Wood isn't garanteed to catch fire if exposed to flame for a few heartbeats (6 secons, the length of a combat round).
Certainly not good oak (in my limited experience).
Admittedly thatched houses are very likely to catch fire, but as I understand the risks is generally overestimated.
That's an interesting argument.
I suppose I used the wrong term: In my view the wood will start being on fire when the ritual is completed, so it won't exactly catch. It will simply continue to be on fire; unless it has a good reason to go out. Because the ritual has made a perfectly natural fire, and there's fuel in that natural fire, so obviously the fire is burning that fuel (just as a natural termite would start eating the wood).
An unnatural fire has the problem of things needing to catch, because it won't be sticking around.
By the CrIg guidelines, "igniting" something and "doing +X" damage are entirely different effects.
This is hard to completely reconcile, but I think mainly due to the legacy echoes from other RPG's (where a "fireball" had an x% chance of lighting stuff on fire). Igniting something is a function of 3 things - fuel + heat + time applied. If you pass a matchhead thru the flame of a candle quickly enough, it won't ignite. The flame is "hot enough" to ignite it - it just requires longer exposure.
In the case of magical fire that causes damage, this is easily handwaved - "magical fire, damage is done but nothing catches, done". But with a ritual... it's more complex, but neither, not "catching" nor "not catching", is guaranteed by any means.
I have to say it doesn't sound like you've lit a lot of fires in your time. I say this because exactly the opposite is true - a fire will go out unless it has a good reason to burn.
6 seconds is only enough time to catch tinder, candles, fabric, etc. on fire, not wooden beams and walls. Things like furniture will be scorched, but hardly burst into flame. Some things will light, certainly, but not "the buildings" themselves, not without a more lasting source.
To continue burning (as opposed to smouldering out), more requirements are needed - and a generic wall may or roof beam or piece of furniture may not meet those. (Tho' a thatched roof easily could...) But you might take a blowtorch to the side of a dry timber all day long, and it may never do more than get a nice bank of coals that then soon go out when the torch is gone.
I'm not sure I would assume that fire that is capable of causing damage is incapable of igniting things. I think as gamers we tend to gloss over all the ramifications unless they are important to the story. Combat's already complicated enough without having to worry about all the side effects but generally I assume combat fire has the potential to catch flammable things on fire. Some times to character's advantage sometimes or to their disadvantage other times.
So sitiations like this.
"You want to light up those casks of oil with a Pilume of Fire? Well let's make some rolls and see."
"So your casting a BoAF into the covenant library? Does your character happen to have common sense."
From a pure rules standpoint it can't be the first time a Higher base effect encompassed a Lower base effect. As far as time required to catch things on fire, as anyone who ever leaped through a bonfire can tell you, the same idea applies to damage.
For my purposes I would say that a +5 fire (base effect 4) has some chance (make a roll) of igniting something very flammable (base effect 3). More intense fires (Higher base effects) will have an equal chance of igniting less flammable things (One base level lower than the damage level) or better chances of igniting more flammable things (Bonuses).
Either way blanketing a city even in +5 flame has the potential to set fire to a lot more things then wood. Trash, dry foodstuffs, roofs, rope, cloth, lamp oil, candles, parchment, hay face it a medival city is literally a tinder box. A +40 fire in that regards is probably overkill.
They don't need to ignite in six seconds, the fire isn't going away. Ritually created fire is natural fire, and will therefore burn if it's on a flammable substance. See pages 142 and 143 of the core book: Tremulous Vault of the Torches Flame and Leap of the Fire. In Ars Magica's medieval paradigm igniting something is essentially having a natural fire which is on it. In the real world those spells wouldn't ignite any fuel: In Ars Magica they do. Ars Magica doesn't run on real world physics.
Ritual CrIg creates natural fire. So you've got wooden beams with natural fire on them, aka wooden beams which are on fire.
You're missing the distinction between a momentary creo ritual (which creates something that lasts as long as a natural version) and a momentary spell (which creates something that lasts at most 6 seconds).
Love this one, works like a charm. I'd need to up it to arcane range but it would also stop the cities spellcasters (alternate setting, blah blah) from casting spells to try and escape their doom. Or at least give them a -10 penalty on top of not being able to breath.
On the ignem option, I like the idea of a fire either doing high damage repeatedly, or of a large area effect doing low damage but for diameter duration to get some normal fires going. Kinda liking King reapers argument too. Momentary ritual fires burn if they have fuel, they don't need to naturally catch things of fire to continue burning. I hadn't looked at it that way.
Creating one big fire probably would cause lots of small fires to catch but. Looking at the CrIg setting things on fire guidelines I like what you could do with them. I kept these as touch because I like the idea of a magi safely standing just over the boundary as everything goes to hell.
