Lean-to Questions

[rbowie]

I'm starting small with my first timber frame project.  Looking to add a lean-to to the side of my garage.  I am planning on lag bolting one side to the garage wall which should eliminate the need for bracing in that direction. 

I have a couple questions.  First, of the two options I'm showing in the picture, is there one that is better than the other? 
Second, does anyone see any issues with the rafter connection that I've shown?

Thanks,

Richard

  

  

  

 

[Jay C. White Cloud]

Hi Rbowie,

Looks good over all, and should be fast/fun to cut.

• Option #2 is real nice. I would not consider option #1 as it is configured, (poor ledger connection-not as strong as #2.)
• If you want to try to do braces for the practice, go for it, other wise you could use a pass through brace, and be fine for this structure.
• Place rafters on top of rafter plates in notches and peg/lag from top.

Good Luck!! and have fun.

Regards,  jay
[/list]

[rbowie]

Thanks for the recommendations Jay.

By "pass through brace" are you meaning use a through mortise on the post?

Richard

[Jay C. White Cloud]

Hi Richard,

Yes just a simple through mortise.  For your project 40 mm x 150 mm (1.5"x6") would be fine.  You would cut the mortise at 190 mm (7") high and wedge from the top. 

The one thing I did forget to mention, is making the tenons on the beams that connect your wall bent to existing architecture could be pass through tenons/or free splines and wedged, on the free standing bent.  Below are some example photos of free spline joinery I just did for a pasture run in shed.


 

[Dave Shepard]

Nice drawings. I like seeing hand drawn stuff. 8)

[jueston]

jay,

in the piece in that picture are they both the same species of wood or is it a softwood beam and a hardwood tenon?

[Jay C. White Cloud]

Hey Justin,

I think I owe you and email too. 

The beams, 5.6 m (18'), hand been some reject hemlock we had sitting around and didn't want to mill anymore 6 m plus.  For ease of assembly and tenon strength we used some really nice sugar maple.  There was three of them in the frame, with three free tenons.

I'll send an email soon.

jay

[rbowie]

Jay, those free splines scare the bejeezus out of me with my skill level.  I think I'll take your advice and use a through mortise for this first project. 

Do you typically use free splines when you aren't able to achieve adequate tenon strength from the beam wood species? 

For the through mortise wedge, should I be using hardwood?  I will be using lodgepole pine for the remainder of the frame.

Dave, thanks for pointing out the hand drawings....I think it's a dying art!

[jueston]

For the through mortise wedge, should I be using hardwood?  I will be using lodgepole pine for the remainder of the frame.

you can do the full mortise with pegs or with a wedge depending on what kind of look you want, but either way the pegs or wedge should be made out of dry hardwood, you don't want those to shrink. as the green wood dries around the dry hardwood it will tighten up a little, but if the pegs or wedge was to shrink it would create a loose joint and reduce the strength of the frame.

[Jay C. White Cloud]

Hey Everyone,

I did forget to comment on your drawings, I was a little taken back by seeing some nice isometric grid paper in use, that was cool.  I make anybody I teach learn to draw and CAD both equally well, not to mention good drafting calligraphy.  I'm almost out of iso paper, where did you get yours, online?

Now for your frame, don't be scared of nothing, that joint is way easier than it looks.  I'm not saying you have to use it, but it's not really that hard.  I will share the caveat, that I use line layout systems, not mill, or scribe rule, which makes layout much more accurate and gives you the option of employing templates more readily.   That ties into your bracing question.

In European frames, scribe rule and edge rule are the layout methods and oblique bracing is the bread and butter of the craft.  Now, in the older-original methodologies of the craft, going back thousands of years, like in most of Asia's history of timber architecture, Center Line or line rule layout is the the norm and bracing a frame is achieved through members running horizontally, rendering a frame solid but more flexible in seismic events.

Do you typically use free splines when you aren't able to achieve adequate tenon strength from the beam wood species?

Yes that is one of the reasons. There are others reasons to numerous to list here that are specific to different circumstances, like too short of beams/logs, ease of frame assembly, mixing species to render different effects in the joinery, etc.

For the through mortise wedge, should I be using hardwood?  I will be using lodge-pole pine for the remainder of the frame.

As Justin pointed out, the pegs and wedges should be of dry hard wood.  However, if you don't have dry, just be aware that the wedge will shrink and need to be re-tightened.  One trick that we still do to this day, is pop them in 150 to 200 degree oven.  This drys them out fast, then we place them in oil.

