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Author Topic: Sizing rafters for timber framed porch.  (Read 430 times)

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Offline Mainecoast

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Sizing rafters for timber framed porch.
« on: September 16, 2021, 11:45:37 AM »
Hi, I have a client who is requesting a timber framed porch with roof. I値l just keep this as simple as I can. Due to building restraints (window locations and foundation location) I知 thinking a 3/12 pitch is the steepest I can go. And even then I知 worried the rafter tails will hang in the view. So my over all run is 142 inches and my hight is 126. 

So if my quick math is correct the rafter minus the tail will be about 191.342...I live on the coast and we don稚 get much snow. The roof will most likely be metal standing seam. I知 trying to figure out what size rafter I値l need. I壇 like to keep them as shallow as possible to keep as much head room as I can. 

So Imagine a shed style roof coming off a house and landing on a single bent. I壇 like to keep the top beam on the bent as high as possible but even at 3/12 It will be about 7 to the top of the rafters..so less than 6 under the bent. 

Hopefully I致e explained this correctly but I知 wondering if this just can稚 happen given the restraints. 



Offline Don P

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Re: Sizing rafters for timber framed porch.
« Reply #1 on: September 16, 2021, 05:25:13 PM »
You really should have 7'+ under the carry beam, especially if anyone is walking down stairs under it. Can you step down onto the porch floor, lowering the floor accomplishes the same thing as raising the roof.

Is there a need for that width, narrower will help. Standing seam can go lower pitch than that, although I think I'd put Ice and Water on the whole thing first.

Can you articulate the roof in any way(s) to get out of the problem?

Can't do a thing with "not much snow", get a design snow load number.


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Offline Mainecoast

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Re: Sizing rafters for timber framed porch.
« Reply #2 on: September 16, 2021, 07:46:18 PM »
 I suppose I could lower the deck a bit but the foundation they poured is rather high so I壇 have to lower my floor joists and to do that I壇 probably need to reduce them on the ends and have them set down below the top plates. 

I figured 7 below the top plate/carrying beam would be ideal. The client had the foundation poured before I came onto the scene. So it痴 already set in place. I could do less pitch but that痴 going to look bizarre. I can usually come up with creative solutions but I知 starting to wonder if this one might not work out the way she was hoping. 


Maybe I can talk her into an open roof sort of like a pergola. 


Snow load based on a few searches comes out to be 50-60psf 

Offline Don P

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Re: Sizing rafters for timber framed porch.
« Reply #3 on: September 16, 2021, 09:04:55 PM »
I just checked that 191.342" rafter, it sounded long, that would be close to an 11/12 pitch
I come up with 142 x 1.0307(Line Length Ratio, secant, for 3/12)=146.3594 (3/8"=.375)

There's a million ways to skin the cat but LLR's are real handy to have in your head on a job. 1/cos of the pitch angle, work that out ahead of time and memorize it, then when reality isn't matching the picture on the box, whenever you have the horizontal measure, multiply it by the LLR to get the plumb to plumb rafter length. 

There's a calc in the toolbox doing exactly that here;
Rafter Length Calc (forestryforum.com)

If its poured you can bolt the floor to the concrete in order to lower and support it, Simpson Titan screws in the 1/2x6 range should be capable shooting from the hip, just an option, not trying to push that way over another.

Uh-huh :D, "some snow" to a Texan means he saw some snow one time, wind controls design at 20psf, around 100 mph. "Some snow" in Maine is 60psf, snow controls.

We can do a quickie on the rafter load, snow falls vertically, use the horizontal run, call it 12'. If the rafters are spaced 2' on center, then each rafter support a tributary area of 2' x 12', or 24 square feet. I'm conservative 60 lbs per square foot snow load + 10 psf dead load (roof material weight) = 24 square feet x 70 lbs per square foot total load=1680 lbs load on each rafter. 

