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Understanding a frame

Started by Jim_Rogers, August 27, 2016, 09:26:41 AM

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Jim_Rogers

Here is the back story.

A client hired me to bring my sawmill to his yard and saw up his pine trees for his screen porch addition that he intended to build on the back of the house.
I did that.
I also sold him more timbers as his logs didn't make enough timbers for his entire frame design.
I drew up his frame from a set of plans he purchased on the web, that really didn't meet his needs.
After all the frame pieces were on hand, this client hosted many weekend workshops where I taught him, and several students, how to cut joints for his frame.

We were planning on having a workshop to help him erect his frame, but timing didn't work out and he had to use the crane company when they were available and he and the crane crew erected his frame without me being there.

Here is a shot of the ridge beam assembly being flown in by the crane on raising day:



 

Here is a shot of the erected frame taken by the client at the end of their day:



 

The next day, I arrived to view the frame. Here is a shot of the frame that I took:



 

Here is a picture of the concrete post support:



 

Here is a shot of the rafter tails overhanging the wall plate on the left side of the frame:



 

Here is a shot of the rafter tails on the right side of the frame:



  

and:



 

And here is another shot of the frame and future floor:



 

Ok, so did you see it?

The client told me that there was a minor problem with the rafters on the right hand side facing the frame.

You'll notice a gap between the rafter seat and the plate. Some of these gaps were as much as 3/8" and he didn't understand why this happened.

I wrote an email to him afterwards and suggested that during his work of adding the roof decking to take some careful measurements to try and figure out what or where we went wrong.

As a designer I would like to know so that I don't make the same mistake again, if possible.

Now the first reason why these rafter show the gap could be that the rafter seats called a crow's foot, were not cut correctly. But, I didn't want to say that first, as that may not be the case. He cut all the rafters based on my drawing and he assured me that the were all cut correctly. However, I never personally checked them to verify that.

So, what are some of the reasons why this happened?

One is that the ridge beam, which is a rough sawn timber, was actually wider than the requested 8". If the ridge is wider than 8" this would push the rafters out and create the gap.
I asked him to accurately measure the width of the ridge beam to see if it was wider than 8" at the point where each rafter attached.
Next the king posts and ridge beam assembly could be installed non plumb. If the ridge beam was leaning to the right to fit all the left hand rafters (possibly set first) then this would push the right hand rafters out and create the gap.
I asked him to verify if the king post or ridge beam post was truly plumb.

Next if the right hand wall was not plumb then the plate would be leaned in and this would create the gap. I asked him to check the posts for being plumb.

If the tie beams were cut too short (shoulder to shoulder) then the distance from plate to plate may be too short and this would create the gap. I asked him to accurately measure the plate to plate outside dimension to verify if the plate was in the correct spot.

Lastly I asked him to verify if the rafter seat was cut in the right location as if it wasn't then that would create the gap.
I noticed that one or two of the rafters were seated correctly at the side of the frame nearest the house.
And as looked like the gap grew a bit as you got further away from the house end of the frame.

Recently, I received an email from him telling me of all his investigations of the frame.

And this is what he said:
"I did measure the porch, and found that the corner post was off plumb- the bottom was out about 1/2" - 3/4", which sprung that corner of the plate inward."

The lesson learned here is that one small thing in one spot can effect other parts of the frame.

Be sure before you secure an post to it's supporting concrete that the post is plumb and online with the correct distance measurement. Measure the posts outside to outside and make sure that at the foot of the post is the same dimension as at the top of the post and at the point where the tie beam joins. All three dimensions should be the same if all your joints are cut correctly.

I hope this story has helped you to understand how important it is to have everything line up right.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

TimFromNB


Carpenter

I've had similar issues due to a foundation being slightly out.  Errors are always compounded too, they never seem to get better. 

PRC

So, when something like this happens, and as it looks like it's not realized until the end of the project....

1. Do you say, well, that side is gonna be 3/8" off.

2. Start disassembling until you get to the problem area, correct the issue, and re-assemble
I have no idea what I'm doing, but if I keep reading and making mistakes, I just might get somewhere.

Jim_Rogers

As you set the frame up you check each dimension and ensure that they are right before you permanently secure it to concrete.
Verifying that the post was plumb would have done it also.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

rasman57

Great info Jim!  Thanks for posting and adding pics.

Brad_bb

Those sonotube piers are too small(diameter).  Also, is there a larger diameter footer under them?  I'd want a pier at least twice the diameter of the post.  I'd want it wider than that being that it's sticking so far out of the ground.  And it needs to be reinforced with rebar.
Anything someone can design, I can sure figure out how to fix!
If I say it\\\\\\\'s going to take so long, multiply that by at least 3!

