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Collar ties or rafter ties: we are not sure © Daniel Friedman at InspectApedia.comRoof Framing with Collar Ties & Structural Ridge Beams

Roof Framing Suggestions, Canadian guidelines:

Canadaian roof structure framing suggestions, codes, guides for gable roofs, low slope roofs, cathedral ceilings. This article excerpts from & provides copies of roof framing suggestions from Canadian building codes and the CMHC Wood Frame House Construction Guide.

Framing any roof but particularly a gable roof, low slope roof, and cathedral ceilign roofs requires collar ties, rafter ties, and possibly a structural ridge beam to support the roof and to prevent ridge sagging and wall spreading.

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Intermediate Roof Collar Ties as Compression Supports: Canadian Roof Framing Specifications

Sagging rafters and wall spread on low slope roofs © Daniel Friedman at InspectApedia.comProper placement & connections of collar ties and ceiling joists are required for gable and hip roofs in order to avoid sagging rafters and worse, collapsing roofs. Here we describe Canadian roof framing guidelines.

This article series describes and illustrates the different types of support that prevents roof sagging and wall bulging at buildings, including definitions of collar ties, rafter ties, and structural ridge beams.

Without the proper support of rafter ties or a structural ridge, a typical gable or sloped roof will sag downwards while pushing the building walls outwards towards a catastrophe.

We include sketches of collar ties, rafter ties, and structural ridge beams as well as illustrations of collapsing and collapsed structures where these roof rafter ties were lost or omitted.

[Click to enlarge any image]

A cathedral-ceiling roof framed with no structural ridge and no ceiling joists (sketch shown above) risks suffering two problems: sagging rafters, worse on low-slope roofs, and outwards spreading of the building walls as the roof loads press down on the structure. High collar ties won't prevent these problems.

Below we illustrate that particularly for low slope roofs (slopes less than 1/3), even if ceiling joists are present to prevent wall spreading, a high collar tie may not prevent sagging in rafters.

Sagging rafters on low slope roofs (C) Daniel Friedmamn

Al Carson, Carson Dunlop Associates, Toronto, was kind enough to explain that Canadian authorities have a different view of roof sagging and wall spread prevention than that offered for collar ties and rafter ties at ROOF FRAMING TIES & BEAMS.

Below: a "collar tie" placed closer to mid-span on long rafters on a low slope roof, combined with the protection of the ceiling joist connections, can prevent both rafter sagging and wall spreading.

Alternatives to this construction such as use of a structural ridge beam or a supporting dwarf wall or knee wall supporting rafters at mid span and resting on a partition wall below can also afford this protection.

In the sketch below we think that the "collar tie" can be in compression (rather than tension) when it is placed at any point from mid-span on rafters to locations in the upper portions of the roof structure.

How to prevent rafter sagging on low slope roofs © Daniel Friedman at InspectApedia.com

Instead of high collar ties or low rafter ties, the Canadian CMHC's Canadian Wood-Frame House Construction guide shows use of an upper 1/3 span horizontal member between opposing rafters, designed to serve as a compression member to prevent rafter sagging.

These compression members, called "collar ties" in Canadian literature, are placed "near the mid point of the rafter" to help prevent rafter sagging. If the compression members would be longer than 8 feet in this location, they should be braced to resist buckling.

For rafters in roofs that slope 1:3 or more, intermediate support is usually provided by 38 . 89 mm (2 . 4 in. nominal) collar ties nailed to the side of each pair of rafters. Since these ties are in compression and subject to buckling, ties more than 2.4 m (8 ft.) long should be supported against lateral deflection.

This can be done by nailing a 19 . 89 mm (1 . 4 in. nominal) continuous member at right angles to the collar ties near their centre with three, 76 mm (3 in.) nails at each end (see Figure 83). - CMHC 2014

Note: Both collar ties and ceiling joists nailed between rafters at the top plate are also used in Canada and are shown in the CMHC's guide in the same figure.

The Canadian text in the CMHC Canadian Wood Frame House Construction Guide (live link given below) suggests that those authors, describing collar ties as "intermediate roof support" but the document's illustration (Figure 83 given below) shows the collar ties in the upper 1/3 of the rafter length, where those members might indeed be in compression rather than tension. Our associate, Al Carson points out that

The rafter span is the horizontal projection of the rafter rather than the rafter's actual length. That can explain why a steeper roof resists sagging more than a low-slope roof. For different-pitched roofs using rafters of identical length, the steeper roof rafters will actually be spanning less distance.

