oak frame behavior question

Hi All,
I'm getting ready to cut a small frame for an outdoor kitchen out of white oak and am wondering if anyone has pictures or anecdotes to convey just how much movement and opening to expect around joints? Because its just a naked frame the joinery will be quite on display, so I'd like to be prepared for what might happen visually. My experience to date has been more with softwood.

Does it help the case that the frame will sit air drying under its own roof, with no sudden drying caused by a heating system as in a house?

Whats a good timeframe between cutting the joints and raising?

Finally, is there any problem or benefit to be had from oiling the frame once its up?

Appreciate any advice, thanks for your time.
jon santiago

In my practice, I tend to fully house the joint abutments as much as possible to help conceal the shrinkage margins, also I tend to graduate widths of timbers. In oak the shrinkage is troublesome with braces so housing is essential to hide the change of angle and drift from checking in boxed heart braces, so free of heart in braces is helpful. You can oil the timber on the horses, right after cutting. End coat the joints with Anchorseal. Keep the timbers in shade. Erect frame as soon as you can. Drawbore the pins.

Roger is right on. In my experience oiling and sealing end grain help slow the drying process and that has to be good.
No direct sun on timbers if at all possible and raise it as soon as you can or the raising is twice as much work as you are trying to fix problems. It doesn't take long for the stuff to start moving.

Good design is key -- don't stack deep beams, avoid posted ridges where the post sits on a deep beam, etc.

If the timbers are going to sit more than a week or two before you cut them, you should seal the end grain of the raw timbers to cut down on end splits.


Also, use the absolute best oak you can get as it moves less (shrinks the same but twists and bows less).


It is common to see joinery gaps of 3/8" and checks up to 3/4".

Green oak can be a bit of a heart breaker, but to me, nothing else is as pretty and as solid looking as oiled oak timbers.

hmm...i do have a posted ridge in the design on top of a 9" deep beam. the tie beam will shrink away, and the ridge will want to settle, rafters will thrust? how big will that movement be and can i mitigate it somehow?

Jon, in your scenario, I would look at shrinkage across the frame and look for differences in shrinkage from the total accumulated at the ridge vs at the plate. Plus deflection under load of the tie can effect the position of the ridge.

If you have a building in service that you concerned about, set up a monitoring plan and track changes. If it appears that joint failure is ongoing, get the building surveyed by someone with relevant experience and is licensed to do so.

thanks roger. to clarify, this is just a design in process not yet built. its a small outdoor structure (12x12). perhaps im overthinking it

What is the depth of the ridge? Is it the same depth as the plate? If so, you can disregard that since they will drop the same amount. If the ridge is deeper than the plate, (say 12" vs 8") you have the shrinkage of the difference compounding the problem. In this example, that's 13" total of oak -- could be as much as 5/8" or 3/4".

If they are the same and you are just looking at 9" of oak shrinkage that would be more like 1/2".


I try not to do posted ridges in green wood. You can leave a gap between rafter plumb cuts and don't screw opposing rafters to each other - just to the ridge. That way when the ridge settles, the rafters don't thrust.

But with 1/2" of ridge drop, you can't really leave gaps that big. You would need a gap of 2x(tan roof pitch x 1/2". That would be 1" if it is 12 in 12 and 3/4" if it is 9 in 12.

Where posted ridges are used, this concern is well known

The solution has always been to let the rafters rest unattached on top of the plates and purlins, pegged or nailed only to the ridge. Then as the roof structure settles, the rafter ends will simply slide over the purlins/plates and not thrust on them

I do not believe a ridge supported roof system is a bad concept. I hold that the relevant shrinkage in the ridge is the portion under the bearing not the entire beam, so in a simple example, if a ledger is applied to ridge four inches up from the bottom of the ridge, then the shrinkage that effects the the rafters support is that four inch portion. Also keep in mind the ridge will shrink in width, so the likelihood of severe thrust at the rafter foot will be limited. The same applies to a mortised and/or housed rafter, it is the height of the bearing not the total height of the beam that would concern me.

I cannot think of a reason to affirm Davids no connection at rafter and plate.

Roger,

This is the way it is done in the cultures that traditionally use a ridge support style of roof framing. There are a lot of differences between a ridge roof and a truss style roof with plate support. We have to be careful to keep these differences in mind.

I focus on ridge roofs, it's my own personal favorite and it is the most versatile possible way of framing a roof. It's the only practical method of framing a shallow roof (thrust is too great otherwise)

In the life of the roof, 'floating' rafters help to ensure the overall integrity of the frame. Stiffness is attained by bridging or decking from rafter to rafter, if uplift is a major concern (It shouldn't be, the gusts that strike hung roofs in the Bernese Alps are matched only by hurricane or tornado force winds) then you can simply strap the rafters to the plate.

