Guardrail strength

It’s easy to look at a deck’s guardrail and know if it meets the code’s size requirements.  The openings in the guardrail should be small enough so that a four-inch diameter sphere can’t pass through, and it should be at least 36 inches high.  International Residential Code sections 321.1.2 and 321.1.3

But it’s usually not possible to look at a guardrail and know for sure if it meets the code’s strength requirements, although it’s often possible to determine with some certainty that a guardrail isn’t strong enough.  Here are the rules for guardrail strength for a residential deck from the International Residential Code Table 301.5 Minimum Uniformly Distributed Live Loads.

A guardrail must be able to withstand a concentrated force of 200 pounds applied at the top.

Sometimes the top of a guardrail also serves as a handrail.  If this is the case then the guardrail must withstand that 200 pound force applied in any direction along the top.  If the guardrail is not also a handrail then that 200 pound force can be applied either horizontally away from the walking surface or vertically straight down.

The infill components of a guardrail must withstand a horizontally applied load of 50 pounds over an area of one square foot.

It’s generally not possible or feasible to test a guardrail to these specifications.  Certainly no city inspector is going to do this, even if he was able to lug around the calibrated equipment needed.  And no home inspector is going to do this and risk damaging the seller’s home (or hurting himself).

But engineers working for suppliers of deck hardware or for academic institutions can build guardrails in their labs using various techniques, and then very accurately measure the loads required to damage them.  This can give prescriptive guidelines for how to – and how not to – build a deck guardrail.

These prescriptive guidelines describe the various ways to build and attach a guardrail.  A city inspector will typically want to see that the guardrail was built using one of these techniques.  I won’t go into those details here (it would take a looooong time), but of course they’re all available online.  Here’s one very good place to start looking:  How To Build Code-Compliant Deck Railings & Posts (buildingadvisor.com)

Note that these rules apply to deck guardrails made from any material – wood, metal plastic composites – and also guardrails that use cable systems.  Guardrails that include glass have some additional rules regarding the type and strength of the glass.