The Roof

My grandmother's childhood home in the Thyrennian Sea off Sicily.  This is the street side.  The flat roof collects rain water and stores it in bell shaped cisterns on the other side of the house.  The recessed roof surrounded with parapets were plastered to waterproof it.

 

 

 

 

This is the opposite side of the house above, facing the sea.  There is one cistern about where Signora Nardi is standing, and one in front of the railing of the porch farter along.

 

 

The roof is a very weak structure.  Pressure from snow and just plain gravity is always pressing down on the structure.  The flatter the roof, the less strain it will take, and the bigger the rafters or beams you will have to use.  In the deep south or in desert areas, it might be wise to build a parapet by raising the walls a few inches above the roof, line the resulting box with rubber or some such and collect rain in a cistern for the garden and drinking water.
On the other hand, the higher the angle you put on the roof, increases the area exposed to the weather, expense in materials, and strain on the building from wind.  The plus side being increased area for solar cells, snow slides off faster, and usable space inside is created. 
Pressure on the roof, pushes out on the walls.  In past centuries, the framing members were enormous.  This added to the stiffness of the top plates of the walls. But even then, weight from above tended to Bow Out the upper part of the walls.  The solution to this is to tie the opposite walls together so they have nowhere to go.  If you are hoping for cathedral, or in our case chapel ceilings, this presents a problem.  All those cross ties are going to become a visual problem.
Personally, I do not like cathedral ceilings.  You lose precious storage space, loft bedrooms and intimacy of the space.  If you are claustrophobic in your small space, this may be the only way to get around it as you will certainly feel like you are in a bigger space.
Please note trussing systems in the illustrations.  Also, be aware that even the best cathedral ceiling will still put a bit of undue outward pressure on the walls, just from the flexibility of the framing members where there are heavy loads on the roof.
Style will also help to dictate roof design.  If you want a nice Gothic cottage, it would be better to build higher.  Italianate needs a lower roof.  A house may appear squat if it is too low, ridiculous and ready to fall over on its side if too high.
Draw possible solutions to scale on graph paper, and take a good critical look at the resulting house.
Draw the front as well as the gable ends.  A very tall roof will make a little house appear to be ALL roof. 
This is helped by adding a gable on the front of the house to raise the visible wall at the middle of the front of the house, or even in two or three spots and minimize the appearance of a huge expanse of shingles.
We often forget in our pursuit of living room, satisfying codes, and getting enough light into a house, what the house looks like. 
You may not give a rat's rear what it looks like, but the time may come when you want to sell your squat house with oddly arranged and asymmetrical windows and doors to someone who does care. 
The paramount rule in Real Estate, sharing equal importance with Location, Location, Location, is Never Build an Ugly House.  In all that you do, try to offend the fewest people possible.  A spaceship house may be fascinating to 3% of the population, but you may eventually be trying to attract as close to 100% as possible when your realtor has your house picture on his wall.
My sister's house is a prime example.

 It uses a technology that is a bit foreign to the average person, but you have to admit when you pull into the drive, that it is a very pretty house!
By the way, the house that attracts the most buyers is the soft yellow house with white trim and black or forest green shutters....Just a statistic from my Real Estate days.

Gambrel and Mansard houses will give you more standing room on the second or attic floor.  Gambrel is not an unusual design, and probably will not turn potential buyers off, but a Mansard or Second Empire building may limit the saleability, not because it is ugly, but because it is unfamiliar to most.  Also, unusual roof designs are really a lot more work to construct.

The first job is to decide what you will build.  Second is to build the gable end walls.  The gable end walls are actually just another roof rafter Truss, with nailing surfaces for the sheathing.

There are several ways to continue with the upper level of the building. 
One is to construct another floor with rim joists etc., just like you did on top of the sills on the first level.
The second is to just install the roof rafters on top of the upper plates, which will either meet each other in the middle of the roof, or meet at a ridge pole which runs between the two gable ends.
The third is to construct Trusses that will rest on top of the upper plates.  This is what is illustrated below.

