Let’s Talk About Spacers and Shims

A few weeks ago, we looked at pier cap blocks, so now it makes sense to talk about spacers and shims.

Just so we are on the same page, a spacer can be used to fill in the space between the cap block and the frame (chassis) if that space is more than can be shimmed (1″). Sounds simple enough, right??? Well think again.

Double block pier, concrete footing under the ground vapor barrier, double 4″ solid masonry cap blocks, 2″ hardwood spacers and hardwood shims. Nice job!

The Model Manufactured Home Installation Standards describe a spacer as “Hardwood plates no thicker than 2” nominal in thickness or 2” of 4” nominal concrete block…”. (3285.304(c)(3)). But the individual manufacturers’ installation instructions often provide more options, and sometimes confusing details.

Most manufacturers allow 2” x 6” hardwood spacers. One spacer for single stack block pier, two spacers for a double stack block pier (one per cap block). If you stick with this, you will be in good shape.

 A recently revised instruction manual now defines a spacer as “hardwood, southern pine, or Douglas fir larch dimensional lumber 1x or 2x (2 layers maximum)”.  This is the first time I have seen pine in the same category as hardwood, not to mention allowing the spacer to be 1” thick. So, for those of you that like to use 5/4 pressure treated decking boards as spacers this appears to be one of the first DAPIA approvals for this method.

2″ hardwood spacer on a 4″ solid masonry cap block. Too bad the strap is loose.

When checking your installation manual, look at both the table and the text. Many instructions will specify “Nominal 2” thick boards” in the Pier Minimum Specification Table. BUT, if you turn to the text at “Install Shims” generally on the next page, they allow hardwood or concrete, not just 2” thick boards.

spacer text

Pressure treated hardwood dimension lumber is mentioned in a few manuals, but I have yet to see pressure treated hardwood. Maybe it’s a regional thing, but where I come from, hardwood is not pressure treated.

I even saw a few manufacturers’ installation instructions that don’t mention spacers. Just be sure to double check with your manufacturer to be certain that you are following their instructions.

When it comes to the overall size of the spacer, most pier illustrations show a 2” x 6” spacer board. While the installation instructions don’t specify the length of the spacer, the illustrations appear to show them the full length of the pier cap (16”). 

While there appears to be variation in the materials prescribed by different manufacturers, make sure to stay away from plywood or OSB! These materials will delaminate rather quickly and will certainly cause problems.

When it comes to shims, the installation instructions are more straight forward.

Use proper size spacers and hardwood shims! Not cedar shims like this!

Hardwood shims, 4” x 6” x 1” thick, must be used in pairs, and cannot occupy more than 1” total between the cap block (or spacer) and frame (I-beam). And like the spacers and cap blocks, a double block pier would require two sets of shims, one set atop each cap block.

Plastic Shims

Most manufacturers allow for plastic shims of sufficient capacity. I believe that plastic shims are a great choice for piers in porch areas, where water can pass between the decking boards and lead to premature decay of wooden shims. Also, the plastic shims are grooved so that they cannot slip apart. Again, I did see one manufacturer allows southern pine or Douglas fir shims in addition to hardwood. But generally plastic is the only exception to hardwood shims.

Hopefully, this post will encourage you to examine your current practices regarding spacers and shims, and to be certain you are following the manufacturers’ installation instructions. If you have questions, call the manufactuer’s Quality Assurance Manager and ask him to clarify their requirements.

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