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Author Topic: Need help designing roof over deck  (Read 1267 times)

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Offline HunterDan

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Need help designing roof over deck
« on: January 24, 2017, 08:30:04 pm »
New member here looking for some advice on a roof over my deck.  My plans are to extend my existing gable roof of my house out over my back deck.  I want to build an open gable design similar to this.

Here is a picture of the house.

The span from the two top plates is 24' and the roof is a 10/12 pitch. My plan is to tie in a ridge beam and two plate beams (12"x6") into the house that will extend 14' out over the deck and sit on 10"x10" posts.  What I need help with is figuring out what size rafters I will need, the spacing of the rafters, and also if the 12x6 beams will be large enough.  My original design was to use 4"x8" rafters.  My area has a snow load of 15psf.  I will be using t&g and asphalt shingles for roof.  All beams will be sawn from doug fir.  I have been reading this forum for a while but I am not having any luck finding the right calculators or figuring out how to use them.  Here are a few quick sketches I did to show my plan. 



Online Briankinley2004

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Re: Need help designing roof over deck
« Reply #1 on: January 24, 2017, 10:20:52 pm »
Is your existing roof timber frame?  If so you should be safe just matching it seeing the span looks the same. I'm not an engineer nor am I familiar with snow loads but with 14' I would think one center timber rafter at 7' would suffice if your decking is t &3G 2x and the rafter is properly sized

Offline HunterDan

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Re: Need help designing roof over deck
« Reply #2 on: January 25, 2017, 09:01:21 am »
Existing roof is timber frame.  That was my original plan but I think it would be easier to tie in 3 large beams to the house rather than 13.  Also 14' would be the total span of the center ridge beam from house to post.  The span for the rafters would be 12'.  Sorry for the picture quality I am trying to post this with my phone since I don't have internet on my cad machine.

Offline Jim_Rogers

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Re: Need help designing roof over deck
« Reply #3 on: January 25, 2017, 12:21:11 pm »
My first concern is shrinkage. That appears to be a log home unless it's just log siding.
I would not try and make the two roofs in the same plane. The porch roof should be self supporting with posts at the wall of the house. And usually at a different elevation level. like lower by a couple of inches or more.
When the new porch roof plates shrink when they dry out the whole deck roof is going to come down some. It may open up gaps at the location of the two roofs meeting.
Put the new roof lower and up against the wall and let the old roof overhang the new roof.
I would imagine that this is not something you wanted to hear, but that is my opinion of how to do it and have the best luck with everything.

Jim Rogers
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Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Offline HunterDan

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Re: Need help designing roof over deck
« Reply #4 on: January 25, 2017, 05:05:42 pm »
You're right that is not what I was wanting to hear.  The house is built with D style logs no siding.  I planned on notching out next to the existing roof plates and under the ridge beam fastening to each.  Anyway I could get around the shrinkage issue?  Could I go with a laminated ridge beam and roof plates and still use timber beams for rafters?  I'm trying to make this look as good as possible with as much exposed timber framing as possible and keep the same roof line. 

Offline Jim_Rogers

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Re: Need help designing roof over deck
« Reply #5 on: January 25, 2017, 06:18:34 pm »
Anyway I could get around the shrinkage issue?
If you do what you're planning on doing then you're adding weight to the logs that support the house roof. This will make them settle more as they continue to dry. The porch roof supported by posts are not going to settle that much. Figure 1/8" in 12" of plate height. I don't see how you're going to get around that.

Quote
Could I go with a laminated ridge beam and roof plates and still use timber beams for rafters?

You can do whatever you want but you're house may continue to settle and the porch will not settle much. Laminated timbers most likely won't shrink much at all.

Quote
  I'm trying to make this look as good as possible with as much exposed timber framing as possible and keep the same roof line.

I do understand that, but there are (or could be) issues you're going to have to deal with in order to keep the same roof line. If you don't care if the gaps open up then build it the way you have designed. But be aware that the may open up and if it does then there could be times where rain water runs in. If that happens then you may have some rot issues. It may not but it could.

It's up to you.

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

Online Briankinley2004

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Re: Need help designing roof over deck
« Reply #6 on: January 25, 2017, 07:46:56 pm »
If your not concerned about the appearance of them on he roof there are roof expansion joints you can use. I have used them on school additions but they were not log structures. The architects typically put them in floor and wall too along the point of tie in

Online Don P

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Re: Need help designing roof over deck
« Reply #7 on: January 26, 2017, 08:07:51 am »
Remember shrinkage and settlement are two different things. The wood will shrink, the building may or may not settle that amount. In handscribed log walls there is another thing causing settlement, the cope spreads and the edges of the cope compress. That is not the case with D logs, the bearing is wider, essentially flat on flat. If a log checks, the wood has indeed shrunk but the overall height of the log may not change, the shrinkage dimensional change is largely in the check. There are a number of construction details that can prevent a log wall from settling from log to log fasteners, solid window and door bucks to the caulk and nails in the trims.

Is the gable wall solid log all the way to the peak or is it the more typical 8' of solid logs and log siding in the gable? If the latter there is no appreciable settlement above the 8' level. Next, how old is it? if more than a few year old it has done the majority of shrinking and then whatever settling it will do. Look around the patio door trim. How much has the wall dropped in relation to the trim?

I've done porches similar to this on log home kits with no callbacks. Those homes went through initial drying and settlement after the porch was built, and yet it has worked without putting in post adjusters or anything special. This house has more than likely already dropped about all it is going to.

That said, trying to match an existing roof plane rarely produces a sweet flat plane. I would do something though, the splash off that deck is going to do in the lower courses of logs on that house.