This is only a general representation of the truss construction.   The actual measurements noted are correct or withing a few fractions of an inch.  The angles are correct.  There will be variations in measurement depending on variations in construction that you used, dimensions of the lumber, little mistakes etc., so you must confirm measurements by checking your actual structure.  Large measurements, like advice on length of members to purchase is essentially correct, just little variations in your building and such decisions as how long you want your eaves to be.  Numbers in the picture refer to text. 

In this case, we will build simple trusses, that will sit above each of the wall studs, and above the end  walls.  This will keep everything very simple, and we will rely on mouldings or a flat board to give a little drip edge for the gable end, and we will extend the rafters a bit beyond the wall upper plate to form eaves.

In a simple building, we could easily build the gable end with the wide part of all the studs parallel with the wall, but in this case, we are trying to make an insulatable wall, so we will build all of the trusses, and the end two will have the ends of the studs cut out to fit into the truss members.

You will need ten identical trusses.  The top of the trusses where the two rafters meet will be joined together with a triangle of plywood on either side of the truss., except the two end trusses which will rely in the exterior sheathing to join the outside face of the truss.  This will all become clear if you look at the illustration.  Also, to simplify the upper ceiling joists, they will bypass the rafters and simply nail to the sides of the truss.  This will alter our very regular measurements a bit, but it is a minor headache.  Again, there will be a minor alteration of the pattern for the gable ends to give us a nailing surface for the floor at both ends.

To begin with, we need a large flat area to build the trusses.  They will be wider than 12 feet, so we will either work diagonally on the subfloor we have constructed, or find another large flat area to work, such as a neighbor's garage floor.

We are going to default to a 90 degree angle for the roof.  We will discuss Rise and Run elsewhere, as it really does not impact us in this design.

Cut all of your rafters and joists at once, or have someone working on them as you construct the trusses.  This will keep you from jumping up after each truss and changing gears.  Best to do one job and keep the stock of parts to work from as all of the parts will be identical for the most part.  Again, you will make your life simpler if you have bought nice straight lumber, and have not left them sitting around in the weather while you get around to this job.  Use a Skill saw, Crosscut saw or Power Miter saw with a very large blade to do the work.  Check to see that the ends of the rafter lumber are sound, and unsplit, unless the angled cuts will remove the splits.  Cut off the ends if they are imperfect., and this would prompt you to buy longer lumber if there are lots of split ends.

Before beginning, just a word of warning.  Never measure a piece of lumber to be cut by laying the last one on top of the new one and tracing it to be cut.  In this way, each successive cut is a pencil width longer, and your cut pieces will slowly creep up in length.  If you must trace something, make a paper pattern from brown paper bag paper, or brown wrapping paper.
I suggest that you lay out a pattern for each cut piece on the floor of the subfloor and use that for each member. 
Also, lay out the shape of the completed truss on the floor, and make sure that each one is identical to the first one, once you have made sure it fits properly.

You do not have full length measurements on the plan.  You will need probably a minimum of 2x8x12 feet unless you can find 11 foot long boards somewhere.
You must decide just how much overhang you want.  If you need sun in the building, you will keep the rafters short  printing out the illustration will give you a scale drawing that will come pretty close to the right lengths.  If you are trying to avoid the sun, you may want to make the rafters longer to overhang the wall and shade the windows etc.  They can be uneven, with the side toward the sun long and the side toward the north short.

First cut the tops of all the boards at 45 degrees where they will join together at the ridge of the building. (1)

Lay two of the cut boards with the 45 degree cuts together making a big V on the floor. Check for square with a framing square.  The intersection will be 90 degrees.

Measure down about 8 feet 5 3/4 inches from the inside of the angle on the first board. (2)

 Measure the same on the second board and mark it.

Now measure the distance from one mark to the other.(3)

It should be 12 feet.  If it is not, check to see that the 45 angle is butted snugly together and measure again.
 
If it still does not quite make the 12 foot spread, you will have to adjust the distance down the boards or up the boards to make it fit.  Make sure you do equal moves on each rafter till the 12 feet has been hit. 

Nail or screw the plywood triangle to the joint at the top of the two boards as shown. (4)

Check measurements again to see that it has not moved.

Lay the floor joist across the V of rafters with the bottom edge on both of the final marks at 12 feet apart.
Tack the joist to the rafters with one or two nails in each end.  Do not nail them all the way in. 

