Framing a Stick-Built High-Posted Cape
This is a framing question and not necessarily ‘green’ in nature. Hopefully someone can steer me in the right direction regardless.
I am interested in building a stick-built high posted cape with ~3′ knee walls on the second floor to maximize living space without going a full two stories. I have read that rafter ties should be placed no higher than 1/3 of the height from the top plate to the peak of the building to prevent potential spreading of the walls (unless one uses a structural beam). Even with a 12:12 roof , complying with this recommendation would require that the building be at least 30′ wide to achieve a full 8′ ceiling on the second floor (given the 3′ knee walls).
Given the above, and the fact that I am considering a 26′ wide building, is the only option to use a structural beam? Or are there other viable framing options?
Thanks for any advice.
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I'm in the land of 2 1/2 story houses, I can tell you the last thing you want is any knee walls or collar ties. These complicate your build, make insulation difficult, are impossible to air seal around and only reduce the space you have. They are very hard to take out down the road but are easy to design out on a new build.
You can use the floor joists to tie the rafters together but than these also end up poking through your air barrier creating air sealing issues.
The best way really is to bite the bullet and install a structural ridge. No knee walls, no collar ties about the only thing you need is a couple of LVLs and some posts. Most reasonable length houses, you can have two of the posts in the outside walls and need only one or two in the middle. Sometimes best to put a beam across the house where the stair comes up to support this post instead of running it down the basement.
If you drop the beam bellow the rafters and clad it in wood you can even make it a nice architectural feature.
I think I need to clarify my use of knee walls in this situation. I am calling the portion of the exterior walls that extend ~3' above the second floor deck a knee wall. In other words, my exterior walls along the eaves are ~12' tall. I have probably misused the term knee wall, my apologies.
In any event Akos, it sounds like you would still end up with a structural beam as the solution. Yes?
Rob, most of the 19th-century homes in my town are constructed like this, and its a neat architectural expression that I've also toyed with replicating in a new house. Without benefit of an engineer's stamp or a structural ridge or any Simpson ties, the surviving examples seem to have not spread their walls. What's happening, of course, is the the exterior walls are balloon-framed, so those studs are anchored at the foundation, tied at the second floor, and terminate at the top plate 3'-4' higher which bears the rafters. A nominal collar tie towards the peak helps out and forms a ceiling. Surely a modern version of this could be engineered, even without a structural ridge, as you're looking to do.
Akos is right about the airsealing dilemma--it would be tough to retrofit energy improvements in the old places. But I've envisioned this as an ideal project for exterior wall and roof sheathing forming a continuous air barrier (a la Risinger "Monopoly" house).
Yup, structural ridge if you are going to stick build.
If you can increase this exterior mini wall height a bit, scissor trusses with a raised heel are probably simpler.
Hi Akos,
Any recommendations for over-roofing an existing structure with deep TJIs? Said structure is OLD (circa 1810), and the existing rafters are undersized (~4x6) and overspaced (3'6" OC). There's a ridge-beam, but no structural engineer would call it "structural". It's otherwise a typical 1.5 story raised Cape, with collar ties forming the ceiling of the second floor. The TJIs themselves will easily handle dead and live loads, but the old ridge-beam doesn't have a particularly good path for structural loads.
Thanks
Lots of people talk themselves into inhabiting the attic of a house. You need some extra space, and what are you going to do, buy a new house? Too hard. Easier to just condition the attic, you think. But then you don't have much natural light, so you put up dormers. Then things start leaking, so you have to re-do the flashing a few times; Maybe you end up with one seam that just won't take, so you dump roof goop all over it every few years. Then in winter you start to get a little ice damming, so you put down ice & water shield and maybe some heater tape, and you eventually just get used to that being there as well.
In a new build, though...
Why not just build out full-sized walls, and put roof trusses on top? Cheap by comparison.
What is the limitation that is pressuring you into knee-walls?
Burninate,
Nothing is pressuring me into knee walls per se. I just prefer a lower profile building and really don't need the extra space afforded by a full second story. Perhaps the real solution is to maximize my use of the space within a 1.5 story building, and forego the knee walls.
Rob,
Have you considered knee walls and scissor trusses? That eliminates the need for a ridge beam or collar ties.
At the risk of missing something obvious, why not make the "knee walls" a little taller, until the ceiling ends up where you want it?
From a energy point of view half stories building are a nightmare to air seal the sloped ceilings limit the room you have to provide roof ventilation and insulation all squeezed into depths of your rafters to have any hope of making it fit you are forced to use very expensive ungreen spray foam insulation.
I think if you put any reasonable value on the time you will spend air sealing you could build a full 2 story with R60 flat ceiling truss roof that will perform better for few dollars per square foot if you can move some stuff from the first floor to the second making the footprint smaller.
Walt
Make your exterior wall 4'. Probably no more expense if not cheaper. For a 12:12 roof spanning 26 feet, place your tie at 4' for an 8' ceilling and you are at less than 1/3 your roof height (4/13) This is precisely what I am planning. My house is in a wooded community with unobtrusive houses, and I too want to limit my overall height. I don't see why air sealing and leakage are issues any more than normal. For the sloped part (about 6') I will cut and cobble below a 2" vent space, fill the rest with mineral wool, add an additional 2 x4 perpendicular to rafters and fill it with mineral wool. At least R40. R60 above flat part. I know it's extra work and more expensive compared with a regular 2 story with trusses.
Fred,
I agree. My own house is built much like that. The complexities in air-sealing occur mainly when you have spaces behind the knee-walls, but there is no real difference between short walls and full height ones if everything is framed the same way.
Architecturally there can be some real advantages. Both inside and out the house feels more intimate and sheltering. I wouldn't make concessions to the look I wanted at the expense of having good building assemblies, but I don't see much compromise in your approach.
Thank you Malcolm, I really appreciate the vote of confidence from an expert.
Fred,
It was kind of GBA to add the badge to my posts, but take it with a grain of salt.
I find the hybrid roof a really nice combination. You get the benefit of a low roofline, but the small attic at the top makes venting at the peak easier, and allows some access for periodic inspection. It also means I can clean my gutters from a step ladder.