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Attic Trusses – Room Size


Attic Trusses - Room Sizes

My friend, Loren Power, will be the first to tell you that I made a mistake on his house. He was the one who pointed up into the vast attic space of his home and said, "What a huge waste of space. Why didn't we make some sort of room up there?" Of course this was after the entire roof was constructed and it was too late to turn back. Loren's comments years ago quickly put me in my place. It was obvious that I still had lots to learn about offering options to homeowners.

BIG Rooms!

If your new house will have a steeply pitched roof - say 10 inches of rise for every 12 inches of run (10/12 pitch) or more - then you can gain some very serious room if you upgrade to an attic truss.

For example, if your house is a 50 foot long by 38 feet wide structure, you can get an attic truss that will allow you to create a full height room (8 foot ceiling) that is 16 feet wide by 50 feet long! That is 800 extra square feet. Think of how you could slice and dice up that space into different rooms.

Costing That Space

So how much extra did that space cost to create? My guess is that the national average for an attic truss that will create the space we just described will cost you about $100 more than a regular common truss. For the 50 foot long room you will need about 24 trusses. This is $2,400 plus the cost of the plywood subfloor. Let's say plywood costs $20 per sheet for 3/4 inch tongue and groove yellow pine. We will need 25 sheets to cover our 800 square foot space. This will add an additional $500 to our material cost. We are now at $2,900 plus tax. Let's just say it will be $3,100.

Get out the calculator. $3,100 divided by 800 square feet equals just $3.88 per square foot rough construction cost! that is a HUGE bargain, especially when you compare that to the overall cost of your house. Not only that, this cost is almost purely material cost. The labor to set the trusses is no different than had you not included the room. You would be a fool not to create this space or at least a partial space up in the trusses.

Storage Space Only?

Just this morning I received an e-mail from an individual. He had discovered a hidden storage space up in his attic. The dilemma was whether or not the floor of the space could support the weight of storage boxes. Had the builder installed attic trusses or their step-sister storage trusses, I doubt I would have received the e-mail.


Cutting Common Roof Rafters

How well did you do in high school geometry? If you did fairly well, then cutting roof rafters will be a piece of cake. All you have to keep in mind is the fact that there are primarily two cuts in rafters. The ones that are straight up and down (plumb cuts) just like a plumb line and level cuts (heel cuts). Rafters simply have a series of cuts in them, all of which are either parallel with one another or oriented at 90 degrees to one another.

Pitch - not Baseball or the Tar Stuff!

To cut a rafter you need to know the pitch of the roof. This is the rise and the run relationship. By convention we call it out as the number of inches a roof slope climbs as you proceed 12 inches horizontally across the roof. An example would be a 4/12 pitch roof. The roof "rises" 4 inches in height for every 12 inches it "runs" horizontally.

A framing square will create the series of parallel lines and perpendicular lines if you simply slide it along one of the edges of the rafter. Note that a framing square has two legs. The narrow one that is 16 inches long is the tongue. The fatter one that is 2 feet long is the body. The are oriented at 90 degrees to one another.....now isn't that a coincidence.....?

Note that there are markings - in inches - on both the inside and outside edges of the square. The starting point of the markings is at the corner where the two legs meet. This is important. You must decide to use either the markings on the outside edges or inside. I happen to like using the outside edges of the framing square.

Positioning the Square

Let's say we are doing a 4/12 pitch roof. Find the 4 inch mark on the outside edge of the tongue. Locate the 12 inch mark on the outside edge of the body. Position the square so these two marks are touching the same edge of the rafter your are going to mark. Note how the square is oriented at an angle across the rafter. The tongue of the square is creating your plumb cut where the rafter will rest against the ridge board. The body is creating the heel cut where the rafter will sit on top of the wall. We could mark and cut right now if our span was just one foot. But we aren't building a cat house!

Rafter Length

Let's go back to geometry. The rafter square makes two sides of a right triangle. The sloping rafter makes the third component - that being the hypotenuse. The length of the hypotenuse is what goofs everyone up. How do you figure out the length of the rafter? It is simple mathematics. It has already been done for you if you buy a quality framing square. There are tables on the body of the square that tell you how long the rafter is for the different pitches for each foot of horizontal run.

You can also purchase books, booklets, or fancy construction calculators that will instantly tell you the rafter length as long as you plug in the rise, run and span values. Once you grasp how to orient the square, this is your only other challenge.

Ridge Tail, and Seat Cuts

If you do the pure math to figure out rafter length, you will always be a little long. Why? Just about everyone uses a ridge board. You need to shorten the rafter for half the thickness of the ridge. This often means slicing off an extra 3/4 inch off the plumb cut at the one end.

The seat cut is the notch where the rafter sits on wall. Don't make this too deep. Never cut out more than 1/3 the plumb cut distance into the rafter.

The rafter tail cuts where the roof ends are best made - in my opinion - after the roof is framed. If your wall is not perfectly straight - most aren't - your gutter board will mimic the curved line of the wall if you cut the tails at the same time you make all your other cuts. It is more work to do it in the air, but it is well worth it.


Truss Setting Guidelines

Every year many construction workers get killed or seriously injured from truss assemblies that collapse suddenly during construction. The most common problem is that the trusses were not braced properly and sufficiently during the erection process.

All truss manufacturers will gladly supply you with a booklet showing how to brace trusses as you set them on top of the walls. A simple 2x4 or series of parallel 2x4's providing the 16 or 24 inch spacing isn't enough! You need additional diagonal braces that make X's on the top chord of the trusses. Larger trusses often need similar diagonal bracing on the vertical center post web member too. the larger the truss (both height and span) the more complicated the bracing. Gravity, wind, the motion of workers, and an unexpected bump by the crane can easily topple a truss roof during construction.

Complicated Truss Roofs

If you are setting a hip roof made from trusses, you will probably have girder trusses, special hips, etc. There is a certain order in which the components must be set. Not only that, you work from the two long ends of the roof towards the middle.

The truss manufacturer will almost always provide you with a set of erection instructions to help you. If you are a rookie, I wouldn't think of trying this without some experienced help.

Column B203

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5 Responses to Attic Trusses – Room Size

  1. Hello,

    My husband and I are embarking on a new build/renovation to an existing 24' by 36' story and a half cape cod with knee walls. We are planning on our addition to be 24' wide by 26' in length, to add a larger living room in the main floor and an extra bedroom and bathroom in the top floor. My question is; How do you calculate what the actual "living space' will be for the top-floor area? We are going to be building storage space into the kneewall part, but are having a heck of a time figuring out the top-floor area for bedroom/bathroom sizing. Any suggestions or help would be greatly appreciated...Many thanks, Heather

  2. i need help designing an attic room on new construction. house will be 60 feet long by 28 feet wide. I need to knoww what size the room will be at what pitch roof

    • Carol, your question requires lots of typing, plus I have some questions for you so I can give you the correct answer(s). I only do pithy answers here in the comment section. If you want to protect the investment you have in your house and not waste time or money *hoping* you make the right decision, you should talk to me on the phone for just 15 minutes. It'll be the best investment you've ever made in your home!

      • I just installed 28' wide attic trusses over my garage, and the attic room was 15'-11" with 5' knee walls and 8' ceiling at the center. roof pitch was 12/12. hope this helps.

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