# need help on rafter figuring with calculator



## nailamatic (Nov 10, 2009)

The current project I'm doing has a 12 on 12 pitch in the front. The back is on a 4 on 12. The front wall is 9 ft plate height plus 17 1/8 inch for web truss and plywood. The back wall is 8 ft higher than the front. Can someone tell me how to figure out my rafter lengths on the construction master calculator? Also, could you tell me how and why it works? Is there anywhere besides the field that I can learn from different types roof examples? A website? Anything? Also, I want to learn how to figure an entire roof from the calculator. I've seen very few that can do it!! I would love to be able to cut all my hips/jacks/valley/broken hips/etc... from a calculator. I still figure my roofs somewhat the old school way and I think I could save myself some steps if I get some more insight. Thank You for any help! It is much appreciated!!


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## dave_dj1 (Mar 16, 2010)

it's called the Pythagorean theory.
you should draw a cross section , sort of to scale, to give you an idea of what you are trying to achieve. Handy Andy says "always try and picture your work". 
You need to solve for where the two roofs meet, the ridge will be off center I think, if it's centered then it makes it easier. On the construction master, start with a given, the distance from the outside of wall to ridge center, this will be the run, then press in 12" pitch and this will give you the diagonal length of the rafter, the height will be the rise. Just hit the rise button after you are done. If you keep hitting the diag. button it will show you the angle in degrees. There is a cheat sheet that comes with the CM. Make yourself a simple one to start with to give you some practice, a 24' wide building, 6/12 pitch. Half the building, 12' will be the run, 6" pitch, then the rise and diag. will appear.
Hope this helps.
Forgot to add that to get your jacks, you will have to tell the CM if they are on 16" ctrs or 24" ctrs. Hips and valleys are just normal rafters that have a longer distance to center so to speak, think if roofs as triangles. whether they be plumb or laid over (as in a valley or a hip) There are many books on rafters and roofs, swanson speed squares used to come with a little book. 
I was at a lumber yard recently and the salesman was trying to get the height of the center of a truss with a 5/12 pitch, the truss was 20' wide, He pulled out an old book that showed this info, I had already done the math in my head (half the width, 10' times 5" equals 50" for the rise.) I hope some of this makes sense.


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## JustaFramer (Jan 21, 2005)

Hip/Val's aren't normal they run 16.97" instead of 12". :shutup:


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## CanningCustom (Nov 4, 2007)

that was a pretty darn good explanation as far as im concerned, i can't explain it to my guys in a way that is simple enough for them to grasp. As far as cutting a standard roof, with hips,valleys,broken hips (same pitch) with a construction master is fairly simple you just have to understand the the centers and where to pull spans, and their remainders. If that makes any sense.


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## Warren (Feb 19, 2005)

Don't forget to figure for the seat! Best bet for someone who doesn't do this all the time is to do it old school. Step it off with a framing square. There is a bit of a learning curve with the calculator, and I am sure if you don't use it often you will probably not retain the info. I have always done it old school either because I was too cheap to buy the calc originally, or because I just love doing all the math. I have two construction masters that were given to me at various times and I have never used either one.

PS. Hope the 17.97 was a typo, Framer. It is actually 16.97:thumbsup:


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## JustaFramer (Jan 21, 2005)

Warren said:


> Don't forget to figure for the seat! Best bet for someone who doesn't do this all the time is to do it old school. Step it off with a framing square. There is a bit of a learning curve with the calculator, and I am sure if you don't use it often you will probably not retain the info. I have always done it old school either because I was too cheap to buy the calc originally, or because I just love doing all the math. I have two construction masters that were given to me at various times and I have never used either one.
> 
> PS. Hope the 17.97 was a typo, Framer. It is actually 16.97:thumbsup:


Good catch on the H.A.P. and why my hip/val rafters are alway f'd up. :laughing: It's been awhile since I last sticked a roof in especially a hip/val.


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## Kent Whitten (Mar 24, 2006)

JustaFramer said:


> Hip/Val's aren't normal they run 17.97" instead of 12". :shutup:


I think you meant 16.97". We forgive you :laughing:


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## nailamatic (Nov 10, 2009)

Yeah that helps. I can figure simple roofs. But the roofs with different wall heights throw me for a loop. 9 ft wall height in the back of house and 13 ft in the front get me.


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## Warren (Feb 19, 2005)

nailamatic said:


> Yeah that helps. I can figure simple roofs. But the roofs with different wall heights throw me for a loop. 9 ft wall height in the back of house and 13 ft in the front get me.


