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
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