Rooftop equipment supports in Manchester, NH: dunnage, grillage and RTU frames, set and welded to the engineer's drawings. Each post and beam goes where the drawings show, so the unit lands on steel that can carry it. Call (603) 660-2011, or send the support drawings with the unit schedule.
Rooftop dunnage is steel that holds equipment up off a roof. Posts and beams carry the unit's weight past the roof deck and into the building's structure. So the membrane and the deck don't take the load. On a commercial roof it usually sits under heating units, fans or runs of pipe.
A rooftop unit, or RTU, is the packaged heating and cooling box you see on store and office roofs. Those units are heavy. So each one has to sit on something that can hold it.
Many units come with a curb from the manufacturer, a box that sits on the roof framing. Dunnage is the other way to carry one, because the unit goes on its own steel posts and beams. Those land on the beams or columns below.
The difference matters on a weak roof. A curb puts the unit's whole weight on the framing right around its edges, whatever that framing is. Dunnage can reach past that framing to stronger steel. So for a heavy unit, the engineer may well pick dunnage.
On an old roof, look underneath first. The load has to reach steel that can take it. So the old joists matter as much.
That's why the work often starts with knowing which joists are up there. The Steel Joist Institute keeps a form for identifying an existing joist from field measurements. Joists that come up short get reinforced first, which the page on bar joist reinforcement covers.
Grillage is a grid of steel beams laid across several joists, under the unit and over the roof framing. A heavy unit sitting on just two joists would put its whole weight on those two alone. On a grillage, its weight spreads out over more of the roof instead. That's the whole point.
An RTU frame is the frame the unit itself sits on. It's sized to that unit's footprint, so a new unit gets a frame drawn for it.
Both went onto one roof at a Massachusetts supermarket, where a general contractor brought us in for the renovation. That job covered the joists under the frames too.
Swapping an old unit for a new one doesn't always mean new steel. But when the footprint or the weight changes, the old frame may not fit. The engineer decides whether it stays.
Some equipment needs more than a frame under it. A steel platform gives the people who service it a floor, so they aren't working off the roof deck. It can wrap around a unit, a fan, a tank or anything else that needs regular service at height.
Once a platform is up off the roof, someone can fall from it. So it needs guardrail and a safe way up, and both come with their own rules. Those live on the Handrail and Guardrail page and the Ladders and Roof Hatches page.
We set and weld the platform steel to the drawings, the same way we set the frame under the unit.
Piping and ductwork cross a roof too. Pipe supports are the stands and frames that hold those runs up, so the pipe never rests on the membrane. Inside a building, the same kind of steel carries pipe from the structure.
We set them at the spacing and height the drawings show, whether the run is pipe or duct. So the pipe doesn't sag, and its weight lands where the engineer planned it.
Rooftop steel touches three trades, so the order matters. Ours goes first.
We set and weld the support the engineer has detailed. Then the roofer flashes every post that goes through the roof, because each one is a hole in the membrane. The mechanical contractor sets the unit on the finished steel once the roofer is done around the posts. The general contractor keeps that sequence and decides who goes when.
Even a few posts mean hot work. So we sort out the permit and the safety plan before any welding on that roof. The commercial welding page goes further into hot work inside a building.
Rooftop frames are one piece of the other steel work in buildings we take on. That runs from joists under the roof to lintels in the walls.
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No. A light condenser can sit on a pad or a manufactured stand, so welded steel would be overkill. A heavy unit on an old roof is different, because its weight has to reach steel that can carry it. The engineer makes that call from the unit's weight and what's below.
It depends on the job, so there's no figure we can print. The unit's weight and footprint, the support type, getting steel onto the roof and any joist work all move it. Your own number is free, written up for a big roof and given by phone for a small one. The drive to your site is already in it.
The engineer does. The support is part of the structural design, so it's sized on the drawings with its connections below. We set and weld it to those drawings and the engineer's specs. Structural inspectors then check the work against the same sheets, during the job and at the end.
Call (603) 660-2011 once the units are picked and the support drawings exist.
Then send the drawings and the unit schedule, with the weight of each unit if you have it. Tell us how we get onto the roof and where the job is. Add when the units are due, so the steel can go in ahead of them.