Putting the horizontal support or collar tie (in Canada) at the "midpoint" of the rafter length makes sense, since locating the collar tie or "rafter tie" at any point other than the middle of the rafter's length will fail to minimize the rafter span.

That document's illustration (shown just below) presumes that ceiling joists are in place - a feature that, if properly connected to the rafter ends and wall top plate, will indeed prevent wall spreading.

The ceiling joists will certainly be in tension, not compression. For low slope roofs or where there are no ceiling joists, all sources require other roof support such as a structural ridge beam or a supporting cripple wall, knee wall, or dwarf wall. - Ed.

To prevent the rafter ends from moving outward, nail the ceiling joist to the side of each pair of rafters (Figure 83). - CMHC 2014

Roof Framing Details, Canadian CMHC Wood-Framing guide - InspectApedia.com

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Canadian Building Code for Intermediate Support for Rafters & Joists

Three horizontal rafter tie or support locations define collar tie, rafter tie, mid-span tie, and compresson, netural, or tension forces (C) Daniel Friedman at InspectApedia.comIn our sketch the mid-span rafter tie is shown as under "neutral" force. That's really only true in comparison to the forces on the higher collar tie in the upper 1/3 of the roof or the lower rafter tie in the lower 1/3 of the roof.

In practice, temporary loads placed on the center of the roof such as from snow, workers, or bundles of shingles during re-roofing will press inwards and the mid-span rafter tie will be under compression.

Below is an excerpt from the Ontario Building Code (consistent with the National Building Code, I believe). I added the Imperial measurements in blue.

Ontario Building Code 9.23.13.7. Intermediate Support for Rafters and Joists

(1) Ceiling joists and collar ties of not less than 38 mm by 89 mm (2x4) lumber are permitted to be assumed to provide intermediate support to reduce the span for rafters and joists where the roof slope is 1 in 3 or greater.

(2) Collar ties referred to in Sentence (1) more than 2400 mm (8 ft) long shall be laterally supported near their centres by not less than 19 mm by 89 mm (2x4) continuous members at right angles to the collar ties.

(3) Dwarf walls and struts may be used to provide intermediate support to reduce the span for rafters and joists.

(4) When struts are used to provide intermediate support they shall be not less than 38 mm by 89 mm (2x4) material extending from each rafter to a loadbearing wall at an angle of not less than 45° to the horizontal.

(5) When dwarf walls are used for rafter support, they shall be framed in the same manner as loadbearing walls and securely fastened top and bottom to the roof and ceiling framing to prevent over-all movement.

(6) Solid blocking shall be installed between floor joists beneath dwarf walls referred to in Sentence (5) that enclose finished rooms.

Keep in mind that a raised-ceiling or a cathedral ceiling will not be framed with ceiling joists positioned as shown in figure 83 in the CMHC document. For framing a cathedral ceiling or a raised-ceiling brought higher than the bottom 1/3 of the roof rafters you'll want a structural ridge (shown below) or other specially-engineered framing design.

See ROOF FRAMING TIES & BEAMS.

Structural Ridge Beam © Daniel Friedman at InspectApedia.com

Supporting Low-Slope Roof Rafters - CMHC Advice

A roof that slopes less than 1:3 should be vertically supported at the peak.

This can be accomplished by providing a 38 × 140 mm (2 × 6 in. nominal) ridge beam supported at 1.2 m (4 ft.) intervals by 38 × 89 mm (2 × 4 in. nominal) vertical struts (see Figure 84). A load-bearing wall may be used instead of the ridge beam. Since the ridge support does not result in outward thrust of the roof members, continuous ties between the lower ends of opposing rafters are not necessary.

Ridge beams are also required for steeper pitched roofs when the outside ends of the rafters cannot be tied together to resist thrust.

Intermediate support for rafters in roofs that slope less than 1:3 is usually provided by a dwarf-bearing wall (see Figure 84) built in the same way as a load-bearing partition, except that a single top plate may be used where the rafters are positioned directly over the studs.  - CMHC 2014

Both the CMHC guide and other expert sources offer other approaches to supporting rafters in pitched roofs, including struts nailed between the rafter side and a load-bearing partition below.


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