Ah yes, Lodge Pole Pine, beautiful Pine species and perfectly acceptable for your frame.  Some trees of this species can have pronounced twist, try to avoid them and don't worry it is obvious.

Now your bracing through mortises, if you choose to use them, that is really simple.  On top of your post, you are going to have your rafter plate, (I would recommend housing the post into it,) from there come down 100 mm (4") and this will be the top of your connecting girt that ties to the other post against the preexisting architecture.  From the bottom of the housing for the connecting girt, you need to come down a minimum of 100 mm (4") and place your through mortise for the horizontal brace beam.  As you get closer to making your final choices, I would recommend making final elevation drawings, a cut list and joint maps.  I would also avoid using a tape measure, make a story pole instead.  Good luck, let us know if you have more ideas/questions.  Can't wait to see photos of finished frame!  See drawing of Bent Elevation for and example below:

[Jim_Rogers]

One thing that worries me about your design is the risk of the post splitting as your tie is too close to the plate.

There should be at least 8" of post between the top of the tie and the bottom of the plate.

This is the reason why:



You should consider moving the plate up or the tie down until you have at least 8" between the two.

Just my thoughts on your design.

Jim Rogers

[Rooster]

Jim,

If the top of the rafter is secured to the existing building, will the rafter still push the top of the post over causing it to split?

Didn't you just post this photo in a different thread that will have the same issue? 
I've been kind of slow these past couple of days...I must have missed something! This is me, dazed and confused...


 

[Jay C. White Cloud]

Unless you get an "awful strange wind," as Ed Levin put it, you do not get out ward thrust loads in any assembly that has full ridge support, as you do in most Asian designs and porches, which is what you have in your case.

Now with that said, Jim's observation about your tie beam's proximity to the top is dead on, with or with out "out ward thrust."  You would be better to capture the top of the post with some form of three way tying joint, or moving the tie beam down 200 mm (8"), but that is only if you are not going to use the through tenon I suggested.  With the through tenon on your tie beam, you will capture the wood that could shear at the top of the post.

Regards,  jay

[piller]

Great thread here.

Jay:  how do you keep your mortiser stable when mortising at angles to your workpiece?   
   
Please post more info/examples about the horizontal through bracing, I'm not familiar with that and it seems interesting.  Do you have to use wedges which can tighten the joint to compensate for shrinkage?  I would think that the joint would need to be kept quite tight and or you would need a wide post to provide lateral bracing.

Also, you mentioned "Place rafters on top of rafter plates in notches and peg/lag from top".  Could you elaborate on this connection, would you notch both the rafter and the plate?

I'm not going to throw out my tape measure but I'll have to look into this story pole thing. 

Thanks,
Chip

[Jay C. White Cloud]

Hi Chip,

A few folks have ask me to expand that discussion and a few other things I've mentioned.  I don't want to do it on this post thread of Rbowie's detracting from his project discussion, so when I get a chance, I have several folks that have also asked about Asian timber framing, so maybe a new post topic is in order, "Asian Design Methodologies in Timber Framing."  Look for that.

Also, you mentioned "Place rafters on top of rafter plates in notches and peg/lag from top".  Could you elaborate on this connection, would you notch both the rafter and the plate?

This I should answer.  You just notch the rafter and ridge plates.  It does not have to be very deep, maybe 30 to 40 mm (1.25" to 1.5") in this application, for this design.

Regards,   jay

[Jim_Rogers]

Jim,

If the top of the rafter is secured to the existing building, will the rafter still push the top of the post over causing it to split?

Well, the correct answer to this question is, it depends. It depends on a lot of things. First of all his location is Calgary, Alberta, Canada, and I don't know the snow load there, but if the snow load is a lot, maybe.

Next, he shows the rafters attached by two different methods and I'm not sure if the screws will take the snow load.

Also, he shows this lean-to roof lower than the garage roof. We don't know the make up of this other garage roof. Is it asphalt shingles or metal? If it's a metal roof, or even if it's an asphalt shingle roof; we have to consider snow sliding off this upper roof onto this roof. That load could be a lot when that happens. And it will happen all of a sudden.

And, we don't know if this lean-to is going to be open or enclosed. If it is open then it maybe loaded with some wind load should one of Ed Levin's "strange winds" occur.

Didn't you just post this photo in a different thread that will have the same issue?

I'm not sure about another thread with this photo in it as I just uploaded it for the tie beam question.

I hope this has helped you to understand that we need to consider everything very closely.

