Coastal Metal-Roof Install: Correcting the Crew's Plan Before Penetration
A crew is installing a closed-loop glycol system on a two-story coastal house: four 4 × 10 ft flat-plate collectors (40 ft² each) on a 24-gauge standing-seam metal roof with 24-in. rafter spacing and a design wind speed of 110 mph. The attic reaches 60 °C in summer; the one-way pipe run to the mechanical room is 45 ft with six 90° elbows. The foreman's plan: (1) 1/2-in. Type L copper headers to save cost; (2) 1/2-in. fiberglass pipe insulation outdoors; (3) S-5 clamps lag-bolted 4 ft on center into the metal panels; (4) expansion tank at the highest point in the attic with no isolation valve; (5) no vacuum breaker because "the pump runs all summer"; (6) sensor wire dropped through an unflashed 1-in. hole. The roof is only accessible by ladder. Before work starts, the general contractor asks you to review the plan.
1. Check the pipe sizing and pump implications for this array and run. Show the numbers.
Read the worked answer
Collector area = 4 × 40 = 160 ft². Typical flat-plate design flow is 0.03–0.05 gpm/ft², so the target is 4.8–8 gpm.
1/2-in. Type L copper (ID ≈ 0.545 in., area ≈ 0.00162 ft²) at 6 gpm (0.0134 ft³/s) gives a velocity near 8 ft/s - well above the 4–5 ft/s practical limit for a glycol loop - with friction loss on the order of 20–25 psi per 100 ft, plus six elbows and 90 ft of total equivalent length. A small circulator rated at a few psi of head cannot deliver that.
3/4-in. copper drops velocity to roughly 4 ft/s; 1-in. drops it to about 2 ft/s. For a 45-ft one-way run with six elbows, specify at least 3/4-in., preferably 1-in. headers, and then select the pump from the actual head calculation at design flow, not from a catalog rule of thumb.
Undersized piping is not just inefficiency: low flow produces high ΔT, collector overheating, glycol degradation, and nuisance high-limit lockouts. Correct the plan before any pipe is cut.
2. Evaluate the mounting, insulation, and attic details, and state what must change.
Read the worked answer
Mounting: lag-bolting S-5 clamps 4 ft on center into the metal panel is wrong on two counts. The clamp must grip the standing seam; fasteners must engage the purlin or rafter structure below, not the panel skin. At 110 mph, a 40 ft² collector can see uplift loads on the order of 1,000+ lb each; 4-ft spacing with panel-only attachment is a tear-off waiting to happen. Require an engineered layout: seam clamps at the manufacturer's spacing, attachments into structure, and a wind-load calculation per ASCE 7 for the actual exposure.
Insulation: 1/2-in. fiberglass outdoors in a coastal environment will wet out and lose R-value; UV and birds destroy the jacket. Use 1-in. closed-cell or jacketed EPDM insulation with UV-resistant cladding, sealed at every fitting, and separate the hot supply and cold return so they do not thermally short.
Attic: 60 °C ambient plus glycol at stagnation means the expansion tank and any elastomeric components must be rated for that. An expansion tank at the highest point becomes an air trap; it belongs near the pump suction, supported, with an isolation valve and drain so it can be serviced without dumping the loop.
The vacuum breaker is not optional: it allows air in during drain-down, prevents negative pressure collapsing the collectors, and is needed for filling and venting. "The pump runs all summer" ignores winter, power failures, and pump trips.
Sensor wire: a 1-in. unflashed hole is a leak path. Use a proper flashing, drip loop, grommet, and sealant, and keep the wire out of the roof's water path.
3. The roof is ladder-access only and the crew will be up there for two days. What safety and documentation requirements apply?
Read the worked answer
Fall protection: at two stories (roughly 20+ ft to grade), work at height requires fall protection - guardrails, personal fall-arrest with anchorage rated for the load, or a documented equivalent. A 28-ft extension ladder is access, not a work platform. Plan anchor points, rescue-at-height provisions, and a competent-person review before anyone goes up. Ladder setup on soft coastal ground needs foot stabilization and 4:1 angle.
Other site controls: verify the roof's structural capacity and walkability with the metal-roof manufacturer; use roof jacks or standoffs rather than walking seams; barricade the ground below; check for overhead power lines during collector lifting; and have a hoisting plan for 40 ft² collectors (two-person minimum, or a crane/boom).
Documentation: record the structural attachment design, sealant and flashing details, torque values for seam clamps, insulation and UV-cladding specs, and pressure-test results. Photograph every penetration before and after sealing. Provide the homeowner the SRCC listing information for collectors and system, the glycol type and concentration, and the O&M manual with the annual fluid-test schedule. If any of that is missing, the job is not ready for inspection.