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Garden Pavilion

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Garden Pavilion

Postby SimonB » 15 Mar 2021, 19:44

Hi Everyone, my first post other than saying hello, and feeling a bit sheepish that it's one asking for help, but would very much value the expertise on here.

We are planning to have built a reasonably large pond, and then self build an adjacent garden 'pavilion' that will, ideally, have an extension that overhangs the pond edge.

I feel pretty confident that with enough hands we can complete the construction. I'm well equipped tool wise, even though I don't use them very often, and have read enough projects on here and elsewhere to think that I have a reasonable idea how to go about building it. What I am far less confident about is the design of the thing such that it will stay up (for a good few years).

This is the concept

Concept.03.1.jpg
(231.42 KiB)


After any structural considerations, the final dimensions will likely depend on materials, but for now it's essentially 5m x 14.5m thereabouts. That's based around whole and half length decking boards that we're considering (20mm x 138mm x 3600mm) with hidden fixings (5mm spacing).

We have discussed at some length what the floor finish should be. No floor is a cheap option but perhaps hard to maintain and we get standing water in places in Winter.

Other options are decking and tile. Tile presumably needs a slab. Decking could be on a slab or suspended. I haven't looked into what sort of slab we would need given our ground conditions (clay, and wet for parts of the year) but my instinct is that it would be more expensive than a suspended deck, but would appreciate anyone's view on this.

I have been considering ground screws rather than piles, not least that they seem quicker and easier to install, although accuracy is a concern.

Assuming that the ground support can be made sufficient, where I really would like advice is on the structure.

My thought was each post (140x140 or 150x150) would be supported on a ground screw, and each joist header (I probably have that term wrong) sufficiently supported that it acts as a ledger board, and joist are fitted with joist hangers.

I have looked at the example span tables in the TRADA Timber Strengths and Spans PDF and for a 47x100 C24 joist on 400mm centres I can achieve the 2.4m span between the 'ledger boards'. Does that sound right?

What I am not sure about is the load that will support. This is a structure that will have furniture, BBQs, possibly a wood burning stove and 10 or more people scattered around. The imposed load for the example span tables is 150kg/m2, but get two of me standing next to each other and that figure would be exceeded. Do I need wider joists, or closer spacing?

On to the roof. The plan is ply with an EPDM or similar covering. No additional load other than during construction. I have allowed a 1:40 slope (122mm) and assume that wedges are OK to achieve this?

Again, based upon the example span tables I can achieve the ~4.8m using 47x195 C24 on 400mm centres. What I am not sure about is the 'joist header'. We were inspired to build this structure by something similar a neighbour built, but they just used 4.8m 150x150 posts sitting on top of the posts at each end to hang the joists from. This doesn't seem sufficient to me, given how little material is bearing on to each post, and in less than a year the posts are starting to sag. My thought was to laminate 2 or 3 boards, with no join sitting on a post, along the lengthwise span of the building to act as the 'joist header'. Would that be enough to allow the 4.8m roof joist span? We would really like to avoid additional posts.

For the pond extension I would like to achieve a cantilever effect, even if there isn't actually much of one. I had looked into how to do a proper cantilever, and am concerned about how close to the edge of the pond I could bury a support and the limitation that would put on the extent of the overhang.

The pond company say that they usually have posts sitting in the water, with everything made of composite. The posts then bear onto a concrete slab under the pond liner. I like the idea of this approach since it seems to make the design easier, and with dark coloured materials the posts might disappear enough for the overhang to appear as a cantilever. That being said I have read opinions that one should never ever have posts in the water. That may be due to the effect on the material, or it may be due to the risk of damage to the liner, but the pond company seemed very confident of this approach. Does anyone see any reason not to?

What is the view on where the eaves height would be measured to? The concept design is overheight at the moment, but will modify it once I know what joist sizes I need.

I am sure I am going to have a lot more questions and I hope that you can all bear with me. I really will be grateful if you can help me get this thing built, I'm being chivvied by my other half to get on with it.

Many thanks
Simon

PS I wasn't sure whether to post this in Projects or Workshop Builds - please move if appropriate
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Re: Garden Pavilion

Postby Malc2098 » 15 Mar 2021, 21:36

Wow!