Jim Rogers

[Jay C. White Cloud]

Hey Rbowie,

Jim's recent post responding to Rooster, is one you should read several times and ...really...consider all the factors.  These are exactly the kind of, "what if's," Ed was taking to me about.  You really have to take a step back and consider,   potential loads.  You have a small project and it probably isn't going to be subjected to some of the events I have running through my "noodle," but Jim's points are really important to always consider on any design.

Always take a, "what if this happens," approach to a design, then consider if you can live with the outcome.  We have all over built things at times but "under building," something can be disastrous; not that you small project is anywhere near that.  You would probably be fine if you had just built it the way you originally drew it, but if one of the cases that Jim brought up did happen, you would most likely loose the frame because the ledger and rafter connections is your weak spot.  That's why I liked your option #2 free standing design with some "tweaking."

Regards,  jay

[rbowie]

Jim, the structure will be located in a zone with snow loads of 35.5psf.  And you are bang on that I need to factor in the snow from the existing garage roof.  It's asphalt shingles and an approximate 6:12 pitch.  I will essentially double the snow load on the lean to.

Further, the lean-to will be open and is on the west side of the house which will leave it exposed to the prevailing winds out of the NW.  Also need to factor in uplift on the underside of the roof due to wind.

For the rafters, I will follow Jay's advice and notch them into the top of the rafter plates and secure with lag screws.  I'm also considering putting a bird's mouth at the lower rafter plate to eliminate the need for the lag screws up top to be in any significant shear.

A light bulb also went off about the bent that is up against the existing architecture.  It didn't have any bracing.  I think I will put the same bracing in both bents so that I'm not expecting the garage wall to take the snow load at all. 

I will do up a fresh set of sketches and post them to reflect this. 

Last thing is that I will definitely create a story post for this project as I will have 4 virtually identical posts.

Richard

[timberwrestler]

I agree with Jim and Jay's earlier comment about moving the tie down a little.

The only other thing I'd do is move the upper garage wall ledger down, and have the rafter sit on top of it.  It's easier to cut, and is much better for transferring the load.

[rbowie]

Alright, I've updated my sketches to reflect all the fantastic feedback.

I've decided to use the through mortise with wedges to capture the top of the post in order to prevent the post from cracking.  I've also moved the tie beam down 4" from the top of the post.

I've moved the rafters so that they are on top of the rafter plate rather than in the side.   

  

  

 

[Jay C. White Cloud]

Hey Rbowie, 

Looks real good. Consider the following:

You do not need to wedge and peg through tenon.

One wedge per tenon will be fine for this frame, (unless you like the look of two.)

There needs to be a least 100 mm or better 150 mm of wood past the wedge mortise on the tenon, (4"-6")

You should extend the rafter tails to make the drip line of the structure come past the through tenons at least 300 mm (12".)

Other than that, It looks like your ready to make a story pole, some templates and start cutting.

Regards,  jay

[rbowie]

The build has begin.  I've got a stack of pine in the garage and I've started making wood chips.

I'm trying to be as methodical as possible to begin with.  The first through tenon has been a challenge in getting it dead flat due to the length.  It will be 11" long as I'm using 6" timbers.  I am using a 1" housing and since it will be a wedged through tenon I have allowed for 6" extension beyond the post. 

Is there any tips to help in getting the reference side of the tenon bang on?  Especially with the longer tenon.  The 5" tenon at the other end of the tie beam was far easier to cut.



  

  

  

  

 

[Jim_Rogers]

To insure your tenon is parallel with the layout side/reference side you can place your framing square over the end and set you combination square on the top of your framing square set to the thickness of the body of the framing square (2") and the offset of your tenon (1 1/2" or 2").

This will tell you if the tenon is parallel with the side of the timber:



 

If there is a gap between the framing square and the combination square then you haven't shaved off enough. If there is a gap between the bottom of the combination square's ruler then you have shaved off too much. Be careful next time.

To check the tenon is offset the correct amount at the shoulder use your framing square again. Like this:



Here you see the blade of the framing square is set on the tenon, and the try square is sliding across the timber's surface toward the square. If it hits the square then the tenon is not correct. If it slides over and there is gap you took too much off. It should slide over and touch the square as it does.

Always cut the offset side first. Then cut the tenon to thickness taking off the back side until the tenon is the right thickness. Use a caliper gage to check thickness.
Like this:



 

Set the caliper gage like this:



 

Hope that helps.

Jim Rogers

[Dave Shepard]

That rounded through tenon with two wedges makes it a Dutch leanto. :D

[Rooster]

We call it a "Leaning Dutchman"...."Georiënteerde Nederlander"

8)