Sprinkling of Fireflies
CrIg35 (ritual)
R: Touch, D: Momentary-ish, T: Boundary
As the ritual completes millions of dully glowing sparks waft down upon the town. It's quite beautiful as they swiftly drift about till you realize that they are seeking out any extremely flammable items and setting them alight. Given the conditions of an average medieval city this will likely result in hundreds if not thousands of uncontrolled fires. It would take a great deal of effort, luck and a bit of magic to keep all of the city from being consumed.
(Base 2, +1 Touch, +4 Boundary, +4 size)
Note: This is nominally a Momentary spell. The pretty drifting sparks are a cosmetic effect. You could just as easily say everything as flammable as a candlewick within the target boundary (up to about 2 miles in radius) instantly catches fire.
Rain of Cinders
CrIg45 (ritual)
R: Touch, D: Momentary-er, T: Boundary
As the ritual completes millions upon millions of blazing sparks pour down. It's an awesome light show as they inundate the area igniting anything as at least as flammable as dry wood or charcoal. Barring a miracle or massive magical intervention the city will be burnt to the ground in hours.
(Base 4, +1 Touch, +4 Boundary, +4 size)
Note: I use a chimney starter and oil soaked newspaper to light my all natural charcoal and it still takes a few minutes for it to get burning. So wow that a lot of stuff that's going to be burning.
Downpour of Stars
CrIg55 (ritual)
R: Touch, D: Momentary-est, T: Boundary
An uncountable number of blazing motes streak into the city causing anything that's even barely flammable to burst into flame. The lights are nearly blinding and will work their way in everywhere so the spells victums won't see much in the terrifying moments before they themselves catch fire. Because here anything means EVERYTHING. Candles, oil, parchment, cloth, rope, leather, bread, birds, dry wood, green wood, waterlogged wood, people, diamonds, poorly made iron nails, peas porridge, people, pets, earthworms, did I mention people. Basically if it is now or was once alive it's now on fire.
(Base 10, +1 Touch, +4 Boundary, +4 size)
Wha' abou' li'l tiny rocks?... 
Yeah, I hadn't thought to look under ReIg for examples - hmmmm... from those, you're right!
Of course, the obvious counter-argument is exactly that - that's ReIg, not CrIg, but... meh.
However, it's "new" fire, not an existing, pre-established fire (as in the ReIg examples)... but I do like "ritual" fire having some advantage over short-duration formulaic spells.
Applying any "logic" to magic is often a trap - but in the absence of anything else (before you pointed out the ReIg examples), that's all we had.
But now that we have something else... I'd buy it for a ducat. 
I said diamonds those are tiny rocks. Any jeweler or insurance adjuster can tell you they burn like the dickens once you get past their refractory properties.
Well, they can certainly put scorch marks on a credit card.
Burning Down the Houses (CrIg 30)
R:Sight, D:Mom, T:Group
Ignites the roof of a group of 100 houses.
(Base 4 Ignite something flammable like dry wood or charcoal, +3 Sight, +2 Group, +1 size)
No ritual, no fuss. Cast this spell from a distant point where you can see the city, and let nature takes its course. Fighting the resulting conflagration is well beyond the capabilities of mundanes. If you want to ensure that the whole city is destroyed, you can cast the spell a few times.
Rain of Corrosive Acid (MuAu 40, Aq)
R:Sight, D:Sun, T:Group
Turns rain of a storm into a weak corrosive acid that slowly dissolves anything it touches. It causes the equivalent of a Light Wound every hour to living beings exposed to it, and attacks buildings similarly.
(Base 5 Transform an amount of air into something slightly unnatural, +3 Sight, +2 Sun, +2 Group, requisite for free)
Note: The requisite is free, similarly to what is done with Rain of Stones (ArM5 p.127).
Of course, such a course of action (destroying a whole city) is sure to draw the attention of the infernal.
Stepping into the modern day for a moment- a wood fire takes time to ignite because the wood itself doesn't burn. Heat causes the wood to break down into cinder, alchohol, and methane- the alcohol is forming well above the boiling point, so the blended gas of alcohol and methane burns.
So what it comes down to is what does it mean when you say that a ritual creates a 'natural' fire and how that differs from a non-ritual fire. In fact, I can't find an example of a ritual CrIg spell right off. Given that most Cr ritual effects endure permanently it almost makes me wonder if you could use a ritual CrIg to make a perpetual flame without the need for fuel...
The maximum normal duration allowed for ritual spells is Year, so you could not unless you developed Perpetual duration as a breakthrough. However, if you allow Ring duration Ignem spells, yes, you can do permanent flames with Ignem. Quite low level magic, in fact. We use that quite a lot IMS.
Xavi
Well, it differs in that it isn't sustained by magic and doesn't need magic to sustain it. So it needs actual fuel (unlike an unnatural fire) but doesn't go out at sunset/the full moon.
