Structural Support - Building Problem Solutions
Structural Ridge Beam
by John F Mann, PE
 
Contents
Introduction
Basic Gable Roof
Support For High Ends Of Rafters
Design Example - Outward Thrust Force
Design of Structural Ridge Beam
Modifications To Existing Roof
 
Introduction
 
Although the names are unfortunately similar, a ridge beam serves a much different purpose than a ridge board. 
 
A structural ridge beam is required when independent support for high ends of rafters is necessary. Conditions for which a ridge beam is required are described after discussion of a basic gable roof.
 
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Large PSL ridge beam (center, top) supporting LVL roof joists. End of ridge beam supported by short post on header beam.
 
Note steel straps for wind uplift resistance.
 
 
 
 
 
 
End of ridge beam supported on column within exterior wall.
 
Framing designed by Structural Support for architect.
 
Location: Cinnaminson, NJ
 
 
 
 
 
Two intersecting ridge beams; Avon By The Sea, NJ.
 
House under construction (March 2010).
 
Design by others.
 
 
 
 
 
 
Basic Gable Roof
 
A basic gable (A-frame) roof is typically built over an attic floor, with attic floor joists running parallel to rafters. When low ends of rafters are connected to outer ends of attic floor joists, the resulting triangular A-frame acts as a simple truss to resist roof loads (such as snow). High ends of rafters are typically installed up against a nominal 2-inch thick ridge board.
 
The ridge board is used primarily for construction purposes, to facilitate installation of rafters. For houses built about 100 years ago, a gable roof was most often built without any ridge board; high ends of opposing rafters were actually butted together.
 
For any gable roof built with traditional rafter construction, outward thrust force occurs at low ends of rafters.This can be visualized by considering that rafters forming the two roof slopes try to flatten when subjected to downward load on the roof. Unless there is resistance to outward (lateral) movement, low ends of rafters will move outward horizontally, resulting in collapse of the roof.
 
The only way that low ends of rafters can move outward is if high ends also move downward. Therefore, if high ends of rafters are supported to prevent downward movement, low ends will not move outward. 
 
High ends of rafters will not move downward if support is provided (at high ends) that can resist force from applied roof loads. Such support could be provided by a wall or ridge beam.
 
In general, a ridge beam is used instead of a continuous wall (in attic space) since the wall is more costly and would divide the attic space. 
 
Support For High Ends Of Rafters
 
International Residential Code (IRC) and International Building Code (IBC) requires a structural ridge beam for roof slope less than 3 on 12. See IRC 2006; R802.3.
 
However, a structural ridge beam may be required for several reasons, as described in this section.
 
Support for high ends of rafters is typically necessary for the following conditions;
 
(1) No attic floor (ceiling) joists, such as for a sloped (cathedral) ceiling. This condition generally results in lack of resistance to outward thrust force from low ends of rafters. Although top plate of exterior support walls may offer some resistance to outward force, such resistance is not dependable without specific design, which ends up being difficult. The typical double 2x4 top plate simply does not have much capacity to resist horizontal force as a beam between intersecting walls. Joints in top plate must be considered.
 
Collar ties installed high (above support walls) may not provide adequate restraint to outward movement, similar to the condition without attic floor joists. Careful evaluation of collar ties (also known as rafter ties) should be performed for relatively long span roof and especially when height of collar ties (above low ends of rafters) is more than one-third of the ridge height.  
 
(2) Attic floor joists perpendicular to rafters. This condition may occur when the span of attic floor joists is excessive for floor joists parallel to rafters, perhaps due to lack of interior support (bearing) wall. Although resistance to outward thrust force can be provided for this condition, the use of a structural ridge beam is more often used today. 
 
(3) Low-slope roof, which results in relatively high outward thrust force at low ends of rafters. Connections between rafters and attic floor joists might have to be bolted (instead of nailed) to provide adequate capacity. Use of structural ridge beam eliminates such costly connections. A roof slope of 4 vertical to 12 horizontal or less can easily result in excessive outward force for standard nailed connections. 
 
For uniform load on a gable roof, outward force is equal to one-fourth (25%) the total load (on both slopes of entire roof) times the inverted ratio of roof slope. This result is derived by considering one half of the gable roof as a free-body and taking moments about the ridge (which is a hinge, such that moment is zero). The unknown (to be solved for) is the horizontal tension force in the attic floor joist, which is the same as outward thrust force (applied by rafter to attic floor joist).
 
Total vertical load per linear foot is calculated as (Dead Load pressure + Live Load pressure) times total width (span) of the gable roof (from wall to wall). Pressure is in pounds per square foot (psf).
 
For high slope roof (greater than 8 on 12), dead load pressure might be increased to account for roof slope (divide weight per foot on horizontal by cosine of slope angle). However, such modification is often ignored for design since design dead load is (should be) already conservative. Snow load (for horizontal span) is not modified since it is defined as load per horizontal unit.
 
