Efficiency by Design - Roofs

After all the work of optimizing the rest of the house, you might think we could call it a day and cap our house. But wait, there's more! Efficiency by Design is a thorough study of EVERY part of the house. Why stop early when there's still a roof?

And Here's the First Pitch!

Roof pitches are expressed in several ways depending on where you are building.  Common in the United States is to use a number over 12.  That is, for every 12 inches you go horizontally, a certain pitch will go up a certain number of inches.  A 4/12 pitch roof goes up 4" for every foot it goes out.  It's a rather shallow roof, easy to walk on but still sheds rain in all but the most of monsoons.  It's not highly aesthetically pleasing though unless it utilizes 3' overhangs for a prairie style look.  An 8/12 pitch roof CAN be walked on with the right shoes but also looks much better and can have livable attic space.  This convention can be used for any number.  A 16/12 roof is not unheard of.  A 21/12 pitch roof will give you a nice equilateral triangle.  Though most designers won't even think about it, it is perfectly acceptable to call out a 5.71/12 pitch roof if that would help make the roof plane 20' even instead of 20'-7.5".

In other countries where the X over 12 makes less sense due to the metric system, degrees of pitch can be used.  This is almost simpler on one hand.  But building it in the field requires a different skill set or tool.  A high schooler might immediately understand a 30° pitch roof better than 7/12.

For your convenience here is a simple chart showing the comparison of roof pitches to angles:

1/12	4.76°
2/12	9.46°
3/12	14.04°
4/12	18.42°
5/12	22.62°
6/12	26.57°
7/12	30.26°
8/12	33.69°
9/12	36.87°
10/12	39.81°
11/12	42.51°
12/12	45°
14/12	49.40°
16/12	53.13°
18/12	56.31°
21/12	60.26°
24/12	63.43°
27/12	66.04°
30/12	68.20°
45/12	75.07°

Or you can use this handy calculator.

Istockhouseplans strives to make the best use of your materials.  If you would like us to analyze your plan and convert roof pitches to the most economical use of your lumber, please contact us.

What NOT to do

Due to mounting pressures in other arenas, last week's blog post missed the mark.  But we are prepared this week to offer a small round of funnies.  Please always background check your contractor and make yourself aware of energy efficiency practices.







We are particularly fond of the fixed R-21 door:

Heat Load Calculator

A while back we posted a series on the mechanics of calculating the heat load of your home.  At the end we promised to offer up an Excel file that is set up for you to do your own calculations without getting a headache or hand cramp

The calculator comes pre-filled with info from our Houston 2448.  Everything in light yellow can be modified.  The file or the rest of the cells are not locked.  This should be considered open-source, AKA modify at your own risk.  If you enter any values into white cells, you may destroy formulas.  There are also no fail safes or error checking in here.  Double check your work.

There are 7 components listed: slab, floor, walls above and below grade, windows, doors, and roof.  Each of these has inputs for area and R-value.  Note that windows should be input as U-value.  When inputting wall area, don't take windows or doors into account.  They are automatically deducted from the wall area in the calculations.  Outside design temperature can be modified for the first four items; remaining values are derived from those.

All the work is shown on the following columns.  The UA value, Δt and Btu/hr values are shown.  Indoor temp can be changed to your desired setpoint.  To the right is a little table with all sorts of nerdy calculations in it.  Percent of load tells you which component is losing the most heat.  Cost/hr tells you how much it costs.  In the example you can see that more than half the heat loss in this house is through the walls.  Of course!  There is only R-15 in the walls!  You can also see that increasing to R-21 doesn't do much for that factor.  Increase the walls to R-30 and you can get that component down to about 1/3 of the heat load.  Still high.  Note that the rest of the load percentages change as you change the area or R-value of an item.

Lower down on the page is a place to take leakiness of the house into account.  Input your target or measured ACH50 as well as volume of the home.  You should only change the HC if you know what you're doing.

The final input is for number of bedrooms or potential bedrooms.  This little calc will determine internal gains from humans.  It takes the number of bedrooms and adds 1 person per ASHRAE standards.  If there will only be two people living in your 3000sf house, enter one bedroom for kicks.

Total peak heating load is given near the bottom of the sheet.  The final table gives an idea of how much of what types of heat is needed to keep the house comfortable.  A forced air unit size and efficiency can be entered.  As you can see, even this is WAY too big for the house.  Even 2 1kW cadet heaters will do fine.  In this case we would recommend a 500W heater in each of the bedrooms and bathrooms with a 1kW in the great room.  Still a bit much but at least reasonable.  Perhaps a mini-split heat pump would do for efficiency as well as adding some cooling if you are in the South.

This calculator should be used for entertainment purposes only.  No guarantees about the results or performance of this tool are made or implied.  If you break it, you bought it.  If you find errors, please feel free to let us know.  If somebody who knows javascript is bored, we would be thrilled to turn this into an online tool.

Lumber Sizes

Ever notice that a 2x4 isn't really 2"x4"?  What's with that?   Fact is that the piece of wood started at 2"x4" but is called "rough sawn", that is it has unfinished faces.  The stick is then sent through a planer to smooth the faces and reduce serious splinter casualties.  About 1/4" is shaved off of each of the four faces resulting in a lesser dimension than you would expect.  Besides, who would want to say "one-and-a-half by three-and-a-half"?  Mind the twist at 2x8 and beyond...

