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Author Topic: Ridge beam vs Ridge board understanding/confusion  (Read 3710 times)

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Offline MAD MARK

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Ridge beam vs Ridge board understanding/confusion
« on: March 03, 2020, 08:29:52 AM »
Ive attached an image of what to be a ridge beam vs ridge board. After a little research I am getting different opinions/facts on the matter. 

It seems to me that in most timber framing/post and beam applications you would chose Ridge Beam over Ridge Board as the beam carries some weight as opposed to the board which applies more outward thrust/force. I understand if you didn't have King Post then it would be hard to use a ridge board of any significant girth, less it be a LVL. 

What am I getting right/wrong? 


   

Offline Hilltop366

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Re: Ridge beam vs Ridge board understanding/confusion
« Reply #1 on: March 03, 2020, 09:08:38 AM »
Sounds mostly right.

My understanding is the ridge board is not a structural member, you could remove the ridge board and fasten the rafters together with the same load capacity. If you removed one side the other would collapse. The ridge board just makes it easier to frame and keep straight. The roof with the ridge board is pushing an outward force at the lower end and requires something to tie each side together. The entire roof load is transferred to the walls.

The ridge beam is a structural member holding up the roof joist (not rafter) and taking the load of  the upper side of the roof, the other half is held up by the wall, of the roof load is transferred to the king post. There is no outward force only downward, if you removed one side the other would stay standing.

A ridge beam could have the roof joist fastened to the side of the beam and the post buried in the wall there by looking like a ridge board causing some confusion as to what it really is. 

Offline MAD MARK

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Re: Ridge beam vs Ridge board understanding/confusion
« Reply #2 on: March 03, 2020, 09:27:04 AM »
So, when I look at the picture below. What am I seeing and what would the top beam be called? I see it doing a little of both, what would be the purpose of doing it this way? I did confirm with the builder that Timberlok screws were being used at the top of the "rafter" to fasten to the top beam.





 

Offline Hilltop366

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Re: Ridge beam vs Ridge board understanding/confusion
« Reply #3 on: March 03, 2020, 10:20:47 AM »
Perhaps there is some gray area but I would say that in this case it is a ridge board and rafters, if you took out the "king post" on the end rafters there would be no question. Although the "king post" is holding up the ridge board once the rafters are connected there mostly decoration.

I use quotes on the king post because I think if it really was a king post truss the tie beam would be connected directly to the rafters not the corner post. Also if the king post, struts and tie beam  were needed they would be needed on all of them because the inner rafters are holding up twice the roof load as the end one. (disregarding any gable end overhang)

I have only done a bit of timber framing 20 years ago so hopefully someone with more experience will come along and comment.

Offline reride82

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Re: Ridge beam vs Ridge board understanding/confusion
« Reply #4 on: March 03, 2020, 03:58:25 PM »
That appears to be a ridge board with a decorative/faux truss. The "truss" in that photo isn't tying the top cords, or in this case the rafters, to the bottom plate of the truss but rather to the top plate of the wall. Thats just an engineer's take on it, I'll let the timberframe gurus elaborate on the particulars.

Levi
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Offline D L Bahler

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Re: Ridge beam vs Ridge board understanding/confusion
« Reply #5 on: March 03, 2020, 08:15:02 PM »
A ridge board is used on a stick framed roof, typically, and basically its function is that it serves to give you a solid reference point to measure to, line things up, cut rafters, and position everything. You use a ridge board because it's easier than building all your rafter pairs out "in the air" and expecting them to be at all accurate. (Many custom builders, in fact, will stick frame roofs with ridge boards, because factory built trusses are difficult to get accurate and properly aligned) 
In such a roof, the rafters are exerting thrust on the walls, because the peak of the roof, being unsupported, naturally wants to fall down.

A ridge beam is used to structurally support the peak of the roof and hold up the rafters. It could be a continuous wall (and in Europe this is common) but generally it's a massive beam with intermittent support. In this case it needs to be quite large because it carries a significant load. (ridge board, in contrast, is not engineered to carry any loads at all) 

With a ridge beam, there is no outward thrust. That is unless the ridge beam is itself supported by a heavy truss or something like that but that's a whole other can of worms. The ridge can't come down, so the rafters exert only downward thrust, plus whatever lateral loads come from winds.

Personally I prefer ridge beam roofs. I think it's the best way to build a roof, and you can make some nice looking assemblies. But that's just my opinion so take it for what it's worth.

Like was said, in the picture you have a heavy timber ridge board. The rafters are screwed into the board and into the wall plates. It wouldn't be a great design, I think, for a much larger roof but for this it's fine. The "Truss" doesn't appear to be functional at all, just pretty. Which is fine. Again like was said, no true truss assembly there. Though some timber frame "trusses" also are not continuous and have some engineering "compromises" going on. 

