riven versus bandsawn for wooden shingles

[sdjp]

A 25 mm (1”) thick riven oak board is as strong as a sawn board 50 mm (2”) thick.

I've been pondering this, and I think I must have misunderstood something.

That implies that if one could split off less than 1/4 inch from each side of a straight grained oak board that was 2" thick, then the final board would be stronger than it was to begin with. Is that a correct interpretation?

Because that seems rather counterintuitive. It would make sense (to some degree) if it wasn't possible to split off the 1/4 inch sides - but I've done such splits reasonably regularly in many woods, although not in oak to be fair (not had any pass the shed yet).

Or perhaps there's an implicit assumption that the grain in the 2" sawn board is not perfectly straight (or that it's a crown cut or something), and hence I'm not comparing the right things in both cases. It's not contentious that a quarter sawn board could be stronger than a crown cut, and that be the source of the 'extra' strength when one gets to full blown riven boards? And therefore with perfect grain a 2" sawn board would be nearly twice as strong as the 1" riven - but the numbers given were for the typical case, hence part of what they mean is the extra selection criteria for riven boards?

(To be clear, I _can_ see how a thinner but riven surface board could last longer outdoors, but that was explicitly not part of this statement)

 

[Sgian Dubh]

A 25 mm (1”) thick riven oak board is as strong as a sawn board 50 mm (2”) thick.

I've been pondering this, and I think I must have misunderstood something.

That implies that if one could split off less than 1/4 inch from each side of a straight grained oak board that was 2" thick, then the final board would be stronger than it was to begin with. Is that a correct interpretation?

I think you have misinterpreted my point. Let's just talk about radially orientated grain in a board for simplicity. If you split out from a log a board that's 1" thick, the split will follow the grain giving a piece of wood with uninterrupted long grain throughout its length: the board may not be particularly straight but the grain is, essentially, continuous. On the other hand, when a 2" board is sawn out of a log the saw cuts straight, and cuts through the grain (albeit mostly at an acute angle) in places making the grain discontinuous, therefore somewhat weaker. You could easily envisage a situation where only a 1/4" or so of a 2" thick sawn board might be continuous, and possibly in many cases there could be no continuous grain throughout the length of something like 10' or 16' board. Slainte.

 

[Random Orbital Bob]

So Richard.....would half inch sawn green oak be sufficiently thick to last (say) 10 years do you think?

 

[Sgian Dubh]

So Richard.....would half inch sawn green oak be sufficiently thick to last (say) 10 years do you think?

I don't see why not because European oak is a durable species, i.e., in ground contact it has a life expectancy of up to 25 years. But as a shingle the wood is not in ground contact so should last much longer. owenmcc said earlier in the thread that life expectancy for oak shingles might be as much as 80 - 100 years. I don't know if that's correct, but it seems a reasonable estimate because I'm aware of oak gates around the farm where I grew up that are just now falling apart to the point of complete uselessness, and they've been around since the 1940s or 1950s.

However, I think earlier you may have been confused by two different wood characteristics. You asked originally if shingles were strong enough to last 30 years if sawn rather than cleaved or riven, but strength and durability aren't necessarily linked one to the other. Strength is about a material's ability to resist and recover from stress (i.e., a load applied to a post or beam for instance) whereas durability is the ability to resist decomposition through the actions of things like fungi and bacteria. The fact that one European oak shingle was riven and another sawn from the same log should have very little impact on the wood's durability (decay resistance) because the chemical make-up of each piece is almost certain to be pretty much identical. Slainte.

 

[Random Orbital Bob]

Many thanks Richard. You're right I was confusing strength with durability. I had assumed the 2 were linked and in fact your response has cheered me up because it essentially means I can safely bandsaw the material without being concerned I'm losing anything. The advantage of sawing is obvious, I can get uniform flatness and dimension which will make the laying much simpler. Many thanks.

 

[paulm]

Given that the oak is inherently durable, and that sawn or riven isn't necessarily important to that, perhaps you could safely go a bit thinner than a half inch thick Rob ?

It sounds kind of chunky ?

Maybe 3/8" if not 1/4" seems more instinctive, although based on no specific knowledge or experience at all ! :lol:

Cheers, Paul

 

[Sgian Dubh]

... it essentially means I can safely bandsaw the material without being concerned I'm losing anything. The advantage of sawing is obvious, I can get uniform flatness and dimension which will make the laying much simpler. Many thanks.

Just make sure you don't include any sapwood in your shingles because the sapwood of European oak is classified as non-durable, which is the case for virtually every wood species, as far as I can recall. There may be a few species for which the sapwood is classed as durable, but if that's the case I can't recall the names of any. Whatever, oak sapwood is non-durable, i.e., it rots away fast, and that's all you need to know. Slainte.

 

[owenmcc]

Based on the cedar shingles I've used, the butt (thicker and presentation) end is around the 15-20mm mark tapering to 0-5mm. (So I'd suggest a bit thicker than 1/2".

Double lapping every shingle will certainly help with durability and I'd have thought you'd reach 10 years of lifespan without much bother.

The picture might help to give a sense of scale, although I've read that oak shingles tend to come out wider and longer which would make laying out easier. Also you won't have to do all the string line tapering going on in this picture!

N.B. There's a few thin ones in this photo, with less distance between joints, we were aiming at roughly 2" from overlap on width. So you can either take it as an example to avoid, or a suggestion of where in the roof to hide your small shingles...

 

[woodfarmer]

Many thanks Richard. You're right I was confusing strength with durability. I had assumed the 2 were linked and in fact your response has cheered me up because it essentially means I can safely bandsaw the material without being concerned I'm losing anything. The advantage of sawing is obvious, I can get uniform flatness and dimension which will make the laying much simpler. Many thanks.

It is my belief that riven shingles will last longer because the water wont have hardly any end grain into which it can enter. This is why riven timber is preferred for farm gates and encosures.