Spiral staircase...

[Reiver]

So I'm trying to price up a couple of spiral staircases, 14 steps apiece, 270 degree twist, 2.75m height and 1.2 metre radius: I know that's big but I have history getting stuff up and down them before ?, and then there's what's above..... I have a couple of enquiries out for ready made items, but the replies so far are errrm, not cheap. So I wondered about making them myself, the central pole is not a problem to obtain (friendly steel fabricator), and it shouldn't be too difficult to cut out the steps on the bandsaw and then cut the pole hole on the CNC router. The one thing that seems to be not so easy is obtaining boards of some nice wood (ideally oak) at the required width - the steps need to be 435mm wide at the outside, and at a guess they'd need to be at least 38mm thick? Local merchants and all the online sellers I've found seem to stop around 250...300mm wide. So would the best idea to glue two narrower boards together? or are such pieces of timber actually available? Any advice gratefully recieved, thanks.

[PeterW]

If you have a friendly fabricator then I would get him to weld T sections to the pole to form the tread supports, as a 1.2m oak board will bend. You could still hide that inside your oak - would probably need to be at least 50mm thick though so as not to bend or bow.

 

@Construction Channel is your man for building things from scratch..

[Reiver]

Thanks for the tip: doing it that way rather than threading the treads over the pole would likely make assembly a lot easier too :-) BTW, I was assuming tying the outer ends to the wall - easy since it's circular.

Edited June 26, 2020 by Reiver

[Construction Channel]

The problem with very wide boards is they want to cup (curl up across the grain), 

You could biscuit some boards together but for strength you would probably be better laminating and off setting two layers. 

 

How are you planning to support the outer edge of the tread? 

 

I havent done much work with spirals but if you want to cantilever a 4' plank of a central column it will either need to be very thick (50-60mm), or have a big assed brace. 

The hole in the plank wont give you very much strength because the grain will just want to sheer out so most of the weight will have to be taken by the brace.... I suppose the handrail could offer some support but i really dont know very much about them. 

[Onoff]

 

 

 

 

Just make it up as you go along like I do....

[Reiver]

10 minutes ago, Construction Channel said:

The problem with very wide boards is they want to cup (curl up across the grain), 

You could biscuit some boards together but for strength you would probably be better laminating and off setting two layers. 

 

How are you planning to support the outer edge of the tread? 

 

I havent done much work with spirals but if you want to cantilever a 4' plank of a central column it will either need to be very thick (50-60mm), or have a big assed brace. 

The hole in the plank wont give you very much strength because the grain will just want to sheer out so most of the weight will have to be taken by the brace.... I suppose the handrail could offer some support but i really dont know very much about them. 

Curling is a good point, I guess finding 40 metres of 1/4 sawn timber at that kind of width is dreaming..... or stupidly expensive. Would a couple of thinner boards glued back to back with mirrored grain sort it I wonder? Then also going cheapskate I've seen those boards made up of little offcuts of timber stuck together....but what do they call it? - I've seen versions in oak & beech as well as softwoods. The outer edge would likely be some simple brackets into the wall...... rebating the wall and setting them in would be cool, but very time consuming, I have better things to do with what's left of my life. I was assuming the hole would only be giving vertical support, no torsional moments, but @PeterW's suggestion for a bracing arm underneath makes a lot more sense and would make the treads shorter and easier to make and install, even if the centre pole becomes a more expensive item.

[Onoff]

Make the treads as steel trays then inlay with timber.

[Reiver]

4 minutes ago, Onoff said:

 

 

 

 

Just make it up as you go along like I do....

Love it! very much what I was originally intending, only with no balustrade, fixed to the wall instead..... and probably twice as wide!

Edited June 26, 2020 by Reiver

[Onoff]

3 minutes ago, Reiver said:

Love it! very much what I was originally intending, only with no balustrade, fixed to the wall instead..... and probably twice as wide!

 

Get on Pinterest for ideas.

[Mr Punter]

Hardwood kitchen worktop might work for your treads.

[Reiver]

4 hours ago, Mr Punter said:

Hardwood kitchen worktop might work for your treads.

Cheers! that's the lots-of-little-bits-of-timber-stuck-together stuff I was thinking of - just wish I knew the proper term for it....  And Worktops-Express seem to have some good prices on them, will enquire if they can do custom lengths, as I could get 4 treads out of a 2400 x 600 board.

[PeterW]

11 minutes ago, Reiver said:

Cheers! that's the lots-of-little-bits-of-timber-stuck-together stuff I was thinking of - just wish I knew the proper term for it....  And Worktops-Express seem to have some good prices on them, will enquire if they can do custom lengths, as I could get 4 treads out of a 2400 x 600 board.

it’s called staves or finger jointing. Beware it isn’t that strong across the parallel joints as it relies on glue alone. 

[Reiver]

2 hours ago, PeterW said:

it’s called staves or finger jointing. Beware it isn’t that strong across the parallel joints as it relies on glue alone. 

Thanks!

That's odd, you would have thought they'd use a decent glue; the odd 1 or 2 times I've joined planks side-by-side I've used urea-formaldehyde stuff which seemed to be stronger than the wood itself. However I guess it's a mass-market product, so anything to save a few pence probably applies ?. Shouldn't be too much of a problem in this case as the outer end can be supported along most of it's length and the inner bracket can be made to support where it's most needed, which I guess will be just behind the front edge of the tread.