Now you can see what options you have for the eave overhang. (5) 

Determine how far you want the overhang to project and mark it on the inside of the V to be cut later.  This will also mark the end of the floor joist and the corner to cut off the joist end. (6)  Trace the future cutting lines with a pencil.

Disassemble the tacked joints.

Make the cuts as marked, then make a pattern on the floor, recording the measurements on the pattern.

Reassemble, nailing five times through each joint.  I would also put at least one screw through the joints so they will not work apart in handling.  Turn it over and add a second plywood triangle to the other side at the peak of the roof, and nail the joints from that side as well.

You now have a pattern to follow and should be able to lay boards on the pattern and make identical trusses.  Measure each board before you cut using the measurements recorded just in case.

You will need ten identical trusses.  Alterations will then be made on the gable ends.
Alternative, easier trusses will appear later, but they will be much heavier to lift.  I know this is a bit confusing, but the extra time on these, will save you the lifting alone problems later.

 

This is the gable end with essentially the correct measurements but not to exact scale...you know my drawing skills by now!!!!

Above to the right, are shortened drawings of the wall studs from four sides.  The truss is made of vertical 2x members and you need the 5 1/2 inches thickness to insulate properly and to bring the wider boards out to the same level as the truss rafters,  Since the floor joist is on top of the rafters on the truss, the joist must be notched into the studs at the bottom.  You will also notice the 45 degree top cuts to match the top and bottom of the rafter angle.


I am the first to admit that the instructions for the gable ends are a little difficult to follow.  If you can, it is a good thing, however, if you make an identical truss to the others in the house, then fit a second horizontal piece of lumber into the recess between the legs of the triangle, and nail it into the original horizontal, you can then put two more legs on the triangle, making basically a double thickness triangle.  Then you can just fit studs in between the base and legs of the triangle to look just like a normal wall, were it not for the angle. 

Another option for the gable end, is to use the same measurements as the vertical 2Xs, but lay the members on the side so that the 2x stock is horizontal rather than vertical.   You then add vertical studs inside the triangle of the truss for nailing.  The only drawback to this is that you have no nailing surface for a floor at the gable end.  You would have to add a 2X member to the side of the top plate to use as a support for the ends of the boards.  Some people complete the entire first level of a house, including the floor of the second level before adding the rafters.  This really is viable, however, if you have remodeling to do, it complicates disassembly of the floor if it is sandwiched between the top plate and the roof.

The end gables will have to be sheathed to be installed easily.  Use 6D 2 inch nails to install the sheathing.  With this height of a gable, it would be best to install the sheets of CDX vertically, leaving a tail hanging down far enough to meet up with the wall sheathing that will come up short of the top of the walls.  This will give you plenty of area for the sheathing to attach to both the gable and the wall, thus bonding the two together.  Cut the sheet goods at a 45 degree angle from the corner of the sheet to the opposite side.  Fit the sheet on to the gable truss on the side that has the complete roof rafter showing.  Nail along the edges of the sheet every 6 inches.  Nail any other areas where the sheet goods meet studs every 12 inches(though you may nail closer if you wish).  The edge of the sheet should line up with one of the studs, not quite to the center of the stud.  (you will need a gap between the sheets of about 1/8 of an inch for expansion.)  Cut the bottom edge of the sheet
The gable end has the studs protruding, so there is no place to nail down a floor.  Run a single 2x board along the inner side of the truss to provide a nailing surface.

There are preforated metal sheet brackets that will allow you to attach the trusses to the top plate, but it is quite easy to simply toenail the bottom  member of the truss to the top plate, two nails on each side at the ends and every 6 inches between. 
Stand the trusses up one at a time directly above the wall studs below.  This transfers the weight of the roof to the foundation instead of the top plate.( the top plate can flex and deform the building if there is nothing below the truss to support the weight.
As you install a truss, nail a long board that will reach the full length of the building(if you have a scrap) to keep the trusses in the upright position.  Just tack them in place with a couple of nails to be removed later.  If you do not have a long board, just use several short ones.  Doing this at a diagonal with keep it all square as you go.