General attic storage weight = "Tank Load"?

[epsilonGreedy]

I am looking at a quote for roof trusses where the design has been specified for a "tank load". Is a tank load good enough for an above average amount of general domestic storage in a boarded attic space? Access to the attic will be via a pull down loft ladder.

 

The attic will not have a water tank and a bit of reading indicates a tank in an roof design is assumed to be up to 300 litres. My concern is that over the years the amount of stuff in the attic might creep over 300kg.

 

Background: I asked the truss manufacturer to incorporate attic trusses in the quote because we intend to develop a small storage space 4m x 2.5m up in the attic, this will not have standing headroom because the ridge is only 1.8m due to the 30 degree pitch.

 

The truss clear span is 5.4m and the attic trusses have a predicted weight of 40kg with 147mm x 26mm section timber. My intuition is that 1" wide timber for an attic truss is a bit feeble on 600mm centres.

[PeterW]

It’s fine - tank load is normally across 3 trusses anyway for design purposes on 6x2 bearers. Insulation is normally the bigger problem - 3-400mm unless you use rigid PIR 

[epsilonGreedy]

8 minutes ago, PeterW said:

It’s fine - tank load is normally across 3 trusses anyway for design purposes on 6x2 bearers.

 

 

Yes I was considering PIR under the attic floor. Should I plan to add additional bearers over the top of the truss joists to spread the load before laying 18mm or 22mm flooring?

 

Here is the design for one of the attic trusses. If the 26 means a 1" thickness, this feels too slender to me.

 

 

[PeterW]

TR26 is a timber grading. 

[epsilonGreedy]

18 hours ago, PeterW said:

TR26 is a timber grading. 

 

 

Ah, that makes more sense than 26mm thick roof trusses.