Conventional Roof Framing

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Conventional Roof Framing - Principles That Bring Good Design

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2 NATIONAL RESEARCH COUNCIL

CANADA

DIVISION

OF BUILDING RESEARCH

HOUSING

NOTE

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A. T. HANSEN

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REPRINTED FROM

CANADIAN BUILDER, VOL. XIV, No.

2 FEBRUARY 1964, P. 46-48

OTTAWA, FEBRUARY 1964

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. I X 4 CROSS T I E W H E N C O L L A R T I E

SLOPE 5 4/12

-

2 x 4 L O L I AII T I E r o ~ H A F T E H SUPPOIIT

OR G R E A T E R

WHEN RIDGE I S UNSUPPORTED

y 2 X 6 RIDGE

SLOPES 4 / 1 2

4

_{2 X 4 RIDGE SUPPORT -4'-O"O.C}

OR GREATER _{2 x 4 COLLAR TIE FOR}

RAFTER SUPPORT

1.4

1 x 4 CROSS TIE WHEN

I:I-

MINIMUM NAILING ( S NAILS)

'

1,l

COLLAR TIE EXCEEDS 8'-0" MAY BE USED WHEN RIDGE

I S SUPPORTED

-

2 x 6 RIDGE

S L O P E S LESS 2 x 4 RlOGE SUPPORT 4'-0"o.c. MUST _{BE USED WHEN SLOPE IS LESS THAN}

T H A N 4/12

W

DWARF WALL TO

DESIGNED AS ROOF JOISTS 'SLJPPURT R A T r F R S

11. _-

WHEN THEY CARRY PART OF ROOF LOAD WHEN ROOF SLOPE IS LESS THAN 4/

12

FIGURE I C O N V E N T I O N A L ROOF F R A M I N G

-

Principles that lbriing

good design

By A. T. HANSEN

Research Officer, Housing Section, Division of Building Research, NRC Although wood trusses are becoming more and more common in roof framing, the traditional method of framing using joists and rafters is still used in a large percentage of house roofs. With some roof shapes the framing can be- come quite complicated and confusion sometimes arises as to whether a par- [icular arrangement will provide adequate strength to support the expected luads. It is important, therefore, to have an appreciation of the principles that

should be followed in obtaining framing arrangements that will provide adequate strength.

Two principles must be followed to obtain adequate roof strength: the rafters must be strong enough so they will not break in bending, and the rafters must be prevented from spreading apart when the roof is loaded with snow.

Rafter sizes

Joist and rafter span tables in the Housing Standards show the maximum safe spans of various sizes and spacings of members for various grades, species. and snow loads.

The spans are measured as the hor- izontal projection of the rafter between supports. 'These supports may consist of

the wall plate, ridge, dwarf walls, 2- by

4411. collar ties (for roof slopes of 4/12

or greater) or ceiling joists such as in

1 '/2-story houses.

Collar ties

Although the term collar tie implies a tension member, collar ties are seldom effective in preventing the spread of rafters under a snow load. Collar ties are used principally to support the rafters at intermediate locations in order tc reduce the effective rafter span; the ceiling joists act as the real tie for the rafters.

Collar ties act as compression members or struts when the roof supports a snow load and will therefore have a tendency to buckle if they have long unrestrained lengths. To reduce this pos- sibility, collar ties should be at least 2-

by 4-in. material and they should have

a continuous strip nailed at right angles near their centres when their length exceeds 8 ft.

The compressive force on a collar tie increases rapidly as the roof slope de- creases and for this reason collar ties are not considered to provide adequate rafter support if the roof slope is less than 4/12.

R o d loads on

ceiling joists

When part of the roof load is carried

by the ceiling joists-e.g., where a dwarf

wall is used to help support the rafters- additional loading must be allowed for in ceiling framing. Ceiling joists are designed for only relatively light loads and any appreciable increase from roof loads may cause excessive deflection and lead to p!aster cracking.

One rule of thumb where ceiling joists help support roof loads is to in-

crease the ceiling joist sizes 1 in. when

the roof slope is 4/12 or more, and to

design the ceiling joists on the samc basis as roof joists (roof framing members that support a finished ceiling) when the slope is less than 4/12.

It can be shown on a theoretical basis that increasing the ceiling joist depth

by 1 in. will not guarantee satisfactory

performance. Wherever possible, therefore, dwarf partitions in attic spaces should be located as close as possible to a supporting partition. At the ends of hip roofs, where only a few rafters are to be supported at mid-span to reduce the unsupported length, it is not always possible to locate these supports close to a partition. The loads from the rafters in these cases should be distributed across as many joists as practicable.

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I

I I I H E A V Y LINE INDICATES P O R T I O N OF RIDGE T H A T SHOULD

B E

S U P P O R T E D P O S T S SHOULD

BE

LOCATED I AT T H E S E POINTS T O

SUPPORT ENDS

O F

_RIDGE

I

b

a

-

_-

a, c

-

Jack rafters at locations marked "a" and common rafters in locations

-

- - -

- -

- - -

marked "c" do not depend on ceiling joists to prevent rafters from spreading _-

b

and may be fastened to ceiling joists with minimum nailing.

