Roof Framing

Definitions for Rafter Framing

1. Plumb Cut - A cut on a rafter that will be vertical or plumb when the rafter is

in position in the roof.

2. Level Cut - A cut that will be level when the rafter is in position.

3. Pitch - The relation between the height of the roof and the width of the

building. A roof of one-fourth pitch would be one-fourth as high as the width

of the building. A half-pitch roof would be half as high as the width of the

building. Pitch is sometimes given in degrees, but carpenters generally would

not understand this as well as if the pitch is given in the above way.

Carpenters have another way of speaking of the pitch of a roof; they speak of

a 6—12 roof, when they mean a one-fourth pitch, because they use these

figures on the square to mark the rafters for a roof of that pitch. Other

common pitches are 5—12, 8—12, etc., which means roofs of 5/24 and 1/3

pitches respectively.

Photo owned by Michael Tiemann

4. Rise of Rafter - The vertical distance a rafter rises for each foot of horizontal

distance covered.

5. Run of Rafter - On a common rafter this is usually 12", corresponding to the

above rise. On a hip or valley rafter the run is 17".

6. Unit of Rafter Length - The length of rafter necessary to span one unit of run;

that is, to cover 12" of run of a common rafter and 17" for a hip or valley

rafter.

7. Ridge - The horizontal piece at the highest part of a roof to which the rafters

are fastened at their upper ends.

8. Hip - The place where two roofs meet forming an external angle.

9. Valley - The place where two roofs meet forming an internal angle.

10. Common Rafter - The ordinary rafter in a roof of two slopes.

11. Hip Rafter - The rafter where two roofs meet at an external angle.

12. Valley Rafter - The rafter where two roofs meet at an internal angle.

13. Jack Rafter - A rafter that is intercepted by a hip or valley rafter.

14. Bird's Mouth - The notch of a rafter that fits over the plate.

15. Rafter Tail - That part of a rafter that projects over the wall.

16. Valley Metal - A strip of rust-proof metal from 14" to 20" in width that is

used to carry the water that is concentrated where two roofs meet in a valley.

Metals used are painted tin, galvanized iron, aluminum, and copper.

17. Cornice - The finish at the place where the roof joins the wall. The complete

cornice has three parts— the part farthest from the building at the ends of

the rafters is called the fascia, the horizontal part is called the plancher, and

that next to the building is called the frieze.

18. Gutter - A wood or metal channel at the ends of the rafters that catches the

rain water and carries it to downspouts that convey it to the ground.

19. Shingles - Small pieces of material, wood, asphalt and paper, asbestos and

cement, plastic, etc., that are nailed to the sheathing to form the covering of

the building.

20. Built-Up-Roof - A roof made up of several layers of asphalt saturated paper,

usually 15 lb. per hundred square feet, and mopped on with asphalt or coal

tar pitch, to make a waterproof roof. Often this is covered with a layer of

gravel, 3 lbs. per sq. ft. to keep the wind from lifting it, and to keep the rays

of the sun from damaging the materials below.

Roof Framing

Framing of rafters is not nearly as hard as it is sometimes supposed to be. There are,

however, a few fundamental principles that must be learned.

The slope of a roof is often referred to as the pitch. This might be given in

degrees, but most carpenters might have trouble converting this into the forms

familiar to them. Among builders pitch is denned as the total height of the roof

divided by the width of the building. Thus a building with a 1/4 pitch would have

a roof one-fourth as high as the width of the building. If the building is 24' wide,

the roof would be 6' high, measured from the plate line to the ridge, or highest

point of the roof. Similarly, a half pitch roof would be half as high as the width of

the building.

Photo owned by Jim Derby

Carpenters also have another way of designating pitch. They refer to the numbers

used on the square to mark the rafter, such as a 6—12 pitch; this is the same as a

1/4 pitch, as these are the numbers used to mark the rafters for this pitch. Please

refer to the diagram for the numbers to mark the various pitches.

Roof framing is naturally divided into the following parts: common rafters for roofs

of two pitches, hip and valley rafters for intersecting roofs, and jack rafters to use

where common rafters are intercepted by hips or valleys. The only tools required

for marking rafters are a sharp pencil and a carpenter's framing square. Sometimes a

steel tape is useful for checking the lengths.

After the joists are in place and the backing is well nailed, you are ready for the

rafters. If you have a simple two-pitched roof, the rafters are easily cut and can be

put up with little or no difficulty. The rafters in such a roof are called common rafters.

