How long for rafter boards?

[handirifle]

I am embarking on a final project to finish off our build. We started the house in Sept of '09 and moved into the house in March of 2010. I have done a number of other projects since then, but the final one is to build a pergola over the spa pad.

The pad is 18' x 18' poured and reinforced concrete. The 7x7 spa is already on it and running. When we poured the concrete I added the steel supports for the 6x6 pergola posts, into the pour.

The span is slightly less than 18' (say 17.5) and the design will be gable roofed with 3 pairs of open rafters spaced evenly, with standard rafters in between. All spaced 4.5' on center. The roof will be corrugated steel panels. Pitch will be 6-12, to match my house and workshop.

My questions is this. I am pretty sure I know how to lay out my rafters, but with a 9 ft span, and I'd like to leave about 18" overhang for a decretive cut, how long of a board should I start with for the rafter boards? I am thinking at least 12 ft but don't want to buy too short (for obvious reasons) or too long to keep costs of waste as low as possible.

Can I find the desired length by laying out the framing square for the 6-12 and multiplying that by 9 (for the span length) since I would repeat that layout 9 times for the template board? Then add the overhang? Would that work?

The design will be similar to this.

Ours will be painted, again to match the house and workshop which are nearby.

 

[handirifle]

Also, since the front and center spans will be the entire 18 ft (the rear has a post in the center) I was also thinking doubled 2x10's. for the bottom boards.

 

[CNC Dan]

(run x run) + (rise x rise)= (length x length)

So do the multiplication first, then the addition to get your answer then take the square root of that answer.

 

[schmism]

rafter lenght = 1/2span/COS (pitch angle above)

assuming an 18' span outside to outside
9/cos(26.5) = 10.05' now add in your overhang.

That will get you fairly close, but remember you have to take into account the width of your ridge beam, assuming your span is outside to outside (not inside to inside) depth of a birdsmouth will change your overhang dimension as well as weather or not you need to consider the fascia being included in your overhang dimension. (ie you want 2' overhang, is that with the fascia or not.

see here for allowable span for framing material. (says you can get away with a 2x6 SP #2 with light snow load for the above 10' span)
Maximum Span Calculator for Joists & Rafters

 

[handirifle]

WOW great stuff. No fascia, as it will be all open, and the ends trimmed in a decorative cut, to be determined. So, it looks like 12' lumber will do it (18" overhang, I think will do it). That's good cause I am sure $$$ go up when getting longer. Plus I can haul 12' stuff on my 8' trailer, with the tailgate down. Might have to prop them up over the front rail for weight dist, but that's ok.

Snow load here is light to non existent. Wind would be more of a factor than snow.

As for a ridge beam, I am KINDA thinking only a partial one. Let me explain. Like the one in the picture the front and back (of the building) the rafters will butt against each other. Their will be three joist/rafter sets like this. Front, center (9ft back) and back. In-between each of these will be a standard joist/rafter setup, so they will work out to be 4.5ft on center. So from front to back, will be, a rafter pair, single rafter, rafter pair, single rafter, rafter pair. Now, the front rafters of the front pair, will butt against each other. The BACK rafter of the front pair will have a beam, and that beam will run all the way to and through the back rafter of the back pair. The very back (or the facing rafter if looking at it from the back) will butt against each other once again. I want this so the ridge beam doesn't show. I know I could use another board to cover the front pair, but this seems like it will look nicer.

Hope that makes sense.

Another question, since I will be cutting a birds mouth at each rafter, and I will be running double joists in front, center and rear, do I need joists in the single rafter positions? They would span from the birds mouth to the rafter beam. All rafters will also be connected with the 2x4's running front to back (not sure what they are called) the ones that provide horizontal support for the roof.

I ask because leaving those joists off would add to the openness of the structure.

 

[handirifle]

Another design question.

I have 2 choices in the design, as I see it. I can make the joists long enough to sit on top of the horizontal header that will connect the posts, and then the rafters would not have a birds mouth cut. Or I can run the rafters in standard fashion with the birds mouth cut, and butt the joists to the header boards, but then I would have to use a joist hanger for support, right?

I think my preference visually, would be the longer joists sitting on top of the header. If I go that route, would I just toe nail the rafters into the header? Or does this require a different style joist hanger. Keep in mind, these headers will be sitting vertically not horizontally like on a house.

Your words of experience and wisdom are much appreciated here.

 

[ace10]

I've had good luck using this calculator:

Rafter Calculator - Animated Visual Rafter Calculator with Large Diagram - Imperial

 

[handirifle]

I've had good luck using this calculator:

Rafter Calculator - Animated Visual Rafter Calculator with Large Diagram - Imperial

That's a cool calculator. I like the visual part.

 

[BHD]

a couple of good ol boys went to the lumber years to buy some lumber for a shed they were building,

and they said to the clerk "we want some 4x2 and some 6x2s for a shed were wanting to build", and the clerk said you mean "2x4 and 2x6s do not?". and they figured for a few minutes and said "that would work just fine",

So the clerk said "and how long do you want them?", and they responded, " a long time were going to build a shed",


I would guess you want your rafters for a long time, as well, that is how long.

 

[ace10]

That's a cool calculator. I like the visual part.

Yup. I print out the page and keep it next to the cutting station. Keep me from messing up simple stuff.