises. And be careful to select your wood such that the joints, particularly the mortises, avoid knots. Have fun. Working with green oak is an absolute pleasure.
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Re: Planning a pergola

Postby greeno » 19 Apr 2021, 16:44

Mike G wrote:Ridgeons will do green oak, but need a couple of weeks. Ask around locally, because you might get keener prices from a little local sawmill. When you specify it, make sure you state, in writing, that it is to be sapwood-free.


You're just down the road from me, i haven't used them before but there's these guys down the road from you:

https://www.cottenhamsawmills.rocks/

I'm trying to find a job to do that would need a visit to them. Let us know if they're any good.
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Re: Planning a pergola

Postby AJB Temple » 19 Apr 2021, 22:53

Our Pergola will be removed when I get round to it. 8 metres long, and rickety. It has just had some repairs with timber fix screws. It used to be a wisteria embellished walkway to what the estate agent Knight Frank (who should know better) described as a "garden office" and which I described as a knackered shed and demolished in short order.

In the case of this one, looking at the picture, I would avoid having the top at the same line as the gutter on the house - if anything I would go higher. It will create more drama.

If you really want shade close to the house, whilst digging out simple footings, maybe consider planting a frost hardy vine in an enriched planting hole.
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Re: Planning a pergola

Postby Cabinetman » 22 Apr 2021, 17:54

That 4 m span could be helped if the posts went higher and you had stainless steel cables going down to the centre of the crossmember, these would also help prevent wracking. Ian
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Re: Planning a pergola

Postby spb » 22 Apr 2021, 18:39

Interesting thought there. That would mean a different and less robust joint between the post and beam, though, to let the post come through continuously.
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Re: Planning a pergola

Postby Cabinetman » 23 Apr 2021, 11:01

Nice big tusked tenons, could be a nice feature.
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Re: Planning a pergola

Postby spb » 04 May 2021, 17:46

Well, Ridgeons have said 21 weeks lead time on green oak, which isn't exactly compatible with being finished this summer. I'll have to have a look around elsewhere.
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Re: Planning a pergola... and building it

Postby spb » 11 Jul 2021, 21:55

Having got a variety of quotes for the timber, ranging from £800 including VAT and delivery to £1200 plus VAT and delivery, the order was duly placed for the former option and it arrived on site a few weeks back. At that point I could only get over there for a day to help unload, but this week I had some time to get started.

First job, on Thursday evening, was to lay out the size and location of the roof timbers. We just laid them out - one in each direction, to minimise the required moving around - and marked those two as templates to copy onto the others.

Image

At that point it was a bit late to start cutting, so that had to wait until Friday. I started with the tenons:

Image

None of the ends were square, and my circular saw needs two passes to get through these, so I decided to just pencil in a square a few mm in, then measure the tenon length from that and trim the tenons square and to proper length later. The 24 tooth blade doesn't exactly leave a clean cut, but none of these edges will exist by the end of it.

While I had the saw set to the right depth for the tenon cheeks, I did all four sets:

Image

Apparently I didn't take photos of the second half of the tenons - 10mm off each side, to make an 80x38mm tenon on the 100x100 post - or of the mortices. For those I drilled through with the spade bit I had on hand (a 19mm one), either in two rows or one zigzag, depending how confident I was feeling about drilling vertically, before finishing with the appropriate sized chisel.

Either way, the first one was soon ready for a test fit, which is considerably more effort with this stuff than it is for furniture...

Image

...not quite. Second time was better:

Image

And that was Friday's work. Yesterday I repeated that process another three times, so the weekend's progress is four M&T joints for the main framework, followed by a client conference about the exact shape of the braces, which will be cut next time I'm over there - current plan is next week.

And if Mike's warnings about green oak and steel weren't enough, this is a brand new chisel after cutting four tenons and one mortice over the course of a few hours:

Image

I have to say, working this green stuff is quite different from seasoned oak. I was initially tempted to use a tenon saw to cut the shoulders, but it just clogged instantly. Chiselling, on the other hand, is far easier than with dry, but drilling is an interesting challenge as the dust/chips all stick together and fill up the hole behind the head of your drill bit quite effectively. It took me a while to figure out the most effective way to approach each new operation, but in a good way.
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Re: Planning a pergola... and building it

Postby Mike G » 11 Jul 2021, 23:16

Fun, isn't it. :D
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Re: Planning a pergola... and building it

Postby spb » 12 Jul 2021, 16:19

It really is. There's also something in the combination of sheer size, knowledge that it's going to move after the fact anyway, and that all the joints will be drawbored that just gets rid of any inclination to perfectionism and lets you get on with it without worrying. I'm now hunting for any possible future project that could conceivably be green oak framed, but I think doing it for a small 1500x800 storage shed would be overkill.

Incidentally, a few design updates have happened after client conferences which are relevant to discussions above. Firstly, the stock for the braces is just over a metre long, which by my reckoning means they'll reach around 700mm from the join horizontally and vertically. That'll reduce the four metre unsupported span to something between 2.6 and 3m, which seems more reasonable given that the structure isn't supporting anything except itself. Second, we decided there's no reason not to bolt the ends of the outermost roof timbers to the wall (via a stainless L bracket) as a belt and braces approach to stability.
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Re: Planning a pergola... and building it

Postby Mike G » 12 Jul 2021, 19:49

That's plenty strong enough for a pergola. With green oak, you soon get used to asking yourself constantly "what's stopping this twisting?", because that's what it will all want to do.
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Re: Planning a pergola... and building it

Postby spb » 17 Jul 2022, 19:31

I'd forgotten how long it is since I last posted here. The project wasn't abandoned, but between my health and other things going on, didn't get finished before the winter. It all got unpacked around April and chipped away at over several weekend visits, until two weeks ago all the pieces were done and the village handyman (who regularly does the gardening and so on) booked to help raise it on Monday. This was the state of things on Sunday evening that week:

Image

After getting to that stage we were told that the helper had just tested positive for you-know-what, so raising wasn't going to happen. With a looming deadline of the client's birthday party to get it finished, we decided the best thing to do was to have a pergola raising party and get the whole family to help out. That was today, and the result was this:

Image

Image
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Re: Planning a pergola... and building it

Postby Mike G » 17 Jul 2022, 19:45

Well that looks grand. I hope you are proud of yourself. I prefer a little curve to my braces, perosnally, but it doesn't make any difference to anything other than aesthetics.
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Re: Planning a pergola... and building it

Postby spb » 17 Jul 2022, 20:43

There is a (very) little curve, though those photos don't really show it. It's a little more visible from the house side, but even then it shows more in real life than the photo:

Image

Either way the client is happy, and I can ignore the mistakes and gappy joints long enough to be happy with it too.
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Re: Planning a pergola... and building it

Postby Cabinetman » 18 Jul 2022, 01:17

Jolly good, just time to throw a tarp over it for a bit of shade! I hear it’s going to get a bit hot in Blighty soon.
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Re: Planning a pergola... and building it

Postby AndyT » 18 Jul 2022, 07:22

Lovely. I predict that the neighbours will be wanting one now - it just looks so right for that house.
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Re: Planning a pergola... and building it

Postby Andyp » 18 Jul 2022, 07:25

Impressive Stephen,
Having built a much small one in softwood I can appreciate the effort that went into making and raising that.
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Re: Planning a pergola... and building it

Postby NickM » 18 Jul 2022, 08:17

Very smart.
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