To determine their length, first find the total width of the building from the

outside of one wall to the outside of the opposite wall. We will call this distance

the total span of the rafters. Half of this distance will be the total run of each

rafter. The run is the horizontal distance covered by each rafter. This is divided up

into the unit of run, which is one foot or 12". Suppose the total width of the

building is found to be 33'-8". Half of this would be 16'-10". Suppose the rise is 5"

per foot. Start at the top end of the rafter; find the number 5 on one side of the

square and the number 12 on an opposite side. Place these two numbers against

one edge of the rafter and mark along the 5" side to get the plumb cut, at the

top end of the rafter.

Step off along the edge of the rafter the distance between the 5 and the 12 a

total of 16 times, plus 10/12 of a time to find the length of the rafter and make

another plumb mark, parallel with the mark first made at the top end. You now have

the length of the rafter. This mark will be the outside of the wall.

A notch is then marked on the rafter called the bird's mouth. It can be at any

position on the rafter, but is usually a little less than half the rafter in depth.

With 2" x 4" rafters the bird's mouth usually extends to within two inches of the top

of the rafter, and with 2" x 6" rafters it is usually three inches from the top of the

rafter to the bird's mouth, except when a very low pitch is used. The bird's mouth

should not extend more than the width of the plate on which it rests.

When you have one rafter cut, use it for a pattern to cut the next one. When you

have a pair, try them to see that they fit perfectly. If so, use your pattern to cut the

rest of the rafters with confidence.

The cut at the highest point on a rafter where the rafter fits the ridge is known as

the plumb cut. There is also a short plumb cut on the rafter at the outside of the

plate where the rafter rests on the wall.

The cut on top of the wall plate is known as a level cut, and the notch is known as

the bird's mouth.

The part of the rafter that extends over the plate is called the tail. You can cut the

length of the tail now, but some prefer to put the rafters up and then cut them to a

string after they are in place. This latter method often results m a roof line that is

nearer straight than cutting them before they are in place, but if they are carefully

cut they will come out all right either way. In getting the length of a rafter it is

necessary to reduce the length of the rafter by half the thickness of the ridge board

before cutting. There is no mystery in this rafter-cutting business. Just picture in your

mind that the rafter is in place, then the square would be held with the long part

level and the short part plumb, and the marks would be made accordingly.

If you want a hip roof, there are a few other small problems that need attention.

Find the width of the building and its length. The width governs the rafters the

same as it does in the ordinary two-pitch roof. Find this width and proceed to cut

the common rafters in the usual way, making allowance for the thickness of the

ridge.

You will not need very many of them. To find how many, subtract the width of the

building from the length and add the thickness of the ridge. This will give you the

length of the ridge against which the common rafters rest. If the rafters are two

feet apart, divide this length by two feet and add one pair of rafters to get the

number of pairs of common rafters required.

The length of the ridge will be the difference between the length and the width of

the building plus the thickness of the ridge. It is often advisable to put the ridge

up longer than is required and to cut it off afterwards. Put the ridge in place and

hold it by nailing it to the common rafters.

The hips are next cut. Do this simply by using the same rise as you did for the

common rafters, in our case 5", and against this use 17" instead of the previously

used 12" for the run, and proceed in the usual manner. This will give you the exact

length of the hip rafter. Do not allow for half the thickness of the ridge, as you are

measuring from the long corner of the hip rafter, which almost exactly compensates

for the thickness of the ridge.

To get the side cut, try this simple method, which always works and is easy to do.

Make the plumb mark as for the common rafter, using the 5 and 17 as you did to

get the length. Then take a short piece of 2" lumber, the same thickness as the

rafter, and place it on the plumb mark making another plumb mark parallel with the

first mark but with the thickness of the 2" lumber between the two marks. You now

have two plumb marks parallel and with a space equal to the thickness of a piece

of 2" lumber between the two marks.

Now square across the edge of the rafter at the place where the second mark

intersects the top edge of the rafter. Join the far end of this last mark with the top

end of the first plumb mark. This gives the angle on which to cut the hip rafter for

the cheek cut, or side cut. This sounds complicated, but a little study of the

accompanying chart should make it seem as simple as it is.

This method works with any pitch, and with any cheek cut whether on a hip, or

valley, or even a jack rafter. This can be proved geometrically, but we shall not

take the time to do it here. Try it and see how easily it works.

The jack rafters are just common rafters that are intercepted before they reach their

full length, where they come up against hip or valley rafters.

Make the bird's mouth for the hips the same as on the common rafter, but make

it so the rafter is slightly lower than the common rafter; in other words, cut it

deeper. To find how much to drop the hip rafter, make a mark across the back of

the rafter using the 5" and the 17", mark along the 5" side, and square across the

rafter, so the square mark intersects the angle mark at the center of the rafter. The

distance between these two marks at the edge of the rafter is the distance to drop

the hip rafter, as that is the rise of the roof in half the thickness of the hip.

your house is ell or tee shaped, you will need a couple of valley rafters and a few

jack rafters to complete the roof framing.

