11ant
2025-05-25 14:54:40
- #1
I am "just" a construction consultant, not a structural engineer. And the original poster is already convinced by reason. But for him and the readers, I will gladly explain it in more detail nonetheless:
No, not just any danger – but that the roof structure collapses.
He should please retrain; in his profession, he should know (and even be able to explain better than I can).
Suspenders instead of a belt – but in a configuration where both are needed.
Three times right: 1. there was some reason, 2. statically necessary, and 3. replaceable. And in my opinion, replaceable by supports at least under the main purlins, which would then also have to be connected with (longer) collar beams and transfer loads into the ceiling below.
Or translated into your carpenter’s language: even back then, they liked to work with a lot of wood, but not more (and then unnecessarily more) wood.
If there were a force-lock connection between them and the ridge purlin, that would actually be a structural short circuit.
The job of the collar beams here is to prevent the rafters from shearing outwards towards the eaves by forming a stable triangle together with the rafters on both sides. Because they are offset from the axis of the rafters, they have to do this with half the strength from each side. They are, so to speak, a base of the triangle shifted upwards in parallel (probably because of the use of a concrete ceiling). Alternatively, the triangle could also have been constructed by using the ceiling joists of the floor below simultaneously as lower chords. How the forces in a roof structure act upon one another also has to do with their geometric-physical relationships. Here, there is obviously a barely tolerable shallow pitched gable roof (around 30° or slightly less), and a roof space rather intended as a dry storage or attic room, in which no intermediate walls offered themselves to integrate supports for the purlins. Now you can also explain this to your carpenter apprentice.
For ceiling construction help, squared timbers in comparable positions (but not required in pairs) would have sufficed. I recommend insulating the roof slopes along the entire length of the rafters – thus also beyond the collar beams and behind any knee wall that may still need to be trimmed down – because this is thermally cleaner. The fiddly work of cladding around the collar beams would have been more or less the same as the alternative of installing a ceiling beneath them (a choice between pest and cholera, both time-consuming and inefficient compared to the large surfaces). But thermally, this avoids a cold air cavity between collar beams and ridge. Installing a ceiling and insulating it would mean even more fiddly work days in exchange for the not really worthwhile saved length of insulation rolls.
Is there any danger in doing without these double collar beams?
No, not just any danger – but that the roof structure collapses.
Our carpenter tells us that back then "they liked to work with a lot of wood"...
He should please retrain; in his profession, he should know (and even be able to explain better than I can).
whether the suspenders are in place of the belt. What I almost fear.
Suspenders instead of a belt – but in a configuration where both are needed.
There must have been some reason back then to go to the effort. A structural engineer can calculate that.
These parts are also called ridge straps and are a statically necessary element. The trusted search engine says: They serve to brace the rafters to absorb tensile and compressive forces in the rafters.
Maybe there are alternatives. A structural engineer or carpenter can tell you. I wouldn’t just remove them.
Three times right: 1. there was some reason, 2. statically necessary, and 3. replaceable. And in my opinion, replaceable by supports at least under the main purlins, which would then also have to be connected with (longer) collar beams and transfer loads into the ceiling below.
Or translated into your carpenter’s language: even back then, they liked to work with a lot of wood, but not more (and then unnecessarily more) wood.
The ridge purlin is not supported on the collar beams at all. The collar beams only connect the rafters in pairs.
If there were a force-lock connection between them and the ridge purlin, that would actually be a structural short circuit.
The job of the collar beams here is to prevent the rafters from shearing outwards towards the eaves by forming a stable triangle together with the rafters on both sides. Because they are offset from the axis of the rafters, they have to do this with half the strength from each side. They are, so to speak, a base of the triangle shifted upwards in parallel (probably because of the use of a concrete ceiling). Alternatively, the triangle could also have been constructed by using the ceiling joists of the floor below simultaneously as lower chords. How the forces in a roof structure act upon one another also has to do with their geometric-physical relationships. Here, there is obviously a barely tolerable shallow pitched gable roof (around 30° or slightly less), and a roof space rather intended as a dry storage or attic room, in which no intermediate walls offered themselves to integrate supports for the purlins. Now you can also explain this to your carpenter apprentice.
or do these only serve as help for the ceiling construction? Today we are wondering whether we can do without the collar beams and let the roof slopes extend all the way to the ridge purlin.
For ceiling construction help, squared timbers in comparable positions (but not required in pairs) would have sufficed. I recommend insulating the roof slopes along the entire length of the rafters – thus also beyond the collar beams and behind any knee wall that may still need to be trimmed down – because this is thermally cleaner. The fiddly work of cladding around the collar beams would have been more or less the same as the alternative of installing a ceiling beneath them (a choice between pest and cholera, both time-consuming and inefficient compared to the large surfaces). But thermally, this avoids a cold air cavity between collar beams and ridge. Installing a ceiling and insulating it would mean even more fiddly work days in exchange for the not really worthwhile saved length of insulation rolls.