How big a hole in roof for gas pack?

[tucsonjwt]

I am considering having two gas packs installed on my roof for a remodel. How big would the holes in the roof be for these two units? I am looking at American Standard 1.5 ton and 3.0 ton units. Some manufacturers make it seem that you need a hole about as big as the whole unit (for servicing?)(like 3' x 4'), but I don't understand why you would need a hole any bigger than the supply and return ducts (like 15" x 39"). I am concerned because this is an old house with only 2x4 rafters on roughly 2' centers, and I don't want a structural issue if I can avoid it. Thanks.

[sammy37]

You dont need a hole the size of the unit. I am old fashion, and prefer to use an elbow out the back of the unit instead of mounting it downshot.

[tucsonjwt]

If going straight down, do you still need the big hole?

[gevans]

Nope. You can use two separate holes, one for supply, one for return. They can straddle a rafter if you like, and only need to be large enough for the duct plus the insulation.

[t527ed]

are the rafters big enough to hold it up ??????

[sammy37]

2by4 rafters should be fine, I see many houses around here with two a/c units next to each other and some with 2 units and a couple of swamp coolers on top of that. Makes you wonder how they can hold all that weight, but they do. My house has a heavy tile roof with a composition pathway where my a/c sits and my trusses are only 2by4.

[tucsonjwt]

Thanks. That's what I thought. The gas packs will be separated on the roof - one for each of 2 dwelling units (it's a small duplex) - so the weight will be spread out. This is an 82 year old house with a mostly flat roof. The "craftsmen" who built these old houses constructed what amount to site built roof trusses - 2 x 4 top and bottom chords with 1x6 tie backs on the outside walls with more bracing down the middle. The original deck is 1 x 6 tongue and groove and I put 1/2" CDX over that. I will add additional roof bracing around the units once they are in place. The roof is actually now better than more modern houses I have owned - partly due to actual size 2 x 4s and what appears to be all old growth fir lumber. I just want to minimize the size of the holes if I can.