Name that joint

[tnimble]

For my work shop roof I'm planning on using the joint shown in the picture. We call it a gekamde kruisverbinding. But what's the English name for such a joint?

Normally these joint are sawn and then the sections are almost pounded out with a large chisel and mallet. Or doing no joinery at all making the roof twice its thickness.

Since I'm not a Neanderthal (although at times I may look like one) I want nice fitting clean joints. And perhaps do most of the joint by planing the dados and only chisel the notches.

What (stanley) planes could be used most effective for the dado like parts and perhaps some scraper like plane to clean the chiseled notches in the lower support beam? The beams are 70mm thick so the dados will be 70mm and 30mm wide.

 

[Andy Pullen]

Why are you wasting your time planing joints in a shed roof, get a saw and a sharp chisel, cut them and move on. It's not a piece of furniture. Save your delicate work for something that is worthy of it.
Once boarded and insulated the joint will never be seen again so better still why not just put your rafters in and nog between them ?
Anyways the joint is called a halving joint in its simplest form. Regards Andy.

 

[joesoap]

Yes and a weakened halving joint at that I'd say !

 

[tnimble]

Why are you wasting your time planing joints in a shed roof, get a saw and a sharp chisel, cut them and move on. It's not a piece of furniture. Save your delicate work for something that is worthy of it.
Once boarded and insulated the joint will never be seen again.

Because the tighter and straighter the joint is the stronger the roof will be and also good practice for when doing insight work without sacrificing the rubbish pile.

so better still why not just put your rafters in and nog between them ?

Its a flat roof so I've 3 choices. Using thick beams which results in a thick roof at a high timber cost, put a support beam in the middle causing a very thick roof because of the single beam down the midle but lesser timber cost due to thinner timber, of joint a support beam making he roof very thin and low timber cost.

Going for the first option result in high cost because all beams have to be thick resulting in a heavy roof which in turn thickens the beams.

Going for the second option the beam drop in size dramaticalle because the span of the roof is reducd. But the roof doubles in thickness at the point were the support beam is located. This either lowers the whole ceiling about 20cm or having a beam running under the ceiling where you bump into when moving around the shop.

The third option doesn't lower the ceiling and gives almost the same timber sizes as option 2.

Anyways the joint is called a halving joint in its simplest form. Regards Andy.

A halving joint of cross lap joint is what we call a lipverbinding.

Yes and a weakened halving joint at that I'd say !

This gekamde kruisverbinding/??? is used instead of a lipverbinding/halving joint becaus of its strengst. The height of the support beam in only reduced very ittle giving almost no extre bow when a weight is put onto it. Also the normal beams although having a lot of their height missing right in the middle of the joint remain almost the same strength since the forces in that part are almost only pulling forces in the direction of the grain.

So this joint definitive has its benefits. The thickness of the complete roof construction is only about 1/2" thicker than that needed for a roof almost a third of its size.

 

[Andy Pullen]

I still can't see what what you gain. If you have roof timbers with noggins between it will be just as strong. Seems complicated for complicated sake to me. Do you have something above your roof that means you can't lift it a few inches if rafter depth is an issue for strength. No offence meant.

 

[Keith Smith]

This is a novel joint, I haven't seen it before and it isn't in "Timber building in Britain" by R.W. Brunskill.

I can see the potential benefit with what you are doing but it does weaken the main joist and so you need a good tight joint for maximum strength. I would personally saw down the shoulders then chop the waste out with a sharp chisel; IMHO you don't need any fancy planes.

In Britain we would just run the main joists and cross nail noggins to prevent movement.

Keith

 

[tnimble]

None taken.

Noggins only reduce the width of the beams used for the joist without introducing flex and twist. A support beam under the joists or jointing it in reduce the height without introducing bow along the length of a the joist.

I could raise the height of the shop but then I run into the building regulations of maximum building height for home extensions, greenhouses, sheds etc. I could file for a relief but that a lengthy and costly procedure. And would make me even less popular with my neighbours taking more of their precious sunlight. And all the noise and dust I already produce. (not that I care much, but they will certainly file a protest preventing the relief)

 

[tnimble]

This is a novel joint, I haven't seen it before and it isn't in "Timber building in Britain" by R.W. Brunskill.

Thanks

I can see the potential benefit with what you are doing but it does weaken the main joist and so you need a good tight joint for maximum strength. I would personally saw down the shoulders then chop the waste out with a sharp chisel; IMHO you don't need any fancy planes.

Indeed that why I would like the do the major work just by saw and then creeping up to a very tight fit with a plane. Or even plane most of it and only do the notches with chisels.

In Britain we would just run the main joists and cross nail noggins to prevent movement.

We also use noggins (only not very often) but they almost only stop hositonatal movement and no vertical movement.

 

[OLD]

I would have used a cross halving joint or just used metal joist hangers my self in fact my own work shop flat roof uses metal joist hangers.

 

[Inspector]

tnimble

There are some terminology differences and definitely different construction practices between our continents. For me a timber frame uses large dimension timber beams 15cm and more in width joined largely by mortise and tenon joints locked with pegs, or bolted with steel plates.

The joinery you've mentioned suggests "stick framed" building with dimensioned lumber of 2x4,6,8s etc. They are pretty much never notched and are held together with nails.