Think of it like this. Figure on a rafter sitting on that lowest wall. When that rafter travels far enough to reach the height of the other wall, STOP! The ridge board will be center from this point if the pitches are the same. 

Example. 12/12 pitch,20 span. 8 ft wall ht and 12 ft wall ht. The rafter on the 8' wall must travel 4' to catch up with the start of the rafter on the 12' wall. The c/l will be 12' in from the 8' wall, and 8' from the 12' wall.


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## loneframer (Feb 13, 2009)

When dealing with uneven wall heights, I usually do the math to bring things up to a level playing field first. In the case of a 12 vs. 4, from a level plane, the ridge will be offset 1/4 of the span. IOW, if the span is 120", the ridge would be set 30" from the plateline of the 12/12 side.

I know it sounds like mumbo jumbo, but if you draw it out, it should make sense.

The trick is in making adjustments for different HAPs, which are sure to differ between 4/12 and 12/12, which will change the location of the ridge.

I'd guess that something won't jive with your plans. Either the wall heights, or the exact ?/12.


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## JustaFramer (Jan 21, 2005)

Is the ridge center of the building?


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## loneframer (Feb 13, 2009)

JustaFramer said:


> Is the ridge center of the building?


 Good question. The centerline of the ridge could end up dictating the pitches of the roofs, wall heights, or both.

Many of my framing jobs had to comply with height restrictions, so the ridge height always dictated the other variables.


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## JustaFramer (Jan 21, 2005)

loneframer said:


> Good question. The centerline of the ridge could end up dictating the pitches of the roofs, wall heights, or both.
> 
> Many of my framing jobs had to comply with height restrictions, so the ridge height always dictated the other variables.



Exactly the last frame job I was on was like that. The front pitch was like 2/12 and the back was 9/12 or something like that. Off centered ridge wall heights different. Plus there was a hip on one side. Big cluster ****. The job was pieced out to some mexicans. The ones that are really good at cookie cutter tracts but suck at real work. :laughing:


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## festerized (May 19, 2007)

Have any runs to go with those #’s
I could give you exact #’s if you could send me more info. I would need to know wall heights and runs. Does the plan show an offset ridge or give ridge height.
I need run, rise & pitch of each roof.
Draw a quick sketch and post, let’s make it a contest!
I drew up a quick sketch showing some of the math. Be careful with the plumb cut heights from the 12P & the 4p.


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## parkers5150 (Dec 5, 2008)

it doesn't matter if the ridge is in the center of the building. Pythagorean theorem is based on a right triangle. just treat each side of the roof as an individual project with its own rise and run.


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## festerized (May 19, 2007)

Opps also need to know plumb cut or standing height you wanted.
I need wall thickness, 2x4 or 2x6. Effects rise of plumb cut
I need rafter size,2x8 , 2x10, 2x12
I like to go full bearing 4”. I sheath the walls first
LOL I have a cheat sheet I made up years ago for common plumb cut heights.
*12p with a 2x4 wall:*
2x8 rafters, 7” plumb
2x10 rafters, 9 ¾”” plumb
*12p with a 2x6 wall:*
2x8 rafters, 4 ¾” plumb
2x10 rafters, 7 ¾”” plumb
I don’t have any for 4P roofs


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## loneframer (Feb 13, 2009)

parkers5150 said:


> it doesn't matter if the ridge is in the center of the building. Pythagorean theorem is based on a right triangle. just treat each side of the roof as an individual project with its own rise and run.


 If the ridge is centered over an architectural feature, such as a window or a cantilever, it matters.

I do agree that the ridge location has no bearing on the ability to calculate the roof. I'm just saying that many things may come into play here. Often times the numbers called out on the plan are merely guidelines and the actual numbers could end up being fractions, or inches off.:thumbsup:


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## parkers5150 (Dec 5, 2008)

i agree with you completely Lone. I've seen your work it speaks for itself:thumbsup:. i was trying to approach his question about calculating in general. I find it interesting you mention HAP as a variable whereas i was shown to whenever possible keep it as a nominal constant so that you you have to rip the eave blocks. (diff. strategies:notworthy) P.S. east coast/west coast:clap:


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## JustaFramer (Jan 21, 2005)

Pssst that's Lone not Framerman. :shifty:


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## parkers5150 (Dec 5, 2008)

oops loong day, four beers


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## nailamatic (Nov 10, 2009)

festerized thanks a milion for your help brotha!!