- - - .

b

-

Common rafters at locations marked "b" may be prevented from spread-

ong by adequate nailing to joists (Table 1 ) or by supporting the ridge. I I

HIP R A F T E R S

FIGURE 2

-

Schematic diagram o f typical rafter layout (broken lines).

Rafter ties

and

ridge support

T o prevent rafters from spreading under a snow load, either the ridge must be supported or the lower ends of the rafters tied to the ceiling joists by adequate nailing. At slopes of less than

4/ 12, however, the force required to resist spreading becomes so large that it is ~ ~ s u a l l y impracticable to tie the rafters to the joists because of the large number of nails required. In such cases, the ridge must be supported (Figure 1). This may be accomplished by providing 2- by 4-in. supports spaced 4 ft apart along the ridge if the ridge consists of at least 2- by 6-in. material. When a ridge does not require support, the ridge board may be omittcd if the opposing rafters are butted together. In most cases, however, a ridge board of l - o r 2-in. material is usi~ally provided to aid in lin- ing up the rafters and as a convenience in constructing the roof fram-.

When thc ceiling joists act as ties to prevent rafters from spreading, the nailing at the joist splice is as important as nailing of the joist to the rafter. Since the force required to resist spreading varies with the roof load, roof slope,

TABLE I

No. of 3%-in. Nails to Tie Rafter to Joist at Each Rafter*t

Roof Rafter House Width up to 26 ft. House Width up to 32 ft. Slope Spacing, Design Roof Snow Load

in. lb./sq. ft.S

Up to 3 0 4 0 50 Up to 3 0 4 0 50 4 / 1 2 1 6 3 4 5 4 5 7 24 5 7 8 7 9 11

'Where the ridge is supported the number of nails may be reduced to three i n each case. t N a i l i n g of ceiling joist splice should contain one more nail than shown i n Table I. *The design roof snow load is assumed to be 8 0 % of the ground snow load.

rafter spacing and span, the nailing should also be varied as in Table I (an abridgement of the table shown in Hous- ing Standards) to provide a uniform fac- tor of safety against collapse.

Hip roofs

Although the foregoing description of roof framing requirements is rnost ap-

parent for simple gable rocfs. these requirements can also be applied to hip roofs. If the hip roof has a ridge section, the common rafters framing into the ridge can be treated as previously described for rafters in a simple gable roof with regard to nailing and ridge support. Regardless of the nailing, however, each end of the ridge board should

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FIGURE 3 FRAMING AROUND OPENINGS

be supported on a 2 x 4 carried down to

a bearing partition (Figure 2).

The jack rafters should be securely spiked to the hip rafters and to the wall plate with three 3%-in. nails. Since they arc supported at the upper end by the hip rafter, there is little or no tendency for them to spread at the lower ends. The nailing required to fasten jack rafters to the ceiling joists need not be greater than is required in the case of common rafters framing into a supported ridge (see note at bottom of Table I). This applies to the jack rafters on the sides as well as on the ends of a hip

roof (Figure 2). The hip rafters must be

2 in. deeper than the common rafters since they must carry greater bending loads.

Valley Sections

Valley rafters should also be 2 in. deeper than common rafters because of the greater loads supported. The jack rafters in this case extend from the ridge to the valley rafters. It is usually necessary to support the ridge over the section that carries the jack rafters

(Figure 2). The lower ends of jack'raft-

ers should be securely spiked to the val-

ley rafters with at least three 3%-in nails.

Framing around openings

When it is necessary to space the rafters and joists further apart than the normal spacing in a section of the roof to accommodate a large chimney or fire- place, the opening must be framed in such a way that the over-all strength of the roof will not be significantly affected. It is common practice to double the rafters on each side of the opening if the opening exceeds twice the normal framing spacing. Headers are installed across either end of the opening, spanning between the doubled rafters or joists to carry the loads from the cut members to the adjacent joists or rafters. When the spacing between the doubled joists ex-

ceeds 4 f t , the headers should be dou-

bled as well (Figure 3).

Summary

Because of the variations in the shapes of roofs, conventional framing can be- come quite complicated. I t is essential. therefore, for good roof design that the principles to be followed to achieve adequate roof strength be appreciated.

These are-to provide rafters with suf-

ficient bending strength to support the roof loads, and to adopt measures for preventing the rafters from spreading apart under a roof load.

Although it may be a simple matter to construct a roof frame that looks strong, one must be careful that suf- ficient attention has been given to such matters as proper nailing and ridge support to be certain that the completed

roof will be strong. Although the

strength of the framing lumber is important, adequate nailing and ridge support must also be provided to ensure adequate performance.

This paper is a contribution from the Di- vision of Building Research, National Research Council, Canada, and is published with the approval of the Director of the Division.