Would you allow a ritually created horse to live forever without eating?
keep in mind I am not arguing a position at this point, I am trying to wrestle with this concept...
so if you have a non ritual fire and stick wood into it, would it not create a new mundane fire?
the exception being momentary creo spells- a ritual creo terram creates a rock which sticks around.
No, but neither does that horse simple disappear if it starves to death. You could still use the hide, bones, etc. Better counterexample is that magically created water will evaporate, but even that isn't the same as vanishing.
Looking at Im as well, it seems that there is a general lack of momentary rituals for creating things which are not strictly speaking things.
This was why I stopped posting - I still have to figure out what it means, and how it is handled in relations to MR.
There isn't one in canon.
No. They become natural in all ways. A cow created using ritual CrAn will die of old age. A piece of wood created with CrHe will eventually dry out and become brittle or rot away, depending on enviroment.
It becomes natural.
Not in this edition.
See p. 112 of the core book: "Ritual Creo spells with Momentary duration create things that last as any other thing of that type."
What does a natural horse do when starved of food? Leaves a corpse.
What does natural fire do when starved of fuel? It vanishes, it doesn't leave a corpse.
What does a horse do when it suffocates? Leaves a corpse.
What does fire do when it suffocates? It vanishes.
Those things are what happens to ritual creo-d fire, because it's the same as natural fire.
Ritual CrIm is an interesting one, because you're either creating natural species (which don't last long at all) or you're creating a "natural illusion"... which isn't a thing and therefore never comes into existence.
Unless it is a desert illusion, that then remains there for quite a while. Nice catch about Im there 
I think you're jumping track (perhaps understandably) by applying terms from one part of the rules to another part.
The use of the term "un/natural" for Creo Rituals is usually used in the context of Magic Resistance and Penetration. That's not this.
The use of the term "permanence" is not a default of a Ritual, not a definition of what Creo'd means, but the other way around - nothing can be created permanently without vis (i.e. a Ritual). That doesn't mean that something created via a vis-Ritual has to be permanent - it depends on the target and Form, among others.
As Kingreaper points out, "Fire" is not tangible, nor does it leave any echo of itself when its gone (at least not within the Medieval understanding of physics).
For me, then, the question becomes "Okay, so what's the practical difference between CrIg with and without vis? For a mage and a Player (i.e. in-game and for game balance), why bother with a Ritual if you don't have to?" And I believe that has to be, as the Rego guidelines suggest, that if there is fuel, it's good to go, just as if it had been there all along. This is very diff from CrIg creating fire that causes damage, and there are diff guidelines for each (damage vs. ignition, and also vs. mere "heat" for that matter). The Rego guidelines show what happens when "natural" fire is moved with magic - it "adheres" to the new fuel source as ir it had been consuming it all along, no modern-physics prerequisites of heat or etc., it just "fires up" (pun semi-intended).
And, no, there are no CrIg Rituals that I can point to. But the "logic" that I am applying here is three-fold - 1) my understanding of the magic system, 2) game balance (what is "fair" to a player without inviting abuse etc.), and 3) a "consistency" within the game as a whole, a sense that this is how it "should" work, or at least is the least jarring interpretation of how it might.
A couple nice small rituals, that even a journeyman mage might easily invent in a season or two... what's 4 vis between friends?...
Foundations of Sand
MuTe 20 (Ritual)
This effect turns all stone within a town or city to sand, causing immediate and catastrophic failure of all stone structures of any sort, including bridges, roads, masonry (such as chimneys), statuary, urns and containers, and even smaller items such as whetstones and mortars and pestles.
(Base 3, +1 Touch, +4 Boundary)
Foundations' Flaws
MutTe 15 (Ritual)
As above, but all stone becomes "rotten", losing its structural integrity and load-bearing ability. Any construction of stone that bears weight will crumble sooner or later, depending on the stress upon it. Stone buildings of more than three stories will begin to collapse almost immediately, and lesser constructions will become dangerous within hours or days. Stone tools may fail when used, but containers may not fail until handled roughly.
This effect allows a mage ample time to be somewhere else before the magnitude of the disaster is realized.
(Base 1 +1 Stone, +1 Touch, +4 Boundary)
...and for the more discriminating mage...
Castles of Sand
MuTe 30 (not a Ritual)
This effect turns all stone within a structure to sand, causing immediate and catastrophic failure. inlcuding masonry (such as chimneys), statuary, urns and containers, and even smaller items such as whetstones and mortars and pestles.
Once the spell ends after a few moments, the "sand" that has collapsed returns to a fine gravel of the type of stone in question, and any that has not collapsed returns as (more or less?) normal.
(Base 3, +2 Voice, +3 Structure, +2 Size)