Offline HunterDan

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Re: Need help designing roof over deck
« Reply #8 on: January 26, 2017, 10:59:29 am »
Thanks for the response Don.  The south exposure of this wall and openness to rain and splash back are the main reasons I am doing this.  The home was built in 1999 and the logs are stacked all the way to the peak.  Here is a picture from the inside at the peak.

The purlins are roughly 7x5.5 and from what I can tell are notched out 1-2" as they pass through the wall and sit on top of the logs. 

I have had others tell me the same thing about matching the existing roof plane.  Would you recommend stepping it down a few inches from the existing?  It would be nice to keep the same plane but I would rather do this than have it noticeably not line up. 

I would be interested in hearing more about the porches you have done and any other recommendations you might have for mine. 

Online Don P

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Re: Need help designing roof over deck
« Reply #9 on: January 26, 2017, 10:14:57 pm »
I've had a couple of companies do that full height gable. Here's my take on settlement in that. There is a continuous bottom chord at the base of that roof triangle that establishes a fixed length of that side, the length of the logs which should not change. The length of the roof decking cannot change. I'm seeing a truss that cannot drop. In this case I believe the logs can shrink but they cannot settle. They may gap. I made the conscious decision to treat them that way on one. The gable was tall, an 18/12 and I really didn't want to allow it to settle. The other 3 gable ends in the t shaped house were framed construction. The logs were dry enough that I wasn't worried about much if any gapping. I ran a ledger under the purlins and attached it to each log, locking the triangle firmly. I'm of the mind that the gable is not going to drop and if the logs were going to settle, they have done so. I'd be comfortable saying you have a wall with a fixed height now.

I would be more comfortable stepping the new roof down. I can point out the transition whenever I've tried to add on in plane.

Offline alanh

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Re: Need help designing roof over deck
« Reply #10 on: January 27, 2017, 02:23:47 pm »
 

 
 

 
I just did this one, obviously not the same situation with matching roof lines but similiar design to what your talking. I used 6x6 for the truss and purlins, 6x10 for the ridge,  8x8 for the posts, tongue and v-groove pine is a full 4/4. span is just under 14` as the total length with overhang is 16`. As far as figuring the load...I copied the design from a couple of the log home kits we`ve built over the years. Your mileage may vary.

Offline HunterDan

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Re: Need help designing roof over deck
« Reply #11 on: January 27, 2017, 04:53:15 pm »
That looks very similar to my house design.  The more I think about it I think it might be simpler to just copy my existing design and extend the purlins out to a truss.  There is a truss in the center of my house that I could just duplicate.  I think all purlins are roughly 5.5x7 and the truss beams are close to that.  Here is a picture of the truss inside the house. 




The span between the truss and the wall is about 13' and my porch span is just under 14' so it should all be very close.  There is 7" from the top of the existing purlins to the shingles so if I kept the same plane on the purlins and laid t&g on top I could step down my roof 5+ inches which I think would look ok.  My only headache is how to join the new purlins to the existing.  The existing purlins are notched out sitting on the gable logs and overhang 18".  I will try to measure the size of the notch tonight but would it be possible to join them with two metal plates?

Online Don P

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Re: Need help designing roof over deck
« Reply #12 on: January 27, 2017, 08:17:34 pm »
Would you be opposed to bolting a 4x8 to the exterior tight up under the existing purlins and setting the new ones on the ledger alongside the existing ones and attaching them side by side?

Offline HunterDan

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Re: Need help designing roof over deck
« Reply #13 on: January 27, 2017, 11:05:40 pm »
I like the idea of a ledger.  I actually thought about that only cutting the existing purlins off flush with the house.  Leaving them there would give me something nice and solid to fasten the new ones to. 

Offline alanh

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Re: Need help designing roof over deck
« Reply #14 on: January 28, 2017, 10:54:41 am »
yep, pretty much the exact design I copied and built  from one of our cabins, I didnt want to deal with the plates perhaps rusting outside so I notched and lagged, not smart enough for more complicated joinery... do you have enough clearance over the corners of that window for a ledger and purlins?

Offline alanh

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Re: Need help designing roof over deck
« Reply #15 on: January 28, 2017, 10:55:53 am »
never mind the window comment, just read the post better,

Offline HunterDan

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Re: Need help designing roof over deck
« Reply #16 on: January 28, 2017, 11:12:35 am »
yep, pretty much the exact design I copied and built  from one of our cabins, I didnt want to deal with the plates perhaps rusting outside so I notched and lagged, not smart enough for more complicated joinery... do you have enough clearance over the corners of that window for a ledger and purlins?

Yes just measured from the top corners of the window and I have about 10" so a 4x8 ledger should work nicely.  I think I will still stick to three support posts instead of building a truss to span 24'.  My sawmill can't cut 24' and I don't want to have to deal with joining the two in the center or using steel plates either. 

Offline alanh

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Re: Need help designing roof over deck
« Reply #17 on: January 28, 2017, 01:17:17 pm »
That should work, with either design I would imagine you would want to support the ridge some how,  so the post against the house will run up centered on the window?

Offline HunterDan

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Re: Need help designing roof over deck
« Reply #18 on: January 28, 2017, 02:50:08 pm »
No posts will be against the house.  I will cut the existing ridge overhang off flush with the house.  I plan to support the new ridge beam by cutting a beam to sit between the top two existing purlins below the ridge with a short block in the center for the ridge to sit on like this.

Offline HunterDan

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Re: Need help designing roof over deck
« Reply #19 on: April 16, 2017, 11:10:57 pm »
Well I've been very busy the past few months and just wanted to show the progress I've made on my timber frame roof over my deck and thank those on here for their help.