The same result can be obtained by considering the A-frame as a simple truss, with uniform load converted to point loads at each joint. Net reaction at each support is then 1/4 of total load (not half). By "method of joints", tension in the attic floor joist is then this net reaction force times inverted ratio of roof slope.
 
Princeton NJ; Feb 11, 2010
Snow on residential roof.
Depth of snow is more than one foot. Weight of snow can vary greatly, depending on water content. However, this relatively wet snow may have weighed about 15 pounds per square foot.
 
Design Example - Outward Thrust Force
 
Outward thrust force is calculated for a simple gable roof with the following conditions;
 
Total Span = 24 feet (wall to wall)
Slope of 4 on 12 (both sides)
Dead Load = 10 psf
Snow Load = 20 psf
 
Tension force in attic floor joist, which is the same as outward thrust force from rafters, is calculated as follows;
 
  (1/4) x (10 psf + 20 psf) x (24 feet) x (12 / 4) = 540 PLF
                                         PLF = Pounds per linear foot (parallel to ridge)
 
For rafters spaced at 16 inches, outward force is then 720 pounds, calculated as 540 PLF times (16 in / 12 in/ft).
 
Results for a wide variety of practical conditions are listed in the 2001 Edition of Wood Frame Construction Manual (WFCM), Table 3.9.
 
Depending on various factors, such as type of nail (common, box), size of nail (10d, 8d) and wood properties (species, grade), shear capacity of one nail is in the range of 90 to 110 pounds. Even though the standard Load Duration Factor of 1.15 for snow load can be used to increase capacity, at least six (6) nails are required to resist design loads for each connection (rafter to attic floor joist). The WFCM (Table 3.9A) lists 7 nails required.
 
It is likely that, in areas where roof snow load is 20 psf or greater, a high percentage of houses built with 4 on 12 roof slopes have not been built with 6 nails connecting rafter to attic floor joist. The roof remains standing of course due to high safety factors for design loads and material properties. Roof sheathing may also restrain outward movement of rafters, by acting as a diaphragm (between gable endwalls). However, slippage at low ends of rafters, along with sagging of ridge board, can be seen for many such houses over time, especially if only 2 or 3 nails were installed.
 
For longer total roof span, greater snow load or lesser slope, outward thrust force can easily begin to require excessive number of nails that might split the wood.
 
Design of Structural Ridge Beam
 
Structural ridge beam is required for conditions previously described. However, if connections between low ends of rafters and attic floor joists do not have adequate capacity to resist outward thrust force, then a ridge beam should also be provided.
 
For downward (gravity) loads, design of a structural ridge beam is similar to any other beam (see "Basic Beam Design"). However, for low to moderate roof slopes, the ridge beam must also be designed to resist upward force from wind uplift pressure on roof surfaces.
 
Rafters on each side of the ridge beam are considered to be simply-supported beams. Horizontal span of rafters (not sloped span) is typically used to calculate design loads (dead + snow) on ridge beam even though dead load is actually slightly greater (for sloped span). For high slope roof (greater than 8 on 12), dead load should be calculated using sloped length. 
 
The ridge beam must have adequate supports, typically provided by built-up wood columns. Columns must be sized to prevent excessive slenderness. 
 
Each column must have adequate support from attic floor framing and bearing walls below that support attic floor framing.
 
For most residential construction today, LVL (Laminated Veneer Lumber) type beams are used for a structural ridge beam. However, a beam built-up (assembled) from sawn-lumber can also be designed.
 
For the example above, uniform gravity load to be resisted by ridge beam is 160 PLF dead load plus 320 PLF snow load. For 10 feet spacing of supports, design load applied to each support is at least 4,800 pounds, depending on span conditions (simple, continuous). This relatively large load likely may require reinforcement of attic floor framing and even floor framing below, down to foundation.
 
An often overlooked requirement for proper ridge beam design is resistance to wind uplift force from rafters. Adequate tiedown connections between ridge beam and supports must be provided. Steel straps are typically specified. Wood side-pieces extending up from columns can also be used effectively. Base of columns must also be tied down to resist wind uplift force from the ridge beam. Additional tiedown connections may be required to ensure that wind uplift force is distributed through the building structure, down to foundations.
 
Modifications to Existing Roof
 
A condition that often occurs is raising one slope of gable roof to increase usable space in the attic. The resulting low-slope roof on one side of the ridge board changes the way roof loads are resisted such that a ridge beam should be provided.
 
The existing ridge board might serve as a ridge beam if adequate supports can be installed, such as a wall or closely-spaced columns. However, reinforcement of the existing ridge board (along with additional supports) may be necessary to form a ridge beam with adequate design capacity.
 
New interior supports for the ridge beam must have adequate support from existing (or new) elements below.
 
 
 
 
 
 
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