The Minnesota State Building Code requires deck guardrails to be present when the deck surface is more than 30″ above the ground. Guardrails must be at least 36″ high, and must be designed in such a manner as to prevent 4″ sphere from passing through. This is all pretty easy to build, as long as you own a tape measure. The last requirement is that guardrails be able to withstand 200 pounds of pressure along the top rail. Now here’s the kicker – according to some, the guardrail must also be constructed with a safety factor of 2.5.
A safety factor of 2.5 means that the guardrail must be constructed to withstand 500 pounds of pressure along the top rail. This means that guardrails must be constructed to be ridiculously strong, and most methods of conventional contruction aren’t good enough. A guardrail constructed with 4x4s attached to the deck with lag bolts will actually rip the bolts out of the deck long before 500 pounds of pressure can be applied to the top rail.
The easiest and surest way to construct a guardrail that will withstand 500 pounds of pressure is to use metal brackets that are designed just for this purpose. A couple manufacturers that make brackets just for this purpose are DeckLok and Simpson Strong-Tie. They’re also the ones that promote the safety factor of 2.5 (go figure). According to reports that I’ve read by both of these manufacturers, metal brackets are the only way to achieve the 500 pound rating. I’ve talked to deck builders that say there are other ways to achieve this rating, but I’ve never seen any testing reports that verify this. Nevertheless, most building officials in the Twin Cities area aren’t picky about guardrail details, and they allow guardrails to be built without metal brackets.
To read an in-depth report with several different construction methods and tests, go here. The only problem I have with this article is that the testing they’re doing is based on a single post, and no guardrail can ever be constructed with a single post. As soon as other posts and right angles are incorporated in to the guardrail, it will get much stronger. When I inspect houses and I find flimsy guardrails (like the one in the video below), I tell my clients to repair or replace them. When I find guardrails with 4x4s that have been notched, I tell my clients that the guardrail might not be strong enough to prevent a large, falling adult from breaking it.
Reuben Saltzman, Structure Tech Home Inspections – Email – Minnesota Deck Inspector