L/480 vs L/600 For Residence

BruceWard · Apr 4, 2011

My Floor Truss provider recommended 18" 2x4 floor trusses on 16" centers. These trusses will provide a L/600 floor.

My Wife and I would like to reduce some 17'4" trusses to 12" floor trusses while keeping the longer 22' 8" trusses at 18". The 18" trusses provide additional space for utilities. The 12" trusses will run above and below our living and dining rooms increasing the ceiling height by 1' (6" drop, 6" rise).

The heat and air guy says he can work with the 12" trusses in those areas.

The truss guy agrees that the 12" trusses will provide L/480 but would prefer that we stick with the 18" trusses for L/600. All building standards and codes that I can find are for L/360 for wood floors or L/480 for tile or commercial.

We are moving forward with the 12" L/480 trusses above and below the living and dining rooms.

Here is a diagram of the truss layout below the living and dining rooms, the trusses above are almost the same with the only change being the location of the stairs.

I would appreciate your advice.

Thanks

dstig1 · Apr 4, 2011

What do you need an L/600 floor for? As you indicated, L/360 is pretty common (though not satisfactorily stiff enough for some folks) and L/480 is required for tile work above, as a minimum.

So what advice are you looking for? Will it work? Sure - your truss guy already told you, it would seem, and he is in the best position to know.

It will cause some complications in framing as they will have to make sure to step the beam or supporting wall somehow to take up the difference for the shallower trusses. The risk is that the framers will mess it up as the more unique the framing, the more likely there will be mistakes. You also would not want something like tile to brigde from the stiffer section to the L/480 section as the differential stiffness will likely cause problems (flexes more on one side than the other = cracks).

Dunno if this will help you or not...

BruceWard · Apr 4, 2011

Thanks Dave,

Nothing bridges from the L/600 to the L/480 section on any floor or ceiling.

I accept that the framing will be complicated by the truss change. On the positive side 22' 8" and 17' 4" trusses will be easier to transport and place than 40' truss would have been.

I agree that L/480 is plenty stiff but want to hear other peoples thoughts.

bdeboer · Apr 5, 2011

Have you decided on your floor covering in this area? That may be the thought process behind the L/600.

BruceWard · Apr 5, 2011

The flooring in that area will be wood or carpet.

LD1 · Apr 5, 2011

BruceWard said:
The flooring in that area will be wood or carpet.

L/600 is just overkill and added cost.

For just flooring, especially covered with carpet, I doubt you would notice the difference

dstig1 · Apr 5, 2011

BruceWard said:
The flooring in that area will be wood or carpet.

If that is the case, then make it all L/480.

schmism · Apr 7, 2011

as a previously employment was supplying engineering services for truss fabricators. L/480 would be a min for a floor.

L/600 is generally required if tile is used.

for long spans L/480 may not be sufficient especially when total deflections approach 3/4" of an inch. At which point many people may complain about a "spongy" floor despite being within design parameters.

looking at your layout i can tell you that this will be especially noticeable if you have multiple and or a single off center chase locations in combination with the center support. The ruleing condtion on sys42 that you have shown are unballanced loads across chase openings. So 90% of the rest of the truss will make much better than l/480 or l/640 but for the locations around the chase openings. in which case double top cords are offten used just over the chase locations. This adds minmual cost to the truss while adding significantly to the precived stiffness of the floor

BruceWard · Apr 7, 2011

The span of the 12" L/480 floor trusses will be no more than 17' so the max deflection is .42".

The span of the 18" floor trusses will be around 22'. At L/600 that would be a very similar max deflection of .44.

The subflooring will be 3/4" Advantech that will be screwed and glued. I am hopeful that the 3/4" sub floor will provide some additional stiffness.