Setting out octagon help required - Setting out octagon help required

[Superdamo]

Hi all,

I'm about to construct a pergola base with a Stonemarket Trustone 4.13m octagon kit. It is to be built on a low brick plinth. My question is, I have laid plenty of octagons and bricks but am scratching my head as to how to build the plinth accurately enough so the octagon sits perfectly on it.

The footings will have enough below ground tolerance to allow me to use a pegged string line to plot a circle. But then how I position the bricks accurately onto these footings I don't know. I've thought I could use a slightly smaller circle (to allow for a small overhang of the slabs) to locate the corners of the octagon but how to do this in practise I'm not sure.

Any advice would be very much appreciated.

Kind regards,

Damo

[bobhughes]

If twer me I would lay the flags out first and peg the 8 corners with rebar or summat. Then remove the flags and use a string to get the dims for the brick base.

[lutonlagerlout]

did an octagon last year and thats exactly how i did it bob,also when the flags are laid out dry first, the client can get you to move it that 26 mm one way or another that they normally ask for, or back and forward a half inch,whilst squinting through the kitchen window
your footing probably needs to be 50mm bigger than the octagon so that when you lay your bricks 100mm in from the edge you still have a 45-50 mm overhang
you may or may not need some 45 degree squint bricks for the corners
and lastly for some reason they (or the one that i did) seem a lot harder work than the circle kits???
cheers LLL

[Superdamo]

Thanks guys, the dry lay idea was what I was considering (and you must have met some of my clients re the 5mm this way and that!) but still can't figure how to plot it once the slabs are lifted. I suppose I could string all the way round but one clumsy foot and I'm back to square one. And that won't be easy once the build has started. As for the octagon itself, they are a swine but at least with the overhang I can hide any minor variations.

What about building a wooden template to give me the correct angle as the side lengths are fixed?

Any more info gratefully received.

[Superdamo]

Thanks guys, the dry lay idea was what I was considering (and you must have met some of my clients re the 5mm this way and that!) but still can't figure how to plot it once the slabs are lifted. I suppose I could string all the way round but one clumsy foot and I'm back to square one. And that won't be easy once the build has started. As for the octagon itself, they are a swine but at least with the overhang I can hide any minor variations.

What about building a wooden template to give me the correct angle as the side lengths are fixed?

Any more info gratefully received.

[Rich H]

I think what bob is suggesting is to dig the trench for the footings, dry-laid the octagon, hammer lengths of rebar rod into the trench at the corner points, pour the muck leaving your rods in place as guides you can't kick out. Once the wall is in you can grind/saw the rods out.

Or not. I dunno. :laugh:

[IanMelb]

Maybe dry lay, mark the centre (with a peg or whatever) and perhaps one corner (if you want to keep the corners in the same place). Knock up a wooden triangular template (maybe even on a bit of stirling board cut out with a jig?)

The template should be an isocoles triangle with the two equal sides being the radius from the corner to the centre and the 'opposite' side being one of sides of the octagon.

You could even hook the 'centre' point over the centre peg (put in a loop of string or summat). Once you're ready to lay the slabs, mark out the location of the first octagon side using some something (chalk, sand, spray paint) and the triangle as a template, move the template round by one eighth, mark the next side, move it round again, etc.

[Superdamo]

Thanks one and all.

Your ideas were spot on until the British construction industry stepped in with its own agenda relating to customer satisfaction and doing a good job and decided that despite me paying for the octagon up front 2 weeks ago to ensure delivery at the right time the order was "never received"! So footings have had to be dug without dry lay but with lots of tolerance (so mucho money on groundworks and concrete).

I reckon Ian's stirling board template is a goer as the siting is over an existing plinth which I had to drill a rod in to to locate centre anyway and as long as I can cut the jig accurately will make the build of the bricks quite straightforward but a dry lay of the slabs very tricky.

IT's going to then be a 1000 pound finger cross when laying the slabs that they fit but I reckon that even they can't get a 4.13m diamter with a couple of inches overhang that wrong. Can they?!

Damo

[lutonlagerlout]

"suck it and see"
LLL :;):

[IanMelb]

Good luck! Hope it all goes well.