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## jimAKAblue (Aug 15, 2010)

loneframer said:


> I like to keep them constant as well, but using 2x10 for rafters on both a 12/12 and 4/12 will make that difficult to do without deeply overcutting the 12/12 seat cut. I try to keep the BM flush to the inside of the wall plate. That's where adjusting wall height to make up for the difference in HAP comes into play.
> 
> Making everything work at the top of the rafter will create new issues with the theoretical points at the bottom of the roof plane.
> 
> These are the things that make the job interesting for me, but are often overlooked by everyone else.


Regional differences sure do come into play with this topic. I never gave any weight to making the heels the same on adjoining walls. We always worked backwards from the bottom of the frieze, out to the top of the fascia and then figured out what the heel was based on the projection and pitch. For the last few decades, I was almost always working with unequal pitches and equal overhangs and therefore had different heels. Of course, the varying plate heights would also come into play somewhere in the equation. 

Some of the reasons are interesting to contemplate. For instance, around here (Austin), open eave systems are common. We also have a lot of low sloped equal pitched single story "ranch" homes. That style home would look funny if the hip did not land directly over the exterior corner. Back in MI, we always had closed systems. Often the side slopes would be steeper than the front to back slopes. Locating the hips directly over the exterior corner would be meaningless and was not done. We NEVER gave any thoughts to making the heels equal.


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## festerized (May 19, 2007)

A beginner carpenter needs to know the basic fundamentals. Rise, run and diag. framing squares are great for teaching the rise & run concept, but I don’t recommend stepping a final rafter with it. I showed a few of guys both methods so they could see how & why rise & run works. More importantly show them that if a framing square if not properly applied could end them in trouble.
 Think about it, on a 13’9” run you have 14 time you could be a 1/16” off, that’s 7/8” 
Before i fully converted to a calculator I always stepped with a framing square then checked the diag. with a calculator. I too was skeptical. Ounce it “click” I never look back.


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## festerized (May 19, 2007)

*Full bearing rafters*

Back in the day full bearing wasn’t necessary, code states you must have 2” bearing. 
Now with hurricane clips and cathedral ceilings it’s a must to have full bearing.
I should rewind that a bit. For years we installed hurricane clips on the inside but in the last 2 years architects changed it to the outside, TY to Simpson.
Now we install hurricane clips before installing the level sub fascia.
Understanding heel cut & plumb cuts play an important role in more complex roofs like bastard hips, where you don’t have the luxury to change the wall heights to compensate for plumb cut height differences


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## jimAKAblue (Aug 15, 2010)

festerized said:


> A beginner carpenter needs to know the basic fundamentals. Rise, run and diag. framing squares are great for teaching the rise & run concept, but I don’t recommend stepping a final rafter with it. I showed a few of guys both methods so they could see how & why rise & run works. More importantly show them that if a framing square if not properly applied could end them in trouble.
> Think about it, on a 13’9” run you have 14 time you could be a 1/16” off, that’s 7/8”
> Before i fully converted to a calculator I always stepped with a framing square then checked the diag. with a calculator. I too was skeptical. Ounce it “click” I never look back.


One of the skills that novice carpenters were taught was to minimize the gain when stepping off rafters. 

At this point in my career, I don't think I "could be" 1/16" off 14 times even with my bad eyes. Perhaps I was 1/16" off when I first started fumbling with my framing square....I also didn't run my saw very straight and I couldn't swing my hammer like "Polack Joe" who was the fastest hammer on the crew. But then, amazingly, I acquired and honed my skills. 

We used to lay out all our sill plates with a framing square. If you use a framing square on a 50 long sill plate and gain 1/16" ....you might cause your self some trouble. So, what did we do? We learned to use the tools in a way that delivered accurate results. It's an acquired skill...and one that is relatively easy. 

But...lets talk about that rafter that might in fact be 7/8" long. In 99.99% of the cases, it really wouldn't matter would it? You'd make the pattern and cut all the rest of the rafters exactly like it. You'd stand them up....nail them up... lay the plywood and everyone would be happy and no one would know that you screwed up. 

That's my take on using the framing square. For the way most people frame and then sheath...it doesn't matter. For me, since I always finished everything laying down, I needed to know that every dimension was absolute. Therefore, I was a slave to my simple calculator and used basic geometry to deliver satisfactory results when used in conjunction with the framing square.