So I always do a double take when I see a post on this forum about timbers and realize it refers to the material more than the sizes.

I would not bother with the notches as it does weaken the structure. I would do as the previous posters and nail blocking (noggins) between the rafters. If you want to add more strength then make the roof like a torsion box by nailing and gluing with construction adhesive, a skin of plywood on the bottom, (put the insulation in first) with the same on the top. The lower skin doesn't need to be as thick as the top because you're not walking on it unless you can manipulate gravity.

If you are set on joining the roof with the "gekamde kruisverbinding" then it would be possible to do the joint with a circular saw and router to save time. I would clamp all the rafters and purlins together and clamp/tack a straight edge to run the saw/router against and gang cut them. For the notch along the sides of the rafters, make a template that registers in the notch and cut them with the router and clean the corners with a chisel. When it comes to laying the purlins into the rafters, put some pressure from below to arch the rafter and "open" the joint for a tighter fit. And it is easier to take a few shavings off the outsides of the joint to fine tune it rather than in the notches.

Have fun making the new shop whatever method you chose.

 

[tnimble]

Thank you all.

Without wanting to force any debate or dumping on any of your posts I want to explain how traditonal flat roofs are constructed and why some thing are done.

In the above picture the joist at the left would be needed for the span of the roof. The joists on the right are needed when a support beam is placed under them. the support beam reduces the width of span and takes out a few of the torc caused by wind and not so perfect flatness of the joists and the top face of the wall.

Having the support beam reduces the size of the joists making the roof lighter and therefore reducing them even more. Introducing the support beam costs extra money for materials. But having only 1, 2 or 3 of them is in no comparison to the material costs savings by the great number of joists.

To not have the full extra height of the support beam fitted underneath the joists the beam can be jointed in place. This requires both the support beam and joists to be bigger.

Multiple joints can be used. Either a simple halving joint or a double cogged halving joint. Using the double cogged halving joint the support beam and the joists need only to be slightly bigger. With the halving joint they need to be a lot bigger.

A weight (red) placed directly almost above the joint only causes pull and push forces in the support beam B. Most forces are trying to compress the beam at the upper part. They are handled by the extra material in the top of the double cogged halving joint (see the other above 2 pictures) and by pressing the end grain against the face of the joists A.

A weight (blue) placed directly in the middle between the wall and the support beam B causes mainly pull and push forces in the joists A. Most of the forces are compressing forces in the lower section of the joists A. Th end grain faces of the joint compress into the face of the supper beam B. The pulling forces are much less then the push forces. They are handled by the small remaining section left in the joists by the joint. The thinness is not a problem since the forces are little.

Any weight inbtween causes a combination of the two above forces and their reactions.

Why is there that a difference between the two joints? The double cogged halving joint removes a lot extra wood from the joists, they must be much more fragile. No they ain't and this is why.

In the above picture the forces in a roof are shown. The red and blue forces are from the weight of the roof, its plating and any snow and rain on top of it. The green force is caused by inaccuracies in the flatness of the wood and brick used, by the slight tilt angle of the roof, and by external forces like wind.

The small red and blue arrows along the face of the joist A and support beam B depict the stress forces in the wood.

This last pictures shows a complete roof construction. All the joists rest on the front and back walls, the support beam rests on the left and right wall. The noggins prevent twist creating kind of a a torsion box. However the noggins are not always used since the roof plating prevents them a bit from twisting.

Notice the different timber sizes used for the support beam and the joists.

Nowadays neither support beams neither noggins are used. The labour cost of putting them in is regarded to high. The joist are simple made very wide and thick are replaced by metal beams or concrete slabs that come pre made in various lengths. This plating comes from the factory along with concrete walls with brick glued to them or even painted on sometimes. Due to the weight of it all a large concreate foundation is put in first. With all the prefabricated (stocked) parts its not uncommon for some builders to spend a bout 2 months preparing a site and putting in the foundation and have the whole building up including all wireing (except for gas and water) in only 1 or 2 days with all windows, glass, doors ventualation and roofing fitted.

 

[Jake]

The reduced notch bit is called a "cogged" joint in english, I think (from looking at Brett). The halving bit is a notched joint, and in this case both bits of timber are notched. A "double notched and cogged" joint then, perhaps, athough there must be a more elegant description though, unless the english language lets us down here.

 

[Shultzy]

I agree with joesoap, beam A has been cut 2/3 of its depth so must be weaker than a traditional "halving joint " which is only cut 1/2 of its depth, hence the name.

 

[Sgian Dubh]

I'm planning on using the joint shown in the picture. We call it a gekamde kruisverbinding. But what's the English name for such a joint?.

Notwithstanding all the other discussion about your proposed roof construction, the joint is a 'double cogged halving', or some prefer, 'double notched halving'. Slainte.

 

[tnimble]

I agree with joesoap, beam A has been cut 2/3 of its depth so must be weaker than a traditional "halving joint " which is only cut 1/2 of its depth, hence the name.

You are 100% correct that the beam A is severly weakened in that spot. However with the forces distributed throughout a roof this part is almost not put under any strain and all that strain is in the direction wood fibres are very strong at.

Using this joint in anything else would probalby be a bad idea. When one would make the joints in the wrong direction the roof will cave in very soon. Also when the support beam B is sawn thorugh the rool will collapse soon.