Larch

It can be difficult to establish good, square points in a sonotube situation when there are significant differences in elevation between post bottoms (tube heights).  In this situation I probably would have just assembled and leveled the frame on blocks a few inches above the piers to make sure it was correct, then poured the concrete and lowered the frame into the brackets.  Otherwise it's an elaborate game of plumb bobs and levels. 

On a separate but somewhat relevant note, I recently built a small home on an out-of-level foundation.  The floor itself was dead-nuts flat, but the corners weren't level.  I knew the foundation would eventually be leveled so I couldn't use a level on the job.  Anything leveled would become out-of-level after the foundation was put right.  So all we used were framing squares. 

The job turned out so well I'm tempted to never use a level on a job again if I can help it.  There is so much more play in the bubble of a six foot level than in the light gap of a framing square.  It's so easy to say "close enough" in a level situation but with a framing square you can't fool yourself if you're seeing light through the square.  If your lumber is reasonably straight a framing square will give you a better result than a level over the full job.  And it's easier. 

I used to think it crazy how the old timers were able to do everything so well with just a framing square.  Now I'm wondering how we can get it so close with our modern fixation on nice levels. 

Roger Nair

Larch, the method I learned for foundation construction was to layout the lot with offset batter boards and string lines from the BB and plumb to the ground and place lines in stages for the cut, the footings, the foundation and elevation, all from the batter boards.

Faced with an out of level foundation, I would correct the level before beginning framing.

I honestly do not understand what technique you are using with a framing square to replace leveling devices.  Can you explain further?
An optimist believes this is the best of all possible worlds, the pessimist fears that the optimist is correct.--James Branch Cabell

S.Hyland

Ditto on the batter boards. It's very important to have them as a consistent reference. Any time I have used sonotubes, once I establish my level off the  batter boards I will mark the outside of the tube and run a short drywall screw through the tube. This gives a handy reference for pour height. Then any excess form can be removed after curing. Footings under sonotubes seem to be frequently overlooked, but are a necessity. Whether it's a separate square footing or one of those integral cone shaped footings, it's critical for the stability and soil load bearing of the sonotube.
"It may be that when we no longer know which way to go that we have come to our real journey. The mind that is not baffled is not employed. The impeded stream is the one that sings."
― Wendell Berry

Larch

Quote from: Roger Nair on September 06, 2016, 09:22:51 PM
Larch, the method I learned for foundation construction was to layout the lot with offset batter boards and string lines from the BB and plumb to the ground and place lines in stages for the cut, the footings, the foundation and elevation, all from the batter boards.

Faced with an out of level foundation, I would correct the level before beginning framing.

I honestly do not understand what technique you are using with a framing square to replace leveling devices.  Can you explain further?

I only did what I did out of extenuating circumstances.  I never would have done it that way if I didn't have to.  Maybe I'm overselling the practice simply because I'm surprised it worked at all. 

I thought of this when I saw the pictures of the frame above: nothing beats the purity of the square.  Even an eighth of an inch off on your bracket adds up by the time you get to to the top of your post and when your tube heights are offset because of sloped ground you're trying to set a post base in wet concrete with a plumb bob hanging two feet off a batter board string line 12 feet from the house and you need to get it 100% perfect.  It doesn't look like there is much adjustability in that post base either.  They actually didn't do that bad, all things considered. 

But that's the kind of play the eye can create by trying to get that bubble exactly between the lines on a level or waiting for the plumb bob to stop swinging as you hold it next to a string.  It's all "close enough" when compared to seeing or not seeing a light gap in the blade of a framing square when squaring a wall against the floor.  Especially when repeated over and over again throughout an entire house.  That's the hypothesis anyway. 

Is anyone going to build a hotel or a 10,000 sq. ft. mansion without a level?  No.  But at least for smaller jobs I think some may be surprised what happens when the levels are left in the truck after a few basic baselines are reached.  I know I was and I'll definitely be testing the limits of this technique in the future. 

OldMasterTech

Jim, awaiting the "rest of the story". Did the client correct the error before proceeding?

Great post, a reminder to all skill levels of the importance of square, level & plumb.
All gave some ** Some gave all
Never forget

Roger Nair

Larch, I am uncertain about your difficulty to get accurate readings from spirit levels, other than wondering if you have verified your level or is there another problem?

As to plumb bobs and control lines, I attach the plumb bob to a gammon reel and clip the line from the gammon reel to the control line with a padded hackle pliers.  The result is the bob hangs from the control line and the gammon reel can adjust height of the bob.  This will eliminate all the jiggle and sway from a bob held by hand.  You can also steady a swinging bob by hand and get a quick reading.  This will get you definitive plumb readings.


An optimist believes this is the best of all possible worlds, the pessimist fears that the optimist is correct.--James Branch Cabell

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