Rim Board Thickness, Which One Should I Use?

Weyerhaeuser manufactures a wide variety of Trus Joist Engineered Wood Products that are used for rim board applications. It can get quite confusing deciding which one to use with all the thicknesses available. The table below provides a breakout of products typically used for rim board across the country.
 

Rim Board (Not all sizes may be stocked locally)

Product

Depth Range

1 1̷8” TJ®-Rim Board

9.5″ to 16″

1 ¼” 1.3E TimberStrand® LSL

9.5″ to 24″

1 ½” 1.3E TimberStrand® LSL

9.5″ to 24″

 

Beam material used as rim in certain markets

Product

Depth Range

1 ¾” 1.55E TimberStrand® LSL

9.5″ to 16″

3 ½” 1.55E TimberStrand® LSL

9.5″ to 16″

5 ¼” 2.0E Parallam® PSL

9.5″ to 18″

 

As you see there are a possible 6 different thicknesses to choose from. The two primary reasons for the difference are load capacity and sufficient width on the edge to accommodate nailing from shear walls and the floor diaphragm above. Below, I’ve discussed each option in more detail.

1 1̷8” TJ®-Rim Board

First let’s discuss 1 1̷8” TJ®-Rim Board. This is typically a prescriptive solution to meet the limited vertical and lateral load transfer needs of the simplistic structures covered in the International Residential Code (IRC) for one and two family dwellings. TJ®-Rim board is a code accepted solution via 3rd party code evaluation reports and has associated engineering properties for bending applications that sets it apart from other 1 1̷8” rim board products in the market that comply with ANSI PRR 410-2011, which at present does not permit the product for use in bending applications. This product is rarely used in engineered structures due to the limited lateral load capacity and it does not meet the National Design Specification (NDS) spacing provision for rows of nails (2.5 times the diameter between rows, and 2.5 times the diameter for edge spacing). With only 1 1̷8” width, a row of nails from a shear wall and a row of nails from the floor sheathing do not comply with the NDS provisions when designing outside the IRC.

1 ¼” 1.3E TimberStrand® LSL

1 ¼” 1.3E TimberStrand® LSL products are used in many residential structures that do not meet the prescriptive code rules, where loading conditions demand higher capacities, or where higher quality and performance are desired. It has code evaluated design properties that allow this versatile product to be used as stair stringers, headers and cantilevers in addition to rim applications. At 1 ¼” width, it can accommodate one row of nails from the shear wall (4” on center is the minimum spacing for a 16d nail) and one row from the floor sheathing while complying with the NDS provisions. In addition, lateral load capacity is not limited to the 150 plf maximum toe-nail capacity that is referenced in the building code.

1 ½” 1.3E TimberStrand® LSL

1 ½” TimberStrand® LSL is very similar to the 1 ¼” thickness, but does serve a few unique purposes. Engineers often call out metal connectors to connect the bottom/top of the rim to the wall plates for lateral load transfer. These framing clips utilize a 1 ½” nail. With the 1 ½” rim, the full published capacity for the connector can be achieved.  With 1 ¼” rim, there is a 10% reduction in capacity due to not meeting full nail penetration and for 1 1̷8” material there is currently no published testing or reduction known. The other advantage of 1 ½” rim is higher vertical load capacity (6480 plf for depths of 16″ and less) for multi story structures. Nail spacing limitations are similar to that of 1 ¼”.

1 ¾” 1.55E TimberStrand® LSL

1 ¾” material, although designated as a beam material, plays a crucial part in structures, especially those in high seismic areas or in multifamily/multistory structures. These highly loaded structures tend to have very aggressive nailing patterns for shear wall connections to rim. Many times the shear walls require 2 rows of nails, plus a third row from the floor sheathing. Following NDS provisions, you can achieve three rows of nails into the 1 ¾” section. Another situation occurs when specified nailing uses a 2″ on center spacing for the nails (Which exceeds the nail spacing allowed into the edge orientation of engineered wood products). When nail spacing is exceeded, the wood may split and thus reduce the capacity and no longer meet the intent of the design. A common practice is to make that 1 row of 2″ on center to 2 rows of 4″ on center (staggering the rows is a good practice). With the additional row of nails from the floor sheathing, it becomes 3 rows total. 1 ¾” rim does have more vertical load capacity then the previous products mentioned, but rarely does that control thickness choice.

3 ½” 1.55E TimberStrand® LSL

The 3 ½” thickness are also used in higher demand applications when nailing or proprietary screw patterns cannot be achieved in a 1 ¾” thickness. You can achieve more rows of nails than the 3 that are permitted in a 1 ¾” thickness and you can get a second row of proprietary ¼” diameter self tapping screws into the edge.

5 ¼” 2.0E Parallam® PSL

Finally, I will mention 5¼” Parallam® PSL used as rim material. For many multifamily structures, stairways and mechanical shafts need to meet a 2 hour fire-resistance rating. There are many complications when using gypsum board for fire protection. This is especially true when you need two layers of gypsum board between a hanger and a rim board in the shaft. Besides being difficult to organize with trades, hanger manufacturers do not publish capacities when connected to two layers of gypsum. The IBC, section 722.1 (2012) allows you to calculate fire resistance based on char rate. With Parallam® PSL having similar char rates to Douglas-Fir, the 5 ¼” Parallam® PSL will achieve a 2 hour rating, and still have a core section to support load with no gypsum attached.

Please refer to Technical Bulletin #206 for guidelines for Trus Joist Products when selecting your rim board nail spacing. For additional Trus Joist Rim Board literature, visit our rim applications webpage.

Jim Anderson
Jim Anderson