NOTE: Make sure you read the first three posts (in order!) before tackling the rest, or it could be confusing: Post 1 is Designing the future, Post 2 is Setting up the problem, and Post 3 is Estimating basic requirements.


Wednesday, September 13, 2006

Framing a solution

Framing is one of the most crucial and complicated parts of constructing a house. In turn, one of the most complicated parts of framing is designing the roof. As a continuation of the earlier discussion on green roofs, I’ll give an introduction to some framing basics.

(A quick disclaimer: You shouldn’t start designing until you are satisfied that all requirements are in place. There’s no harm in learning about design solutions, however, and this discussion is intended to give you a further part of the ‘big picture’ of the design process.)

In discussing green roofs, I mentioned that roof loading can be in the range of 2300 – 7000 Pa (50 – 150 lbs/ft^2) compared to 5 – 20 lbs/ft^2 for a conventional roof. This is a significant weight addition that requires non-standard designs for the roof and frame structure.

Since this is such a complicated and important topic, it’s going to take us several posts to lay it out. First, you should check out the extensive tutorials and free technical resources at the American Wood Council. Go through the tutorials and play with the calculators in order to understand the basics – remember, not all of the conventional wisdom applies if you are building a high-load green roof. Also, Miedrn at Snippets&Bits has a good post on do-it-yourself framing books.

As you are digesting the information on these sites and books, here are some Peak Oil-based considerations to keep in mind:

1) Wider joists and studs allow for more insulation and wider spacing. Evaluate using 2x6’s, 2x8’s or larger instead of your standard 2x4’s. The energy savings and strength improvements could allow you to relax the design in other areas.

2) Make a note of the local woods available in your area. If lumber prices skyrocket during building, or you have to make future repairs (a certainty), are your local trees strong enough to meet your design requirements?

3) Shifting climates – What do climate change experts say will be the future weather trends in your area? Will snowfall rates (and your snow loading requirements) increase? Decline? Getting an answer can help prevent under- or over-designing.

4) Exceptionally heavy roofs may require the use of beams or a series of tightly-spaced joists to allow a large enough span with allowable deflection.

5) Consider hybrid systems – Think about a solar collector on one part of the roof and green roofing on the rest. Or shingles/green. Or a mix of green roofing types.

6) Make sure that you have a deep understanding of building structures, particularly your own. More than anything, framing a house is probably one area where you will want to employ a certified professional. If your walls aren’t plumb or your floors or ceilings aren’t level, you will have more trouble with the interior walls windows, plumbing, and so forth. However, if you can’t find a contractor (e.g. post-Peak Oil) or can’t afford one, you had best know all about building and repairing your structure – a poorly constructed frame could fail with devastating consequences.


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