Offline Don P

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Re: Ridge beam vs Ridge board understanding/confusion
« Reply #6 on: March 03, 2020, 09:40:22 PM »
I typed slow but another tome :D

That might be a good place to begin. A truss is based on one or more rigid triangles. A triangle cannot change shape, add a 4th or more sides, pin connected, and the shape can change. The simplest truss then is a pair of rafters with a ceiling joist forming the third side of the triangle. In your first picture of a ridgeboard that bottom tie is missing, so that roof will sag at the least as the rafter feet toggle outward under load.

As the span of the ceiling joist (in truss terminology the bottom chord) gets longer, it tends to sag. If you tie a rope around the peak of the stable triangle and drop it down to the bottom chord and tie it to the bottom chord you can support that sagging member. I'm using that analogy so you'll understand that a kingpost is actually not a post at all, it is a tension member. Looking backward, the oldest surviving kingpost truss is in St Catherine's Monastery at Mt Sinai. They understood very well the mechanics going on, it is not connected at all to the bottom chord, they simply used the dangling kingpost to support compression struts that in turn run up like your fan struts to support the rafters at their midspan. This was built during the reign of Justinian in the 6th century. BTW the oldest surviving copy of the Bible resides in the library there ~345AD. That form of free bottomed kingpost was used frequently in the formwork, centering, for Roman arches. So anyway, kingpost as a tension member. 

For that reason I hesitate to call the post in your second drawing in post #1 a kingpost. I simply refer to it as a post or more correctly a column.

Fast forward to the middle ages, that is all forgotten. Timberframers of that period stood a post on the tie to support a collar beam which in turn supported the rafters. That post was in compression and is called a crown post. It was a miserable solution to the roof support problem since the tie was point loaded at midspan. It is easily intuitive but to put it mildly, this is not a good solution. I tend to call any compression post in this situation a crown post even when it extends all the way up to support a ridgebeam. Shortly thereafter we were back on track ~1600's the kingpost truss, in tension, is showing up again. The top chord heels are locked into the tie in various forms of notches and the kingpost is pinched between the top chords at the apex.

Look at your bottom pic, we've forgotten again, that is a crownpost supporting a ridgeboard, the rafter heels are not locked, the king is not pinched at the apex. The ties are hopefully secured well enough to the plates to arrest spreading thrust creating a simple truss that is rather poorly connected. The plate beams are hopefully stout enough to contain that thrust horizontally at midspan as that is a ridgeboard up top, incapable of supporting half the roof weight. At this scale that can work just fine, the plates can do the work of a ridgebeam, but it is probably not the best understood design on the part of the builder.

I'm just a carpenter so take that all with a grain of salt, I agree with everyone above. I was mainly just trying to give some background to the semantics as I understand it, more grist for the mill.
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Offline MAD MARK

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Re: Ridge beam vs Ridge board understanding/confusion
« Reply #7 on: March 04, 2020, 08:47:13 AM »
I appreciate all the responses. 

In researching all this I was coming across different opinions, thus different understandings. I was in contact with the company of the real photo I took. All they were using were I believe 2 Timberlok 12" screws at the plate beams. And 2 Timberlok 12" screws at the Ridge Board. You can see them in the picture if you look closely. 

This structure almost has the exact dimensions I am looking at doing, therefore I was studying it quite closely. Below is the finished product.  



 

Offline Al_Smith

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Re: Ridge beam vs Ridge board understanding/confusion
« Reply #8 on: March 04, 2020, 11:49:53 AM »
That end on the structure is an example of a trussed rafter .Which I've found is nearly impossible to explain to some people that  it's much stronger than conventional rafters .I gave up even delving into it decades ago .It's all about an understanding of geometry, a sub division of that exact science is trigonometry .They've had that fiqured out in days BA--before Al --- ;D

Offline D L Bahler

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Re: Ridge beam vs Ridge board understanding/confusion
« Reply #9 on: March 04, 2020, 10:27:51 PM »
Replying to Don,
Regarding your truss history, the information about medieval work does depend somewhat on what part of Europe you're talking about. This might describe the situation in England, for example, but for Central and Southern Europe it is inaccurate. In some regions they definitely held on to trusses and used them throughout the Middle Ages. But also make sure we're comparing apples to apples. Vernacular work in the provinces wouldn't have used trusses even during Roman times, as they all had their own building traditions that had nothing to do with Roman engineering. Most of Western Continental Europe's house and barn construction stems back to Celtic high post building, not Italian Roman methods. Most of the Gauls went about doing things they way they always had, and cared very little for the fact that they were now technically Roman subjects. They had fields that needed tilled and grain that needed stored. (in German Hochstudhaus, referring to tall posts supporting the ridge beam, is the ancestor of all vernacular forms, including the log houses of the Alps, and is the original Celtic house.) They later incorporated innovative roof construction techniques from other sources. I think English and northern French and northern German building has a lot more to do with Germanic methods, which were a lot more primitive than their Celtic counterparts owing to the transient and migratory nature of the Germanic tribes before about the 5th or 6th century.