To find the length of the valley rafters merely substitute 17 for the run instead of

the 12 you have been using, and make the same number of steps you took to find

the length of the common rafters. This is because the diagonal of a square 12" on

a side is 16.96", as near as you can mark to 17". Valley rafters are usually the

same length as corresponding hip rafters.

To get the cheek cut, or the cut where the valley rafter joins the ridge, make a

plumb cut in the usual way, using the rise with the run, in this case 5" and 17". The

side or cheek cut will be the same as was explained for the hip rafter.

Rafters must be firmly anchored to the plates, because the wind sometimes tends

to lift the roof. They should be toenailed to the plates with at least two 16 nails

and an eight-penny nail or two. In exposed locations it would be well to use the

small metal anchors that you can buy at the material dealers, sometimes called

"triple-grip anchors," that you spike to the plate and to the rafter.

Rafters should also be well spiked to the ends of the joists, to take the horizontal

thrust of the rafters. Remember the flatter the pitch of the roof, the more the

rafters tend to spread the building, and the stronger must be the nailing and

material to resist it. Don't be afraid of making the roof frame too strong.

Rafters should be securely spiked to the plates, and where they come along side of

the joists they should be thoroughly spiked to them as well. The strength of roof

required depends on the amount of snow expected in any locality and the

intensity of the strongest wind that has ever been known to blow there. Better to

make it too strong than too weak. Wood is remarkably strong if it is well nailed and

fastened in place. It is seldom that the wood is broken by the wind or snow before

the fastenings give way. Use plenty of nails.

All the expensive beautiful work and materials you later put into the house depend

for their value on the integrity of the frame. If the frame fails, the rest of the

house is worthless. Make the frame definitely strong, well nailed and secure. The

roof is one of the most important features of a house, for without it all else soon

deteriorates.

Where snow loads occur, they not only tend to make the rafters sag, but put great

outward stress at the ends, tending to push out the walls. The joists used as ties,

and well nailed to the feet of the rafters, will tend to hold the rafters in position.

That is the reason for nailing the rafters well to the joists. Rafters are also kept from

sagging by being braced from the partitions. When the rafters are securely nailed

in place, and are lined up in good shape, you are ready for the sheathing or roof

boards. If you are using wood shingles you will probably space the boards about 2"

apart, but with all other kinds of roofing, the boards are nailed tightly together,

except sometimes for certain kinds of tile roofing.

Knee braces are often added to rafters to help secure the lower ends. These are

nailed to the joists and to the rafters.

Collar beams, which are pieces nailed from one rafter across to its mate on the

opposite side, help to hold the roof together. Since these take only a few minutes

to put up, they might be a form of reassurance when the big winds blow and try

out the frame you have built. Don't hesitate to use plenty of nails; the way things

are usually nailed, the nails develop less than a fourth of the actual strength of the

timbers they hold together.

To erect the rafters, put up two common rafters on one side, nailing them to the

ridge and to the plates where they go. Then put up the ones that go opposite to

the first ones, nailing them both at the ridge and the plate. You will now have the

ridge in place held by four rafters that can be nailed solidly in place. Check the

ridge to see that it is level, and the rafters to see that they cross the building at

right angles, and brace the ridge in place. Then take the hip rafters and put them

up from the corner until they reach the ridge and nail them to the sides of the

ridge and to the plates at the corner of the building. Put all four hips in place,

and nail them securely. Now put up the rest of the common rafters, taking care to

keep the ridge straight. The jack rafters come next. They will be just like the

common rafters at the lower end, but the top will be intercepted by the hip rafter.

Measure from the plate to the hip for the shortest jack, and cut it to fit where it

goes. The next jack will be just two steps of the common rafter longer than the

first one and so on up the roof. Cut eight jacks of each length, four with the

cheek cuts on the right and four with cheeks on the left. Put them up in pairs,

nailing them securely to the hip and to the plate.

If the building has an "L" or "T" shape, of course you would need to put up valley

rafters as soon as the hips were in place, before proceeding as above. The valley

jacks are like common rafters at the top end, and the lower end of them has a

cheek cut. Once in awhile, you will have "cripple jacks" between the hip and valley

rafters, with cheek cuts on both ends. These will all be one length in any one

situation, as they run between the hip and valley rafters which are parallel with

each other.