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## festerized (May 19, 2007)

That 7/8” is a lot by my standards. 7/8” will throw of your fascia and create a cap at the ridge connection.
This is why you see gaps in the ridge to rafter connect and I have seen many framing contractors fail for not having full bearing.
When you get to the more complex roof likes bastard hips and un-even roofs like this 4p & 12P offset roof it’s important to be within a 1/8”. 
I’m not sure how you guys figure out your bastard roofs but I take the rise from the lower pitch (usually the main roof) to figure the run of the higher pitch. So if your numbers are off it will affect your rafters in several ways


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## loneframer (Feb 13, 2009)

Here are a few examples of where heal cut variations come into play.

I often have buildings that have a given ridge height for several different roof planes/pitches. They also require the fascias to line up at the same elevation with equal overhangs. The only way to accomplish this is to modify the seat cut of the rafters. Some cases require taller walls, or at least the addition of another plate to get good seat cut bearing.
\
The first and second pic are of a roof with about a 5/12 and a 12/12 octagon with equal overhangs and fascia heights. The third pic is of a roof with several different pitches with equal overhangs and fascia heights.


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## jimAKAblue (Aug 15, 2010)

festerized said:


> That 7/8” is a lot by my standards. 7/8” will throw of your fascia and create a cap at the ridge connection.
> This is why you see gaps in the ridge to rafter connect and I have seen many framing contractors fail for not having full bearing.
> When you get to the more complex roof likes bastard hips and un-even roofs like this 4p & 12P offset roof it’s important to be within a 1/8”.
> I’m not sure how you guys figure out your bastard roofs but I take the rise from the lower pitch (usually the main roof) to figure the run of the higher pitch. So if your numbers are off it will affect your rafters in several ways


7/8" is a lot by my standards too...but not if i'm leaning up rafters and they are all the same. 

How many times have I cut the rafters EXACT and they don't fit the ridge perfectly? Almost every time. Why? Because the ridge board is cupped or twisted. Yes...I selected a decent one.....but I didn't pull out my 22" jack plane and put a perfect edge on it. 

How many times has the seat cut not set perfectly....even though I measured everything perfect and cut it with all the skill I could muster? Almost every time.....why? because the plates aren't true even though they are as straight as I could find. 

My point is that a rough frame can absorb a set of rafters cut 7/8" long...but back in the good old days if a saw man cut a set of rafters that were 7/8" long...there'd be a lot of hollerin' and bickerin' all day LOL. The pattern might vary 1/4" by stepping it off and even that in my system could cause serious issues....that's why I used the calculator. 

On the other hand...if a jack is 7/8" off, no one would care. The fit will still fit tight because the framer will nail it tight up to the point where it intersects. It might cause the layout to vary a few degrees....but who hasn't lived with that on a roughed framed roof? 

Really, it's all relative. A 7/8" error on a 16' jack means absolutely nothing. a 7/8" error on the first 16" jack will make the thing look like a kindergardner put it in...if some other compromise isn't applied. 

Anyways, its a fun discussion. I'd love to pull out the old framing square and do a few roof calcs on it to see how close I'd get to the perfect number...just for the challenge. I'm just glad that I don't have to do it any more to make a buck.


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## loneframer (Feb 13, 2009)

In my book, 7/8 could be the difference of walking away, or ripping it apart.

As I stated many times before, Most of my frames had to be under or at 31'. This height is measured from crown of the road at the center of the side setbacks. When you need to stay under the max height, but still achieve a minimum pitch and give the HVAC crew enough room to get a unit in sideways in the attic, the tolerances are very tight. A rafter that is even 1/2" too long on a 3.5/12 with a 22' span, could spell disaster. I have seen more than a few cases where the ridge height was over and the framing crew had to flatten the top of the roof to rectify it. Looks like s#!t too.

In any event, I prefer to do my roof math at home, with coffee and climate control and save my time on site to make sawdust.:thumbsup:


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## JustaFramer (Jan 21, 2005)

Hey Riz,

What lumber rack system is that?


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## loneframer (Feb 13, 2009)

JustaFramer said:


> Hey Riz,
> 
> What lumber rack system is that?


