On the Swiss structures that I have studied, the use of half laps v. mortises depends on how the building is to be finished out, it seems. If the building is to be infilled between the timbers and the timbers left exposed, then long slanting braces (streben) are used, which are mortised and pegged. If the building is weatherboarded or enclosed by means of heavy boards between the timbers, the half dovetail lap is more common (with the profile a bit different than the English version of this joint) The lap joints on these building are pegged as well, some dating back to the 1600's and possibly earlier.

I think more than anything it depends on WHERE the building was built. In Switzerland building style, joinery, etc. varies a lot from one valley to the next. Folk in the Gürbetal (A valley that lies between the cities of Bern and Thun), for example would build a house that looks something like this:

(This picture is of the old Bähler farm near the Swiss village Kirchenthurnen)

Whereas people the next valley over would build something like this:

which is "blockbau", built of squared logs, not timber framed aside from the roof structure.


Mortised and tenoned roof bracing, from a farmhouse built in the late 1700's


A half lap version of the same basic roof system, I don't have a date on this one


half lap wall braces from a day laborer's house built in 1803



Pegged Streben from a Bernese farmhouse built in 1797

I have some better pictures, but don't feel like uploading them right now. Maybe some day I will post the results of my studies into this style of framing and then you can see more pics (That is if anyone actually is interested in seeing that)

There are examples of mortised and tenon/pegged braces in this style that date all the way back to the late Middle Ages, including some of the oldest timber framed houses in Switzerland (which seem to be often located in castle complexes)
There are also examples of hap lap joints built within the last century or two. There are even examples of Streben style long braces with half lap joints, which seems somewhat odd really.

Also note that the short corner braces are not at 45 degree angles, but rather go to the third point of the post, and the third point of the adjoining timber.

This is kind of a long winded answer to your question, Tim, but It's easier to get the whole thing out at once to avoid some confusion.

Why did they use half laps? A lot of it I think has to do with their tradition, and also what they intended the joint to do.

As I understand it, there are times when a half lap dovetail was intentionally chosen in a situation where the joint would be subject to tension, even though it was still primarily a compression member. An example of this would be scissor trusses, all examples of which in English Historic Carpentry seem to use this joint, which the author explains is due to the fact that a mortise and tenon would have failed in this task.







-all pictures aside from the first are from buildings in the Ballenberg Museum in Switzerland, a Museum of rural Swiss buildings, the oldest being a farmhouse dated to approx. 1330. Pictures courtesy of J B Dowse, jbdowse.com, used by permission.-

Great Pictures. The Xbracing in the roof plan is good stuff. And the thatch.

I'm interested in seeing more pictures and analysis.

P.S. As echoed here, shouldn't the pegs in a well built frame just provide assistance in the fit-up? Only good tension joint I've have noticed is the anchorbeam through tenon.

Hi Guys,

As Simon Cowal would say - "I knew that this was a good question" and the follow on questions and comments are equally thought provoking.

Re the original blind pegging question :-

There are some places in timber frame construction where a concious effort was made not to show pegs coming through to the interior of the building. This still applies to day and typically might include where a common rafter is pegged to a clasped purlin and where floor joists are tennoned into a spine (summer) beam.

Re mortice and tenon joints not being being employed on scissor braced trusses :-

This is not quite true. The scissors can and are joined using M&T at each end to the rafters but the ovelap joints are generally made with unpegged lap joints.

See photo of Bishop's Camera roof at Farnham Castle [dd 1375, Moir]:- Scissor braced trusses with collars

It is of course possible to use lap joints at the scissor / rafter connection points but one has to consider the sequence in which the rafters, scissors and collars are to be laid up, cut and assembled and especially where slightly twisted timbers are being used. This can lead to less than tight lap joints. These are some of the most difficult timber configurations to frame. The roof illustrated was a "cost not an issue" construction and employed only the best timbers to form 39 sets of scissor braced trusses set on 16" centres. The rafters feet are not pegged and simply butt into a shallow locaton socket on the wall plate with no overhang beyond the plate. There are no purlins or windbraces i.e. a "sans purlin" roof.

Re Ommitting pegs

It's probably worth examining the interupted butt rafters employed in an early 17th century butt purlin roof in Farnham, Surrey.

See :- Butt Purlin Roof & interrupted Rafters

You will note that the lower sections of the common rafters have been deliberately omitted where a lean too dormer roof has been framed. The lower rafters were designed to be mortice and tennoned and pegged to the underside of the butt purlin but the upper section of the rafters only to be mortice and tennoned with no peg. There is no ridge or windbraces in this roof. I have seen this on other buildings in this town so this practice of completely omitting the upper peg is quite deliberate. Your thoughts on this practice would be much appreciated.