Now pay attention as we mention dimension convention:

1x:

• 1x2 = .75" x 1.5"
• 1x3 = .75" x 2.5"
• 1x4 = .75" x 3.5"
• 1x6 = .75" x 5.5"
• 1x8 = .75" x 7.5"
• 1x10 = .75" x 9.5"
• 1x12 = .75" x 11.5"

(5/4 material is similar but is 1" thick)

2x:

• 2x2 = 1.5" x 1.5"
• 2x3 = 1.5" x 2.5"
• 2x4 = 1.5" x 3.5"
• 2x6 = 1.5" x 5.5"
• 2x8 = 1.5" x 7.25"
• 2x10 = 1.5" x 9.25"
• 2x12 = 1.5" x 11.25"
• 2x14 = 1.5" x 13.25" 

3x: (for those odd structural plates that engineers like to call out)

• 3x4 = 2.5" x 3.5"
• 3x6 = 2.5" x 5.5"

4x:

• 4x4 = 3.5" x 3.5"
• 4x6 = 3.5" x 5.5"
• 4x8 = 3.5" x 7.25"
• 4x10 = 3.5" x 9.25"
• 4x12 = 3.5" x 11.25"
• 4x14 = 3.5" x 13.25"

6x and beyond follows typical pattern as above.

And while we're at it, how about some typical engineered wood sizes.

I-joists are created by standing a piece of OSB or plywood upright and capping it with a 2x flange.  The result looks like a capital serif 'I' hence the name.

I-joist flange widths (varies by manufacturer):

• 1-3/4"
• 2"
• 2-5/16"
• 3-1/2"

I-joist heights (total height):

• 9-1/2"
• 11-7/8"
• 14"
• 16"
• 18"
• 20"
• 22"
• 24"

Laminated Veneer Lumber (LVL) beams are created by gluing several sheets of 7/8" thick plywood together.  Installation is by standing them on edge so that the profile looks similar to |||

LVL widths:

• 1-3/4" (2 layers)
• 2-5/8" (3 layers)
• 3-1/2" (4 layers)
• 5-1/4" (6 layers)
• 7" (8 layers)

LVL heights:

• Any height possible though generally intended to match I-joist material.  Can match dimensional as well.

Glu-lam beams are created by gluing and compressing several layers of post milled dimensional lumber together.  The whole beam is then planed again to create an even surface.  For this reason, glu-lam beams are slightly narrower than dimensional lumber.  *The industry has recently changed to also offer Gle-lams in full 5-1/2" widths as well.  Heights are always in multiples of 1-1/2" due to the size of the plies.  *The industry has recently changed to offer heights that are consistent with solid sawn and engineered lumber as well.  Due to general engineering practice the height should always exceed the width though rare exceptions always exist.

Glu-lam widths:

• 3-1/8"
• 3-1/2"
• 5-1/8"
• 5-1/2"
• 6-3/4"
• 7-1/4"
• 8-3/4"
• 9-1/4"
• 10-3/4"

Glu-lam heights:

• 6"
• 7.5"
• 9"
• 9.5"
• 10.5"
• 11.875"
• 12"
• 13.5"
• 14"
• 15"
• 16.5"
• 18"
• 19.5"
• 21"
• 22.5"
• 24"

Glu-lams can be used as posts as well.  A 3-1/8"x6" glu-lam post is sturdier than a 4"x6".

Lengthy Topic

Today's little bonus post is brought to you by the American Wood Council and their span calculator at
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

Don't get to excited, this is not a full structural calculator.  It is, however, a great alternative to checking span tables and they have an iPhone version, handy for field specification.  Mind the deflection.

Happy designing,

Istockhouseplans

You're fired!

In searching for some material on fireblocking, we ran across this thread on the DIY Chatroom.  Indispensible material.  This is generally the bane of do-it-yourselfers and code officials alike.

http://www.diychatroom.com/f98/how-fireblock-framing-37190/

Istockhouseplans is currently working on trying to fireblock a double 2x4 common wall with raised heel trusses.  We'd like to rock the wall all the way up and then hang the trusses but are unsure that the hanger would achieve strength through two layers of 5/8" type X drywall.  A more viable option would be to nail a 2x4 ledger through the sheetrock into the walls studs.  This would require 1.5" + 5/8" + 5/8" + 1" embedment = 3.75" nails.  While 18d nails might not be common, this is going to require a bunch of hand driven 20d nails.   Those won't exactly fit into a power nailer.  The other option is multiple 2x16 blocking between trusses.  Not really an option though.  Maybe stacking 2 pieces of 4x8 would do it?  Does anybody have input?

Shrimp, stroke, iron, roof

What do these things have in common? They all have a butterfly variety. However, which one can cause extensive damage to your home? Please assume your current neighbor does not play Ina-Gadda-da-Vida at '11' all day long.

The butterfly roof is a nightmare waiting to happen. For those of you not in the know (and we take a chance by giving you the wrong type of armament) butterfly roofs take the idea of a roof and turn it upside down. That is, the peak is really a valley in the middle of your house, and the edges are on the high side. This has the general idea of draining all the water into the center of your house. Oh sure, there are roof membranes and back-up plans and a plan C in case all else fails. But I would like to put forth to you this: Would you try to race your car at 150mph on the freeway at 3am because you have high-speed impact bumpers, a five point harness, airbags all 'round, and a million dollar insurance policy? Why invite disaster?

Now there are some of the neo-modern movement who would argue aesthetics with me. "Mies van der Rohe would love it!" Well good for Rohe. If Rohe jumped off a bridge, would you? Okay, sorry about the motherly retort.

Now, if you live in an area of no rainfall, such as Los Angeles then please, by all means, knock yourself out. But if you have transplanted yourself to the rainy northern parts of the country such as Canada, please consider for a moment your rain and snow situation.

Flat roofs? Same deal. If your drain backs up against your parapet and your scuppers are misinstalled, grab a towel and take a dip. ("What'd he say?")

In the interest of durability and creating a home that will last for you, Mr. and Mrs. Homeowner, Istockhouseplans has taken great pains to ensure that even Laurel and Hardy could build you the best house possible.