David, I'm glad you clarified because "The solution has always been to let the rafters rest unattached on top of the plates and purlins, pegged or nailed only to the ridge." does not equal strapping but still the strapping used in high wind resistant construction is not either floating or unattached.

I would also like to add, in the tradition I accustomed to, framing is done with very well seasoned fir timbers -seasoned before any layout and joinery is done- so movement in the final frame is slight, especially compared to framing with green oak.

The reason for this is the very problem Jon is facing, very exposed framing that must continue to look well fitted years into the future, so we use seasoned wood and work with extremely slight tolerances (like, 1/32" in any exposed joinery, and 1/16" in blind joints)

I would also like to state, I don't disagree with Gable by any means, but I should say that his approaches named are just one way to approach things.

In Bernese framing, we mix elements of log building into timber framing including the stacking of deep section timbers at the union of two stories of a structure, and the seating of ridge beams on top of deep section timbers. These things can be done, BUT you have to design for them. Gabel's stated approach is to avoid these things, so that you don't have to go to the trouble of designing for them.

The floating roof is one example of designing for these things.

Strapping as I mention it is, a steel strap nailed to the plate, passed over top of the rafter, then nailed back to the plate, nowhere nailed to the rafter, so that the rafter is free to slide through the strap if it needs. But this isn't really done. The weight of the roof is going to keep it down.

The Storm Lothar several years back took roofs off of one or two houses, this was an Atlantic hurricane that turned inland and struck Central Europe in 1999 or 2000.

This type of construction uses no manner of birdsmouth or other rafter to plate joint. The rafter passes over the plate, allowing us to easily make generous overhangs (like in the Emmental, where overhangs are about 12 feet)

As a side note that may interest some, when framing became more industrialized in Switzerland, a plate seated rafter roof was adopted, mostly in response to the need of a completely open attic space for open hay lofts and driving floors. But they still wished to preserve the generous overhangs responsible for their houses lasting 500+ years. So they inserted lower rafters keyed into the primary rafters, but set out at a slightly shallower angle so that they could kick past the plate. This became a distinctive architectural characteristic of the Canton of Bern from the 18th century on.

Thanks for these comments everyone. As DL alluded to I ended up with a ridge because I was designing for a shallow pitched roof (4:12) to not make an imposing structure where the space under the roof will never be used. I like the idea of the sliding rafters and would probably strap them for peace of mind. I will have plate and ridge of equal depth so it seems that the only shrinkage the will be relevant is that of the tie beam shrinking away from the bottom of the ridge post? This will result in the same scenario of the peak dropping as the ridge post settles onto the smaller tie beam, no?

DL, are you saying that the full rafter just sits on the arris of the plate? no notches? What connection would you recommend at the peak? tongue and fork, or should they just sit on the ridge and not meet each other?

Thank you Gabel for the mathematical piece of it. are you assuming something around 5% shrinkage? Also I didn't follow how you came to "13" of oak" from the example with a 8" plate and 12" ridge.

very appreciative of this forum and your time, thanks.

Uplift should be considered, an open structure and high winds.

Jon,

Sometimes we just sit them on the edge, some times the whole timber is set rotated to match the rafters, sometimes a 5th face is made to match the general slop of the rafters, and sometimes the plate is only reduced at the rafters, so that they slide through a notch, this is the method to use if the attic space is to be insulated.

At the peak, rafters don't need to be strongly joined to each other, unlike a standing rafter roof, the top connection between the rafters here does nothing. So they can be butted and spiked, half lap and peg, or even just set beside each other, which is how it was done when they used log rafters. You peg or spike them to the ridge.

I missed the part of it being an open structure, in this case I would definitely recommend strapping for the rafters.

I got the 13" in my example from 9" of tie beam and 4" deeper ridge than the plates.

Will/do the rafters sit atop the ridge with a birdsmouth or house into the side of the ridge as Roger is alluding to? If the latter, he is correct and only the distance below the rafter bearing should be considered in shrinkage planning.

If I recall correctly, oak shrinks ±5% radially and ±10% tangentially from saturated to oven dry. (I could have that backwards). While in forestry school, I did learn how to calculate estimated shrinkage for different species and different cuts of timber/lumber between two moisture contents, but observation and experience have informed my estimates now Over the years, we have taken several blocks of different species that were green and measured them, marking the measurements on the face indelibly and then checking them over time to observe actual shrinkage. I also like to take a tape measure when visiting past projects to check shrinkage.

I plan for/expect 5-6% shrinkage in oak from saturated moisture content to equilibrium MC.



Apologies DL, but I don't see how unconnected rafter feet could meet modern building codes with regards to uplift. Especially in an open building.

Gabel,
true, which is why I stick to strapping my rafters. Other more complex methods of a sliding joint/fastener could probably be designed too

With the oak I have used I have been very pleased with the stability and relatively low shrinkage. This has been winter cut and quite nice in grade. Also anchor sealed and oiled. There has been lots of small checks but most not more than an 1/8th inch.