Castles and Churches used trusses all through the Middle Ages, as did wooden bridges, even in the so-called "dark ages", at least in Central Europe this is the case. There are Churches in Switzerland, for example, built in the 6th century that certainly had wooden trusses originally, though these original roofs no longer exist. Notably, these would have been Burgundian projects, built in the "Lombard" style under the guidance of Italian carpenters who would have had access to and some understanding of their Roman prototypes. This era of Roman-revivalism reached its apex in the mid 10th century under the Burgundian king Rudolph II. These, in turn, informed the construction of later Church and Castle roofs, which by the 12th century in the Holy Roman Empire had developed into a very sophisticated system of truss framing that suited the rise of Gothic architecture. (incidentally, most of the older Romanesque churches had their original trussed roofs replaced around this time with steeper vaulted trusses. The Grossmnster in Zrich might still have its original 12th century roof, I'm not sure if it was destroyed in the fire of 1763, or if that was limited to the towers) These are the places where some Roman ideas were held on to, passed down among a very specific class of carpenters. These practices later made their way into house and barn construction, especially as need arose for bigger buildings. In Switzerland this seems to take place in the 15th century at the latest, and many folk legends attribute the shift to trusses in vernacular framing to the arrival of Waldensian refugees from Savoy and Lombardy in the later 15th century, but this is probably not true. Simple kings post trusses become the standard for smaller agricultural buildings, though they rely on lower trusses used to prop up a ridge beam, from which common rafters are then hung. Of course this is all muddled by the fact that there is no single Swiss architecture, and even when some regions were working with quite innovative trussed barns and houses, others were still building iron age high stud thatched barns, along with about every imaginable iteration between the two.

Offline Don P

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Re: Ridge beam vs Ridge board understanding/confusion
« Reply #10 on: March 05, 2020, 08:02:36 AM »
Very cool, thanks DL, more reading to do along those lines :P
OT, construction is interesting and definitely varies. I drove from here up to Chambersburg PA yesterday and back. (brought home a new to us tandem dump, woohoo!) When I pulled into the gas station up there I was across from a beautiful stone bank barn, the rock by the island at the pumps was exactly the same blue dolomite we have nearby here, the same ancient seabed all the way up 81 in the valley of Virginia. It was well worked up there and used for barns and houses, down here that is rather rare, there are examples but much fewer and farther between. I think it is more German up there, more Scots/Irish down here.

Al, that was made to look like a trussed rafter but it is not trussed, that is all faux infill. The kingpost would go into compression under load if called upon, it is incapable of tension as connected, the webs in compression would simply serve to open that joint. It can work but that depends on the bottom chord acting as a beam rather than as a tension chord.
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Offline Al_Smith

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Re: Ridge beam vs Ridge board understanding/confusion
« Reply #11 on: March 05, 2020, 09:07:04 AM »
Thanks for pointing that out .Pictures some times don't tell the entire tale .On this framing deal  most on here are timber framers.Unlike that I'm a steel; framer for the most part .The principles  are exactly the same ,the study of the triangle .The difference being steel is not pinned ,nailed or dove tailed .Over the years it's been riveted,bolted or welded .Any of it works .Although I doubt I'd ever be a fan of driving hot rivets .Some things in history are best just left there .This is 2020 not 1920 . :)

Offline Al_Smith

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Re: Ridge beam vs Ridge board understanding/confusion
« Reply #12 on: March 05, 2020, 09:35:40 AM »
That mention of dolomite brings up an interesting point .This area of Ohio sits on top of a very thick limestone base that reaches from Lake Erie to several hundred miles south of the Ohio river .That's what you find in old foundations because it was plentiful .My three outside patios are Berea white sandstone from quarries near Cleveland Ohio .Recycled from sandstone sidewalks from Toledo Ohio .Tonnage believe me ,took a lot of beer to lay them . 8)
You get into the unglaciated area of the state to the east you get into granite .Not like Vermont but still hard--like a rock  ;D 

Offline Don P

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Re: Ridge beam vs Ridge board understanding/confusion
« Reply #13 on: March 05, 2020, 03:26:08 PM »
To continue the train of thought as I was working today. I would call that braced rater construction, at best. You can see a dscription and pic in the codebook at the end of the rafter span tables section IIRC. It wouldn't take much to make it a trussed rafter set, nail a 2x4 from the peak to the bottom chord, connecting ridge, rafter, and bottom chord together as a tension member and bingo, its a truss. Sorta, the heeljoint would probably not be strong enough in reality with just a timber screw to stand much thrust.
The future is a foreign country, they will do things differently there - Simon Winchester


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