Looking forward to this!
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Re: Garden Pavilion

Postby Mike G » 15 Mar 2021, 21:43

Wow, there's lots there, Simon.

Let's start with the basics. This structure will need Planning Permission if it is near a boundary, exceeds half your garden area, or you live in a National Park, AONB, or World Heritage Site. Or, indeed, if the deck is 300mm or more above ground level. It will probably require Building Regs approval because of its size (70+ sq m). Check with you local council, and get it confirmed in writing (an email usually).

If it does need BR approval, then ignore anything about structure that anyone on the internet tells you. Because to comply with BR you will need to provide calculations from a structural engineer. I'm an architect, not an engineer.

So, I carry on safe in the knowledge that you will only take any notice of this if you DON'T require BR approval.

The first thing that any engineer will say when looking at your drawing is that there is no bracing. If an elephant or a strong wind pushed from any direction your structure would rack. Even more so as your connection to the ground essentially involves a pin-hinge at around ground level ( a bolt onto a pile of some description). An open structure with a solid roof is going to be subject to enormous uplift forces in a strong wind. If a good gale came by anytime, it's likely your structure as drawn would end up in a neighbour's property. This is where having access to load tables but without engineering training or experience is a particularly bad thing. The main forces acting on your building are not operating vertically downwards (gravity).

The next thing to say is that you should design the structure first against the criteria necessary for structural stability, and not around the length of the decking boards. You are mixing up live loads (furniture, people, and so on) with dead loads (the load the structure imposes on itself). Again, I urge you to seek structural engineering input (you'll be forced to if this requires Building Control, as I said).

Don't even begin to entertain the pond company's suggestion of sitting a post onto a pond liner, which sits directly above a piece of concrete. You could count the lifetime of the liner in months if you follow that advice. I would certainly be looking at a cantilevered or suspended structure, with nothing in the water. I designed a similar one once which hung from the adjacent house by two chains, like a drawbridge, but if you want a modern aesthetic, you could use stainless steel wire cable. Again, this isn't a DIY calculation, and you'll need an engineer's input.

Finally, and somewhat brutally (sorry!), I'd say that this is a real missed opportunity. You could have someone design you a really wonderful pavilion-like open building, but there is a danger that this will end up looking like an out-of-place flat-roofed car port if you build it something like your drawing. It wouldn't take a lot to make this much more interesting.
Last edited by Mike G on 15 Mar 2021, 22:23, edited 1 time in total.
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Re: Garden Pavilion

Postby AJB Temple » 15 Mar 2021, 22:18

Listen to Mike.

This design as drawn and described, is going to look horrendous until the wind blows it away. A massive EDPM roof with a slight slope will be super ugly.

To reinforce my "listen to Mike" remark, I used to live in Capel in Surrey. Just about a mile from us across the fields was a lovely old oak barn. This was acquired by a DIY developer (very nice couple but beyond clueless) who stripped the frame and sheeted all around it with scaffold sheeting and then proceeded to install a Horsham stone roof (traditional for the area) similar to the roof we had on our farm house. Horsham stone roofs use great lumps of riven stone instead of tile. Seriously heavy and they last forever.

So now we have a huge oak barn, with a super expensive stone roof, and scaffold sheeting for walls whilst they faffed around with infills and cladding. Then the wind blew. It got past the sheeting as if it was tissue, got under the roof and collapsed the building (which had managed to survive 300 years prior to that) in about half an hour. It was spectacular.

............

Before you go any further, I would set out what the pond is for and what the very large covered area is going to be use for. It is crying out for a more substantial and elegant design.

As with Mike the first thing that struck me from the drawings is the complete absence of bracing. I also think you need to have a serious think about whether you are putting enough money into foundations.

What are you doing with the pond spoil? Using it in landscaping or paying to take it away? This needs to be a cohesive scheme but will be a money pit if you don't think it through or get the build wrong. I have made several large ponds and they are easy enough. I think the cantilever will be easy with steel IF you get the foundations right.

