Compliance with ANSI/TPI 1–2014 requires maintaining member to member gaps at less than 1/4 inch. An exception would be for floor truss chord splices where the limit is 1/16 inch.
Let us take a look at two different situations, the first being a roof truss and the second being a 4 x 2 floor truss.
While building trusses, the code calls for tight joints. In a roof truss where member to member contact exists, the joint calculation for design assumes one member is hard against the other members. In a compression situation, the force will compress the area consuming the gap. When this happens, it creates a change which effectively reduces the member length and changes the resultant geometry. Often this will create additional deflection that was not considered in the original design. Excessive deflection is a design restraint that should be limited in order to achieve a good design.
Sometimes an argument is made that, if the connection is in tension, this gap is not significant and will not contribute to additional deflection. This may be true in a few cases, but certainly not in all cases.
In fact, the situation referred to as stress reversal can occur – and this changes force directions on members. An example would be a truss designed for a snow load but experiencing a wind load with uplift. Comparing the two loads cases, the truss members are either in compression or in tension, depending on the load case. This is why member to member joint gap is always a consideration.
In a floor situation, stress reversal can occur when loading is unbalanced and/or dynamic. In addition, due to the very high loads, compression chord gap of any significance will result in compressing the gap causing deflection. That is why the chord gap in floors is limited to 1/16 inch.
In a situation where member gap occurs, there are several remedies. The most common approach would be to remove the connectors and re-situate the web material chord material so the gap is eliminated. Another option would be to insert a proper metal shim fabricated specifically for this procedure. The shims require a means of fastening that will hold or capture the shim. Most fabricators do not use this method though, because the fix appears to the public like a repair or substandard fabrication.
The bottom line is that gaps will happen – but what we do about them is what actually matters.