Arches

[bluenose]

Hello folks. I need to make two arches, without the need for any process involving steaming/laminating.

I want the profile to be as close as possible to 100mm by 50mm and made from tanalised timber.

I have, hopefully, attached a rough image which indicates where I am trying to be. I will obviously have to construct these two arches in sections which I anticipate fixing together using either biscuits or dowels and exterior/waterproof glue.

If anyone has any ideas on the best way to achieve this and can point me in the right direction I will be extremely grateful. Thank you.

 

[Doug71]

Easiest way would be make them out of 2 x 25 mm thick arches glued and screwed together with the joints staggered.

 

[bluenose]

Thank you for your suggestion Doug71 but surely I will have the same construction problem doing that.

 

[MikeG.]

Never mind.........

 

[sunnybob]

I think youve missed out a couple of zero's.
Do you really want 4" x 2" arches?

Is this load bearing?
A window frame? A wedding arch?
The drawing shows a semi circle, I would make that from 4 identical pieces, with the joints depending on the loads involved.

 

[Trevanion]

I would possibly buy a length of 9 x 2 and cut the majority of the curves out of that rather than trying to join a bunch of 4 x 2s, it just would be time-consuming more than anything.

 

[profchris]

We're guessing because we don't know what they are for.

If these are for May day, garlanded with flowers, then make the arches from ply with scrap spacers in between. No-one will know past the flowers.

If they're load bearing, that's something different.

If they need to stand up to years of weather, maybe supporting roses (and tanalised suggests something like this) then cutting curves exposes the interior which has less/no preservative = rot.

For light load bearing I'd cut pieces from something fairly wide (2 X 8 inches maybe, if these are 50 mm front to back) and half lap the joints, then waterproof glue and coach bolts. Or cut from 1 inch X 8 inches, overlapping joints, glued and screwed. And then soak the cut surfaces in preservative.

Any more clues?

PS I'd not want to butt joint the arch pieces, whatever I added in the way of biscuits and dowels. Outside, this will move enough to open the joints, waterproof glue notwithstanding.

 

[MikeG.]

I assume this is a door frame for a garden gate. There is no way on this planet that I would do that with butt joints. I'd make proper bridle joints, but most people here seem allergic to traditional joinery, so at the very least I'd suggest making it in two layers with staggered joints.

 

[Phil Pascoe]

Hammerhead tenons. :lol:

 

[bluenose]

Thank you profchris and Trevanion, your suggestion is what I had in mind initially but, I have problems when trying to work out the angles at the numerous joints. If you can help in this regard I will be grateful.

The purpose of the arches is to secure a length of 5 foot fencing that runs between my property and that of my neighbour. The fence is reasonably stable at the moment but, I have noticed quite a bit of movement when the wind is strong. The posts are only fixed by way of metaposts that are bolted to the paving slabs as opposed to being firmly concreted int the ground. These two arches would provide the stability I am looking for by being fixed to a fence post and the other side to the bungalow. There is very little by way of loadbearing involved.

I now appreciate that butt joints are a no no and so will go with the lapped joints suggestion. Perhaps I could even go the mortice and tenon route? Any thoughts on that?

Thanks again

 

[Doug71]

Draw the arches full size on some MDF or ply. Draw lines from centre to outside where you want joints, you can get the angles from this. Use the arch you have drawn to assemble arch on. Other option use router and trammel to cut out a full size template from some thicker MDF.

 

[AJB Temple]

Forget tantalised timber. By the time you've cut joints to make it strong enough to last - especially if it needs to support a gate (?) you will have exposed a lot of unpreserved surfaces.

Making accurate strong arches, especially if they need a rebate (?) requires some skill.

Mike is possibly a bit harsh about this forum and joints - lots of people are using Dominos because they are fast and easy. But most work is indoors. He is right though about the joints. Listen to him.

If I am going to put in the labour required to make proper arches, I am not going to do it in softwood - whether tanalised or not, unless it is much larger cross section and painted. I do quite a bit of oak framing on a DIY amateur basis and I like using traditional joints. I may well use dowels as you suggest - but only to draw and secure the joints! I've made a pointed arch frame in oak to fit a reclaimed church door 2.5 metres high and a right job it was too for an amateur. But it did need rebates and a fair degree of accuracy. Took me 4 days. I cursed buying the door!

If on the other hand you are making two arches so you can put slats between them to create a pergola, then I would have a look at wooden and metal kits. Pretty cheap. Not very good, but neither will making them with biscuits or dowel joints.

 

[MikeG.]

If this is to resist sideways forces from wind loading, then I don't think it is going to work. Whatever joints you choose will fail if they rely on glue. Even the laminations will fail (the two layered approach). This will leave the arch vulnerable to failure in a strong wind as the fence acts like a sail and pulls it apart. Why don't you have taller posts and a beam across the top? That way you could make strong joints between straight bits of solid wood, and if designed and executed well you could have a solution that lasts many years.

My preferred solution would be 2 posts, a beam (which overhangs the post at the free end by say 6 inches, shaped), and a pair of arched braces, in 4 inch oak, pegged M&Ts.

 

[Jacob]

As mentioned above, the trad way would be two quadrants cut from solid, bridle joint to the vertical, wedged hammer head tenon to the top joint (for purists!). Simpler than the HH tenon would be one dropped in loose tenon, or two thin ones even better. For outside situation you'd have to cover the top edge with lead flashing or other form of weather protection - thin lath bent round perhaps, well painted.

 

[bluenose]

Well chaps, thank you so much for all of your input. I have decided to change tack on this one. I am moving away from the complications of producing curved arches and instead, I am going to make them sort of (part) octagon shaped. I'm sure you will know what I mean.
I'm convinced that I will find this much easier.
I will use lapped joints, screwed and glued as per the suggestions in some of the replies.

Thanks again folks.