Good luck

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Re: Garden Pavilion

Postby Mike G » 15 Mar 2021, 22:28

I'll just add, Simon, that this could be a really great project, and I don't mean to be sounding negative about it. If you were a client of mine I'd use your sketch as the opening of the design discussion. It gives us a rough idea of the size, and the relationship with the pond. In no time and on the back of an envelope we could be talking about half a dozen variations which would be really fun and interesting buildings.....and we wouldn't be talking about foundations, span tables, and so on for weeks.
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Re: Garden Pavilion

Postby SimonB » 16 Mar 2021, 17:42

Thank you for the feedback and candour, it was exactly what I was looking for, and I appreciate that you don't mean to be disparaging. You highlight several concerns that I had had myself, but getting enough 'surely it can't be that hard, loads of people do this sort of thing' comments made me think perhaps it wasn't.

That being said, this was always a concept, and I fully expected to make adjustments and tweaks. Perhaps including specific questions about materials and loads etc was a mistake, but I just wanted to get a feel for whether there was the basis for a practical design.

The original remit was for a simple, cheap as possible, structure akin to what our neighbours had built. It would be an outside covered entertaining space that got the afternoon and evening sun and had a pleasant outlook of a landscaped pond. Aesthetics were a factor but simplicity and cost were the priority. That's not to say cost cutting.

I had looked at what was available commercially, and I didn't, for example, see a fundamental difference between my concept and this

https://www.summergardenbuildings.co.uk ... 21508.html

but perhaps that is flawed also.

Since the roof would not be seen, the appearance of the roof covering did not seem terribly important. The fascia could be made to look attractive.

Regarding bracing, are you just talking about knee braces? I didn't include them since I was hoping for advice. What if some of the open 'panels' were partially closed in, could that be sufficient?

As to foundations, I would like to think that I'm aware that they're critical and cost wise can make up a significant portion of the overall cost, but I didn't actually go into detail, because I was looking for advice.

Steel for the cantilever... that had crossed my mind, but got the 'surely it can't be that hard' treatment.

So I take what you say on board. Obviously engaging experts adds to the cost, and I am not dismissing that this might be necessary. I'm accused of always over complicating things but that's usually because I think I know how they should be done right, ie making them more difficult and expensive. I suppose as much as anything this was a sort of validation of whether my doubts and concerns were real.

I guess at this point I'm looking to see whether this idea has legs, in some shape or form, and would be possible with guidance from the members on here (assuming BC give it the all clear). We meet all the planning requirements for PD.

Thanks
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Re: Garden Pavilion

Postby Mike G » 16 Mar 2021, 17:55

SimonB wrote:.......We meet all the planning requirements for PD.....


Are you absolutely sure, Simon? Remember, a deck which is more than a foot above ground level anywhere requires planning permission.
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Re: Garden Pavilion

Postby SimonB » 16 Mar 2021, 18:33

The hope was for it to be less than a foot, but that depends on the final structural design, as well as any guidance on how far off the ground any wood should be. We really want to avoid requiring planning permission for fear of losing further PD rights (which was expressed as a possibility following the new build house).
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Re: Garden Pavilion

Postby Mike G » 16 Mar 2021, 20:51

SimonB wrote:.....I guess at this point I'm looking to see whether this idea has legs, in some shape or form, and would be possible with guidance from the members on here (assuming BC give it the all clear).....


It has, Simon, and of course you'll get guidance here. But I'd love to see the design improved.
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Re: Garden Pavilion

Postby Mike G » 16 Mar 2021, 21:20

Simon, this is in no way meant to be a design......just something for you to think about. Look how much more dramatic and fun this building coul become just by sloping the front posts, giving the roof a slope and an oversail, and by segmenting the footprint and making it "curve" around the pond. Crap drawing, but you get the idea:

Pavilion sketch Simon.jpg
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Re: Garden Pavilion

Postby SimonB » 17 Mar 2021, 21:27

Thanks Mike, I appreciate you taking the time to draw that. We were never in any doubt that the structure could be more attractive, but we always recognised that this was likely going to be function over form if we were going to do this ourselves, which was one of the objectives, and were content with that. The pleasure would have come from entertaining in it, and looking out, rather than admiring the structure itself. If we wanted elegance we could simply glance over at the house :lol:

So I'm going to draw a line under this thread since it seems the consensus is that we're not going to be able to achieve what we want without external expertise. Thanks for everyone's input.
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