One must consider how a joint is expected to work. It is not just about tension or compression it is also about moment connection. A M&T joint functions well largely as a result of the shoulders being pulled up tight by the use of a tapered draw peg. It is the shoulders of the joint that are important as this is where load is transmitted so even if a member goes into tension providing there is sufficient preload applied by the peg then the joint can continue to act as a moment connection. A single three quarter inch dia oak peg can sustain a double vertical shear load of nearly 10 tons which means that a significant degree of preload can be applied. Its worth keeping in mind that the preferred mode of joint failure is in peg shear such that tenon relish shear and mortice cheek pull out are avoided. Replacing a peg is cheap and easy.

Re Lap Joint versus Mortice and Tenon

It is highly likely that Tim's surmise about the evolution of timber joints probably progressed hand in hand with tool development. This is true today with certain joints now being made on CNC joinery machines being impractical to achieve by hand (housed taper curved dovetail floor joist to beam joints).

That said there are still places where a lap joint is preferable e.g. as in the case of a cruck mantle beam (low tie cross beam). These joints are generally skew pegged and though the carpenter might anticipate that the lap shoulders will do most of the work it is more likely that this is actually done by the pegs since cross grain shrinkage will gradually loosen shoulder contact.

There is also the well known lap dovetail found at the end of a tie bean where an English tying joint is employed. These unpegged lap joints are known to shrink back and frequently come apart over time requiring the addition of some metal strapping to arrest any further movement. This is classically illustrated at The Olde Bell public house in Henley-On-Thames, Oxfordshire [dd 1325, Miles].

See :- English tying joint failure

Conclusion

It appears that carpenters were and probably still are prepared to rigidly stick to tradition regardless of whether or not there is a well established history of failure in service of a particular type of joint.

What is the alternative ?

Regards

Ken Hume

With the English tying joint, I have gone to a cog, still a lapped joint but removing the shrinkage of the dove tail. This is an exception to the lapped joints missing in my area, and in breaking tradition, I see far more dropped tie configurations.

I hope the OP sees that a through peg hole is the way to go. And if not then a deep blind hole just shy of the other side. If a blind hole is used take care not to drive the peg deep and blow out the other undrilled side.

Tim

Now were cooking.

Hi Tim,

I concurr with your warning about the potential to blow the back face of a timber containing a concealed peg and it would appear that this is not a new problem. Please attached blown back face of a cruck purlin circa 1390's.

See - Blown Cruck Purlin

Regards

Ken Hume

Ken- could you explain that joint, seen in failure there in the picture , please? is it a lapped dovetail brace- or specifically its absence, that we're looking at there?

big spikes make excellent one-side blind pins, in that application and others.

Hi Toivo,

This is a particularly difficult joint to decipher. The purlin "extension" (to the left) is a short length maybe 3 or 4 ft long that is tacked onto the end of an 18 ft boxed heart elm purlin. Its very difficult to say whether this is original or added later because of the smoke blackening and limited access around the joint but it is positioned above a fairly narrow 6 ft wide bay (at ground level) which is still open to the fully hipped roof. To determine exactly what is happening inside this joint would require a little more in the way of joint forensics and unfortunately that window of opportunity has now closed. This is not a perfect world or a subject where everything is or can be revealed on demand in crystal clear detail.

I rather like the full text version of advice clarification given by Donald Rumsfeld :-

"... as we know, there are known knowns; these are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns - the ones we don't know we don't know1...”

Are you now ready for the analysis ?

The large peg that has blown the joint is the "singles" rafter peg and hence the 2 other smaller pegs (to the right of the large peg) appear to relate to the scarf joint. These 2 pegs have been driven from the outside in. To the right of these 2 pegs there appears to be 2 peg hole "shadows" which might relate to this joint and if they do then these would be for 2 pegs driven from the inside out. This might well be a simple step lap joint with the mating faces in the centre of the purlin in a plane parallel to the roof. We must keep in mind that there might well be other features of this joint that we cannot currently see.

Which stage in "Rumsfeld's progress" have you now reached ?

Maybe Mike could produce an "X-Ray" specs view of the likely configuration of this joint using Sketchup.

Regards

Ken Hume

Mike, I think something might be burning to the bottom of the pot, as we read quotes from Rumsfeld.

I have also seen draw bored holes that had a tad too much draw in them, and with a green peg driver not paying attention, pounded the peg through the start of the hole into the tenon and at which point the pointed peg was driven, not through the remaining hole but into the timber and out the other side. This done by the owners teenage son. If you need a sledge to knock the peg into place, at least don't force it. I could not get the peg back out, I left it as it was.

Tim

Hi Toivo,

Comme sa ?

Face pegged half lap scarf joint

Regards

Ken Hume