If you have a section of the roof which intersects the main roof, it will also have

common rafters of its own, and valley rafters and hip rafters. Cut these common

rafters and erect two pairs of them as for the main roof, leaving the ridge long

enough to intersect the main roof. It can be sawed off later, then put up the hips

as before, and nail them in place. The valley rafter should intersect this ridge and

run clear through to the main ridge if possible; otherwise, the two valleys will be

terminated at the lower ridge. It is important to have some kind of support at the

upper end of the valley. Hips are self-supporting, but valleys have to support the

lower ends of the cripples, and the weight of the roof above them. If the valley

crosses a wall, it is a simple matter to put a post on the wall under the valley to

help hold it in place. Support the valleys wherever you can. After the rafters are

all in place and lined up and well nailed, put in the collar beams and the knee

braces. Then check the ends of the rafters to see if they form a straight line for the

eaves. You are now ready for the cornice. Maybe all you will need is a straight board

with a square edge for the first roof board, with a cornice that is entirely open, or

you may wish to put a 2" x 8" along the ends of the rafters, and a 1" x 4"

outside of that extending up to the shingle line, and let it go at that, or you may

want a more elaborate cornice. In any case make the cornice complete and then put

on the roof boards, nailing them securely as you go.

You are now ready for the shinglers, but first you should have the flues,

plumbing vents and all other openings in the roof made, so the shinglers can

do you a good job. To shingle first and then try to cut holes in the roof is seldom

satisfactory; always a nuisance, hard to do, and you can't fix the responsibility for

leaks on the shinglers.

After the sheathing is on the roof, the valley metal and flashing should be

considered. Use only rust-proof metal for these purposes. Painted tin, galvanized

iron, copper, and aluminum are all used satisfactorily for valleys and flashings. Except

on very steep roofs, a minimum width of 20" should be used for valleys. Valley

metal sometimes comes with a rib projecting up in the center of the valley which is

supposed to keep the water that runs down one roof from crossing the valley and

forcing its way under the shingles on the opposite side. This may have some

merit, but the flat metal, carefully placed, will do the work very satisfactorily. Where

two valleys meet at the top, be very careful to lap the metal so the water will not

run down inside the building. Remember that water always runs downhill. A little

soldering is sometimes advisable where the valleys meet.

Flashing around the chimney and fireplace consists of placing pieces of metal similar

to the valley metal into the joints of the brick work when the bricks are laid,

letting the metal project several inches, then bending the metal so it comes

between the layer of shingles to keep water from running through the roof along

the brick work.

Cornices

Cornice has been defined as "any horizontal, molded or otherwise decorated

projection which crowns or finishes the part to which it is affixed as, the cornice of

an order, pedestal, door, window, or house." More commonly the term is applied to

the finish placed on a building at the point where the wall meets the roof. The

treatment of the cornice may be rather simple, or it may be very ornamental.

The roof must project over the wall to prevent the water from running down the

side of the building. A wide cornice may be a good thing in a bright, sunny

climate because of its shade. In a cloudy, foggy damp climate, a narrower cornice

would not shade the building so much, but it must be wide enough to keep the

rain from striking the side of the building. The cornice must also be in keeping

with the style of the building. A Cape Cod cottage would not need as wide a

cornice as a California bungalow.

The modern trend is toward simple cornices. The simplest one is known as the open

cornice. This cornice is usually from two and a half feet to three feet wide. The finish,

if any, is placed on the top of the rafters, and is usually smooth boards or plywood.

It may be made of ceiling or "V" siding if a little more pattern is desired. The ends

of the rafters are cut at an angle or are rounded, or otherwise made attractive.

A gutter can be attached to the ends of the rafters if desired. The shingles project

enough to let the water fall properly into the gutter.

A simple box cornice requires a piece over the ends of the rafters known as the

fascia, and a horizontal board or boards to cover the space under the rafters, known

as the plancher. A crown moulding or a gutter is placed at the ends of the rafters,

and sometimes a bed moulding is placed at the point where the plancher joins the

walls of the building.

The most important thing about a cornice is that the pieces should fit exactly. A

simple cornice is pleasing if the parts fit perfectly, but the most elaborate cornice is

unsightly if the joints do not fit properly.

Keep the cornice simple and put your money and effort where it will add more

liveability to the house. Who notices a cornice, anyway?

A wide overhang on the roof increases the area and expense of the roof

tremendously. To illustrate the expense of building an elaborate cornice, a story

comes to mind, the source of which I have forgotten. A woman was showing a friend

her new house which was under construction. The friend was overawed by the many

wonderful features of the new house, but after several exclamations, finally said, "It's

all very wonderful, but I don't understand why you want so many pieces of life-sized

statuary dressed like carpenters around the edges of the roof." The woman

explained, "That is not statuary, those are carpenters, and they are putting up the

cornice."

To Fasten Gutters to the Edge of the Roof

If wooden gutters are used, merely notch the ends of the rafters to receive them, put

them in place, and nail them securely.

Metal gutters are often held in place by straps of metal spiked to the rafters.

Methods vary with the location. Observe how it is done in your locality.

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