How much unevenness is acceptable in ceiling joists?

[champy]

About a year ago I replaced my garage roof with a new flat roof. As far as I know I used the correct timbers for the 3.6m unsupported span - 45x175mm on 400mm centres with 1 row of noggins. The static load is about 0.25kn per sqm.

I’ve just finished plaster boarding and noticed a pronounced bow in one section. After measuring I discovered one joist is nearly 15mm lower in the centre than the rest. The rest vary by around 5mm. The wall plates they rest on are exactly level and flat and there is no pooling water anywhere on the roof.

Is this amount of variation common? Is it acceptable? Could it have just been caused by wood movement during the preceding year?

 

[johnny2007]

Wood not straight.
I have always put the natural bow up so to have some sort of arch more difficult to sag.
Don't know if it's something roofers do, but it makes sense to me.
Anyhow, unless you want to start removing joists, live with it, it's a garage.

 

[mattylad]

Live with it.
Or take the plaster board down and get the plane out.

 

[big-all]

Wood not straight.
I have always put the natural bow up so to have some sort of arch more difficult to sag.
Don't know if it's something roofers do, but it makes sense to me.
Anyhow, unless you want to start removing joists, live with it, it's a garage.

exactly the correct way to do it as the other level joist and load will drag it more level

 

[tony1851]

Did you use C24 grade timber?

 

[catlad]

Crown up and quality of timber is principal .

 

[champy]

Did you use C24 grade timber?

Yeah properly graded.

 

[champy]

exactly the correct way to do it as the other level joist and load will drag it more level

I never thought to do this but it makes a lot of sense, although the top of the roof is perfectly flat with no variation, almost like this one joist is thicker in the middle. Is it common to have slight unevenness on the underside or is it just shoddy workmanship/ subpar timbers?