Rafter Layout Question

[Joe Wood]

I was hoping someone could help me with a layout question with hip roofs.

I have a building that's 12' 4.5" wide, and 15' 4.25" long.

I'd like to space all the rafters evenly, but I can't figure out how to figure out where to put the first jack rafters, so that my layout each way is even. The rafters don't have to be at 16" or 2' centers .. just evenly spaced all around.

[Joe Carola]

Joe,

I know you said you wanted the same amount of rafters but would you mind if you had this?

148.5/8 = 18.56" (Gives you 7 rafters @ 18.56" O.C.)

184.25/10 = 18.42" (Gives you 9 Rafters @ 18.42" O.C.)

18.56 - 18.42 = .14" or 1/8" Difference.

So one side is 18-9/16" the other is 18-7/16". If you lay the jacks out to butt each other at 18-1/2" you'll never see the difference.

Is it that important to have the exact same amount of rafters on both sides? I assume this roof is all open.

[Erik R]

How did I know the rafter god was going to reply! ;0)

You would never see that 1/8 inch.

Hope your doin well Joe.

Take care,
Erik

[Chippy]

Joe,

Tell me, if you correctly centre the first common at half of the width how do you get equal spacing on the long side?

Chippy

[Richard Birch]

Joe,

I believe this is what you want.

Subtract the width of the building from the length to find the Ridge length. Divide the ridge by the number of spaces that you think will look nice. The result is the o.c. spacing for your rafter layout.

Divide the span by 2 for the centers of the King Commons.

15’4 ¼” – 12’ 4 ½” = 2, 11 ¾” (Ridge length)

2’ 11 ¾” / 2 (2 spaces at ridge) = 1’ 5 7/8” (Rafter spacing, o.c.)

The cut length of the ridge will be the ridge length plus the thickness of one rafter.
2’ 11 ¾” + 1 ½” = 3’ 1 ¼”
Or the length of your building minus twice the effective run.
15’ 4 ¼” - (2 x 6’ 1 ½”” = 12’ 3”) = 3’ 1 ¼” (SAA)

The effective run to calculate the rafter’s length will be the span minus the thickness of the ridge divided by 2. 1 ½” is typically the thickness of the entire roof’s framing components. (12’ 4 ½” – 1 ½” = 12’ 3”) / 2 = 6’ 1 1/2” (effective run)

Do not worry about the placement of the smallest jacks as they will shake hands perfectly at the hip and look just fine because they are evenly spaced with all the rafters. They do not need to be equally spaced from the corners of your building as the over hang will affect the visual aesthetics of the spacing too.

If you were to provide the Pitch of this roof I will post a cut list for you. Please include the level overhang dimension too if you want tail lengths.

[Richard Birch]

Joe W.

I have attached a .jpg of a cut list for the roof I detailed in the previous post. It is for a 6/12 pitch.

This spacing will work for you very well. It places the shortest jacks 1-3/4" to center from the corners of your building which ought to be close enough and look just fine.

[Chippy]

Joe Wood,

Is there a particular reason why you are trying to achieve equal rafter spacing on the length and width?

The only way you will achieve your objective is if the dimensions of the building apply to this formula (which yours doesn't).

Width of building, even multiple of rafter spacings plus the thickness of one rafter.

Length of building, width of building plus multiple of rafter spacings minus thickness of one rafter.

Richard,

Looking at Big Bend National Park


Chippy

[Joe Wood]

Well, I thought I wanted them evenly spaced for appearance, but that leaves a pretty big space between the first jack and the hip, as you can see with pics a & b.

I'm just starting to fool around with laying out a dutch hip. Experimenting with different slopes and where that open triangle area ends up.

Pics c & d are of the first (4:12) roof I modeled believe it or not. It's a rectangle, and I started the refter layout by where I placed the 2 outer main commons. I just choose 6', because it looked good. Then, I started my rafter layout from there, each way.

Guess what ? All the rafters on both sides are evenly spaced at 2'6 13/16 !