Which brings another squirrel to mind. Some snow up north and decks, unroofed outdoor floors. Typically when you see deck or floor joist sizing tables if you look at the footnotes they are probably set up for 40 psf live load. If you live in the land of ice and snow the snow load can be significantly higher than the summertime party load the code was thinking about. If it doesn't get a roof those floor joists are seeing 70psf (60+10) rather than textbook 50psf(40LL+10DL).
The future is a foreign country, they will do things differently there - Simon Winchester

Offline Remle

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Re: Sizing rafters for timber framed porch.
« Reply #4 on: September 16, 2021, 10:48:40 PM »
Building on Don P's suggestion .." Can you articulate the roof in any way(s) to get out of the problem? "
My father would have scrapped the shed roof idea , he considered them an ugly eye sore . Instead , he would have used  a Cross Gable tied back into the existing roof , using the same slope as the roof and tying into at the same level of the ceiling in the existing house. Now the over hangs , rafter tails , new and old all line up as long as the soffit are the same width .
The gable would or could offer a timber frame vaulted ceiling as well in the new addition . Much more architecturally pleasing and structurally sounder than a shed roof has to offer .




Offline Mainecoast

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Re: Sizing rafters for timber framed porch.
« Reply #5 on: September 17, 2021, 06:00:40 AM »
-Hey Don,
I appreciate the time you spent going through my word problem! I will certainly get a grid on the LLR痴 I can see them being handy. I値l compare load charts today and see what I can come up with that might work. 

I grew up north of here by about 4.5 hours. Snow load up there is 100 psf.

-Remie, that plan just won稚 work in this situation. It痴 a three floor house with a gable roof but two valleys on one side. Maybe if it was a bit smaller house. I tried to attach a photo of the clients desired outcome but couldn稚 get it through. So long story short gable porch is a no go. 

After this I致e got another timber frame entry roof to build and that will be a gable but stand alone on the 3 story wall. 


Offline Mainecoast

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Re: Sizing rafters for timber framed porch.
« Reply #6 on: September 22, 2021, 12:08:54 PM »
So...I知 if I致e gone over the charts correctly. 

I could use ewp or white spruce. 
4x8s 2 oc..does that sound about right? 

My plan is a large ledger bolted into the beams of the house. And rafters set into the ledger an inch and lag bolts from the top into the ledger and into the house. They壇 land on a 7x7 carrying beam. And 7x7 posts. 

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Re: Sizing rafters for timber framed porch.
« Reply #7 on: September 22, 2021, 12:33:22 PM »
So I just had another thought. What if I added a purlin plate mid span. I壇 run two posts up from the deck to hold the purlin plate mid span. I壇 effectively cut the span in half and I could use 4x6 rafters. 

Offline Don P

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Re: Sizing rafters for timber framed porch.
« Reply #8 on: September 22, 2021, 06:50:30 PM »
The 4x8's work for me for rafters. 

I did not check your 7x7 carrying beam, it is supporting half the rafter so 1680 lbs/2 840 lbs/rafter. The rafters are spaced 2' on center  840/2'=420 lbs per foot bearing on the porch carry beam. Use the heavy timber calc for this since the timbers are larger than 5x5"

 If you go with the purlin it will be supporting half the rafter load along whatever its span is. It'll be picking up 420 lbs per foot of purlin span. On the plus side the carry beam and house ledger are then only supporting 1/4 of the rafter load each. I imagine the purlin dimensions will size you out of that option. 
The future is a foreign country, they will do things differently there - Simon Winchester

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Re: Sizing rafters for timber framed porch.
« Reply #9 on: September 22, 2021, 09:10:19 PM »
I cut cut a curve into the purlin so give more head room. Or just use a smaller beam. Red oak perhaps. I was thinking a carrying beam sawn on three sides live edge down would look really nice. 

Also using corbels in place of a few of the knee braces. 

I知 meeting with the client tomorrow to chat about options..covered roof vs a pergola style roof with a couple of shade cloths put up in the summer. 


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