 That's the TracRac system. Every component can be removed or repositioned as desired. The rails are attached with compression fittings into the stake pockets, so no holes in the bed rails. There's lots of cool gadgets available as well. I did a few upgrades since that pic. It's a great system for my needs.:thumbsup:

BTW, I have a set of bed rails for a 6.5' Ford late model bed if anyones interested locally.:whistling


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## festerized (May 19, 2007)

I have built a few houses on the island where they build 1” below max height. Just as lone has stated the builder would shoot an elevation off the crown of the street just before we started rafters. 
Then give us “working height” from the top of the 2nd floor plate to the top of the ridge.
Lost most of those pics when my PC crashed, but found this one


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## loneframer (Feb 13, 2009)

festerized said:


> I have built a few houses on the island where they build 1” below max height. Just as lone has stated the builder would shoot an elevation off the crown of the street just before we started rafters.
> Then give us “working height” from the top of the 2nd floor plate to the top of the ridge.
> Lost most of those pics when my PC crashed, but found this one


Exactly what I've been dealing with for over 20 years. Every house gets an "as built" survey, to verify top of ridge and make sure the setbacks were adhered to. I know a few builders who were forced to move sidewalls back and trim the joists back because they were built 2" too close to the property line on a 4' setback.


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## jimAKAblue (Aug 15, 2010)

Yall posted some nice houses but there's way too much siding for my tastes.

I'll vote for the house on the island as the best of the group but I'm a little dissappointed with the details on it at the upper rakes and eaves. Its out of proportion and boring. It almost looks like a cheap aluminum trimmed job...but maybe I need a closer view. 

Incidently, since I would frame and build all those walls on a table out in the yard, I'd need that elevation shot from the builder before I framed the first wall. I'd have every elevation height detailed and know the height of the fascia and ridge before the end of the first day. There wouldn't be any discussions about 7/8" variations in anything. 

But....send me out on a simple box ranch and toss half a bunk of rafters that are cut "long" and Ill be glad to stand them up without comment. 

There is a time and a place for perfectionism and geterdoneism and thankfully I was able to experience all of that in my undistinguished career.


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## festerized (May 19, 2007)

Jim this house was not picnic. Take a close look at the framing on the main house. The front wall (triple window) is tapered in two directions. The front wall had to be planed out well in advance so the floor beams, lower porch and upper deck would line up. 
This house also has a gambrel main roof,& 6 dormers all with the same ridge height.
For me the main house was just another frame.
The garage is framing art work. Lots of time planning out the fascia and double pitch roof.
I only had 3” of wiggle room with the window height. The window is just able a steel beam in a cathedral ceiling.
BTY I could and have figured the exact height of a building before framing the structure.
As stated in my previous post it only takes minutes to get a total rise of the building.


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## jimAKAblue (Aug 15, 2010)

I did notice something was going on with that triple window and deck. I thought I was seeing photo distortion. 

The garage actually looks very difficult. I'm not sure it's a good fit with the main house. You did a very nice job with it though.


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## Joe Carola (Jun 15, 2004)

nailamatic said:


> The current project I'm doing has a 12 on 12 pitch in the front. The back is on a 4 on 12. The front wall is 9 ft plate height plus 17 1/8 inch for web truss and plywood. The back wall is 8 ft higher than the front. Can someone tell me how to figure out my rafter lengths on the construction master calculator? Also, could you tell me how and why it works? Is there anywhere besides the field that I can learn from different types roof examples? A website? Anything? Also, I want to learn how to figure an entire roof from the calculator. I've seen very few that can do it!! I would love to be able to cut all my hips/jacks/valley/broken hips/etc... from a calculator. I still figure my roofs somewhat the old school way and I think I could save myself some steps if I get some more insight. Thank You for any help! It is much appreciated!!


What size rafters and ridge are you using? Are you nailing a single 2x4 plate on the deck for the 12/12 side?


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## festerized (May 19, 2007)

nailamatic said:


> The current project I'm doing has a 12 on 12 pitch in the front. The back is on a 4 on 12. The front wall is 9 ft plate height plus 17 1/8 inch for web truss and plywood. The back wall is 8 ft higher than the front. Can someone tell me how to figure out my rafter lengths on the construction master calculator? Also, could you tell me how and why it works? Is there anywhere besides the field that I can learn from different types roof examples? A website? Anything? Also, I want to learn how to figure an entire roof from the calculator. I've seen very few that can do it!! I would love to be able to cut all my hips/jacks/valley/broken hips/etc... from a calculator. I still figure my roofs somewhat the old school way and I think I could save myself some steps if I get some more insight. Thank You for any help! It is much appreciated!!


How did my calculations work for you?


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