So when I started to lay-out this new 8:12 (pics a & b) with the odd lengths, I thought I'd start the layout with all the rafters being evenly spaced again. Actually, the OC spacing for this roof is only an 1/8" off between the sides and ends (beginners luck?), so I just cheated all the jacks to line up, and evenly spaced all the rest between.

So, I guess there's no Golden Rule about laying out so that everything's even. Seems like I'll just have to play around with each roof until things look right.

I do know now that I need to have my first jacks alot closer to the hip, like between 6" and a foot from the corner, so that's where I'll start my layout next time.

These roofs aren't being sheeted, so I don't need to stick with 16" or 24" layout either.

[Joe Wood]

Here are c & d

[Chippy]

Joe W,

It's not a case of a "golden rule" but one of mathmatical fact.

If you want the equality of rafter spacing on the end as well as the side of the roof then you have to follow the formula of my previous post.

Richard has put the "numbers" to your dimensions and has proved that you cannot get that equality.

The spacing of the rafters are always an issue because of the load carrying capacity (dead and live loads). If you increase either the spacing or the span of the rafter then the rafter size or stress grade of the member may becomes an issue. You just cannot just alter rafter lengths and spacing to make a roof frame "look good" or save cost.

Roof type or style may bring other issues. What you have depicted is what is incorrectly described as a Dutch Gable or correctly termed a hip gable roof or gambrel roof. (OK all you other readers I waiting for you comments and I'll take that on board presently if you wish).

If you are intending to construct a combination hip gable roof, (with the hip at the wall plate and the gable at the ridge) the style of architecture you are trying to emulate becomes an issue. There is the "western style" or the "eastern" style. The eastern style has generally a steeper roof pitch than the western style and the size of the gable end changes. The word "Balinese" roof or "Indonesian" roof has started to be used by the writers of "glossy Home magazines" as a feel good term and not a correct architectual term.

There is not fixed proportions for the span of the gable but it is usually between on third and one half of the span of the main roof.

As an aside, the opposite to a hip/gable roof is a Jerkin Head roof where the end of the roof starts as a gable at the eaves and finishes as a hip at the ridge. gable/hip roof.

Chippy

[HenryP]

Joe Wood:

I'm curious if you are selling a lot of these structures?

What species and what is the cost materials/labor breakdown for the completed project. SF sale price is OK too.

[Tim Uhler]

Joe,

Please post pics of the Dutch hip when you are done. Those sketch drawings look good and I would really like to see the finished product if/when you build it.

[Richard Birch]

Joe Wood,

All your pretty pictures show a square pavilion. The dimensions you gave make it rectangular. So your pics cannot be applied. The other posters are trying to find a common denominator for the rectangle, and that’s fine, but they won’t work with out including the ridge length as a factor. It is the main factor as I stated in my previous post.

I have played around with your project as an academic exercise, just for fun. If you don’t believe what I have to say or do not take my advice it is no skin loss to me. It’s your pavilion. The odd dimensions are throwing it into new uncharted waters though. Sort of.

I maintain my assertion that the only rafter spacing you can use correctly is 1’ 5-7/8” o.c. (half the ridge’s length) or a multiple of it. (2x) or 2’ 11-3/4” (full ridge length). That’s it. No other spacings are correct for equality with the given dimensions.

This spacing is based on the difference between the length and width of your building. The difference is the unadjusted length (2’ 11-3/4”) of the ridge, real or imaginary, depending on how you configure the design of your roof. Full hip, French hip, Dutch hip, Whatever hip.

I did a few plan view layouts,
pic 2 is for a full hip
pic 3 is a Dutch hip with the spacing offset by one half
pic 4 is a Dutch hip with no offset on the spacing.

If I were framing this pavilion, pics 2 and 4 are my favorites because the last jack has a more pleasing placement. All pics show an 18” overhang.

Like I said, just for fun.

[Richard Birch]

You can only upload 2 pics per post here. What's with that?

Here's the other one.

[HenryP]

Joe:

Ooops!

I guess after you get the bill from Richard Birch...the SF price will go up a bit (smile).