Methods for Building deep roof rafters
Hey all,
I’m in the very early stages of design on our pretty good house (CZ5 – central MA) at the moment and am trying to get a better idea of roof construction that we might use. Forgive me if my question is confusing; I’m not sure how to draw or properly explain my question, but I’ll try.
We’re aiming for a farmhouse look with historical elements and have an architect / history buff friend helping with the initial design. Our friend is very familiar with tradition building methods, but is interested in combining original new england style with modern energy efficiency practices. We’re looking at something like a 1500SF cape style house with maybe a dormer or two for character, but we’re still evaluating a number of styles for aesthetics, ease / cost of construction and the ability to cheaply insulate to high levels.
My initial thoughts were to go with 24″ parallel chord trusses with an attic space over the bedrooms. The trusses would be filled with cellulose and the attic space insulated with ~20″+ cellulose, with gable vents installed to allow for ventilation and vapor diffusion. This design would really only work with the simplest of roofs it seems, so we may not be able to add any aesthetic details.
With limited experiences with truss construction, my question is how might one add a large dormer or additional gables easily and still be able to insulate with 20+ inches of cellulose. Some four gable farmhouses are really nice looking, but I’m not entirely sure how to construct a roof allowing for insulation that deep over a sloped ceiling other than trusses.
Some thoughts I’ve had are
– traditional stick frame the dormer/ gable and use plywood strips to “hang” a 2×4 below
– I-joist cut at an angle (may not achieve as much depth)
– Add a gable for aesthetics, but keep it outside the conditioned space (seems like a waste of materials and money)
I’d rather not go for exterior foam on the roof just to keep things simple and cheap.
Thanks!
Isaiah
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Replies
You can build pretty high R value roofs with I-joist. Just order larger ones than needed for span and install a baffle under the top flange to create the vent sapce. For low slope you can insulate with loose fill or dense pack for steeper roofs. You can also loose fill steep roofs if you build a mini attic near the ridge with extra insulation up there to take up any settling in the rafters.
For a complicated roof, you can cross strap on top with 2x3 on flat to create a vent channel over valleys or hips so no rafter bay is blocked. Bit more work but saves on insulation costs as now you are not dealing with SPF or exterior rigid.
I find stuck on dormers wrong on many levels. If you are going to build it, use the space. Bit more work but definitely worth it.
Isaiah,
Architecturally that house looks like it's begging for a small flat ceiling area at around the ten ft level. Otherwise the rooms will feel disproportionately tall. That shaped roof can be trussed, or as Akos suggests framed with I-Joists, or dimensional lumber. The ceiling would probably need to be to0 high for the joists to act as rafter-ties, so if you don't go with trusses you are going to need ridge-beams. If it were me I'd gravitate towards the simplicity of trusses.
From an energy point of view the half story building are a pain to build and seal properly with more surface area per SQ foot than a 2 story.
If the plan is to grow old in this house I say putting it all on one floor.
Very pretty drawing looks a little like a house I lived in once.
Walta
Thanks for the input everyone.
That's a good point about the surface area ratios and sealing. i'm going to be going over all the air sealing details with a fine tooth comb to ensure everything is doable. If it looks like it's going to be too much trouble, we'll probably head back to the drawing board. I also have access to a blower door setup to ensure we get everything right during construction.
My wife likes this design the best currently so I'm trying to figure out how to make it work. She's pretty laid back about the design, with the only requirements be classic/ historic style, bedrooms on 2nd floor for privacy and a porch. My requirements are similar with the added goal of near net zero through a PGH design and PV, with attention paid to material selection.
As far as the trusses go, I don't have really any experience with truss construction, which may be why I'm having difficulty visualizing how the sections would come together. I can picture setting the trusses on the end of each gable and the subsequent trusses moving inwards, but what happens in the center of the 2nd floor, where the trusses perpendicular to each other meet? I feel like there's a significant area around the valleys that have to be stick framed to tie it all together. Sorry if the question is unclear. a drawing would help if i had any artistic ability...
Isaiah,
That's the beauty of using trusses. You don't have to visualize or understand how they will work (Within reason. It does help to know what's possible). As Tim said, you just take it to truss supplier and they will work out how it c0mes together.
What you have to look out for then is that the trusses will have been designed based on the structural demands. They don't design with an eye on optimal insulation depths or ventilation paths. Sometimes you need them to modify their plans to reflect those concerns.
Hi, Take the plans to a truss company for a quote. They will draw it in the computer an plot each truss & show the layout. Then you can work with them to get what you want or move on to stick frame it.
1 1/2 story spaces are hard to air seal when working with trusses. You have just too many members poking through your air barrier. You can do it if you design the space with exterior rigid insulation, but that does add cost. This why you see the recommendation to build 2 story with trusses.
If you design with a structural ridge and rafters, air sealing becomes much easier and you can have the high cathedral ceiling throughout the space.
I'm in the land of 2 1/2 story houses and this type of design is pretty much universal.