I have trusses on four foot centers. I plan on installinf between the trusses 2x4 material every 4 feet so I can hang drywall or some other board for a ceiling. My idea is to install these boards flat rather than on edge so I can use r-30 insulation which is 30 inches wide. Also, there are some places where I have wire that is in the way if I put the boards on edge. I plan to nail or screw each end of these boards between the bottom chord of the trusses.
I know the edge of the board is stronger than when layed flat. WIll I have trouble with sagging? SHould I also space every 2 feet or will 4 be enough?
That's completely unclear. Firstly, the trusses have a SPAN in addition to their spacing, and if you want to add boards inbetween (i.e. parallel to the trusses) those will have to match that span.
To prevent the ceiling from sagging, the parallel members, like the trusses, will have to be sized to suit the span. It's unlikely that a single 2x4 will work, but a 2x2/ half-inch-plywood/ 2x2 composite member might work well.
If, on the other hand, you intend to run boards perpendicular to the trusses, there's no benefit to running them 'flat', you can choose any convenient spacing, and the four-foot span is probably OK with ceiling material and insulation weight with a 2x4...
My old (1976) house had plasterboard ceilings fastened on 16" centres, after about 10 years there were wavy lines in the ceiling where the plasterboard had sagged. I guess that 24" centres would sag faster and deeper.
That's not old at all. My current house was built in 1929, and the prior house 1896. Neither have sagging plaster. (It is wet plaster on lath, not plasterboard, in the original parts of the house. But there is no mystery to making non-sagging ceilings.)
? ... had plasterboard ceilings fastened on 16"
How thick is the sagging plasterboard? I'm guessing that it is simply too thin for a ceiling application.
Watch out! All the lumber at Lowes, at least in Oregon, is green, not kiln dried. It will shrink in all dimensions, including length, as it dries. The 2x4 will also twist as it dries. You might want to look at kiln dried lumber and then pick out the boards that have no twisted as they dried. Yes, they cost a little more than green lumber.
My first house (1966) was much better built. The ceilings, also 12 mm plasterboard, were fixed with reinforced plaster draped over the joists. I was only there 10 years and did not notice any sagging. snip
12mm Gyprock. There is no problem if it is installed properly the old fashioned way. Modern methods using adhesive and tek screws do not support the board from above.
12mm / 1/2" Gypsum Drywall is the international standard, and it won't sag when hung properly - meaning 16" OC joists, fastened with drywall screws properly - 6" OC on the perimiter, 12" OC on the joists in the "field" in the middle. Coarse screws on wood, Fine screws on steel studs. Construction adhesives optional.
You can not cheat and use staples or regular nails, they will not hold. And drywall nails really don't hold that well on ceilings, even the ring-shank style.
You have to use a proper drywall screw gun or other depth control - If you break the drywall outer paper layer all the strength is gone, the screw will pull right through the gypsum layer when loaded.
24" OC joist spacing is OK to put drywall on, but IMNSHO I think it's pushing your luck. Stick in purlins as nailers at all the join points.
If you have a Barn or Shed with roof trusses or joists at 48" OC spacing, you have to put in 2X4 purlins crossways at 16" OC to give yourself something to screw the drywall up to. You simply can't get enough screws on the perimeter of a sheet of drywall to hold the whole sheet up, there has to be some support in the middle.
And you can't go too crazy on the insulation if it's anything heavy like blown-in rock-wool or cellulose. If you use double layers of fiberglass batts, put the second layer crossways, so the weight is mostly on the joists.