Clay and concrete tile is everywhere in Arizona, and it handles the sun better than many roofs. But under every tile roof is still an attic, and that attic still needs to breathe. Here's how tile roof ventilation works, where it falls short, and what helps.
Tile roofs already have a head start
- The Florida Solar Energy Center tested tile roofs against a black-shingle roof in The Measured Summer Performance of Tile Roof Systems and Attic Ventilation Strategies in Hot, Humid Climates. It noted that "Tile roof systems are a popular roofing system in southern climates." The vented, counter-batten tile roof cut ceiling heat flux by 48% compared with black shingles, and the standard direct-nailed tile roof by 39%.
- The researchers explained why: "the tile has a large thermal mass, and the temperature on the back of the tile stays quite low. There is also an air space under the tile from the tile's barrel shape."
- Oak Ridge National Laboratory found much the same in Steep-Slope Assembly Testing of Clay and Concrete Tile Roofs: "All tiles, whether direct-nailed or installed on battens, have a sub-tile venting occurring up along the underside of the tile traveling from soffitt to ridge."
In plain English: air moving under the tile carries some heat away before it reaches the roof deck. That's a real advantage, but it isn't the same as ventilating the attic.
The tile vents, but the attic still needs its own
The air channel under the tile is outside the roof deck. Your attic is inside it. The attic needs its own intake and exhaust.
The Tile Roofing Industry Alliance says so in its Concrete and Clay Roof Tile Installation Manual: "These codes recognize that the proper ventilation is a necessary component of any successful steep slope roof system." It adds: "Generally building codes require that a minimum net free ventilating area for attic vents be a 1:150 ratio of the attic space being ventilated."
That matches IRC R806.2, which reads, "The minimum net free ventilating area shall be 1 / 150 of the area of the vented space." It allows 1/300 when the vents are balanced, with part of the vent area up high and the rest down low.
Where tile roofs come up short
Two details are common on tile roofs:
- A mortared ridge. FSEC notes that "Normal building procedure is to seal the cap tiles to the ridge of the tile roof with a concrete “mud.”" ORNL did the same in its tests: "the ridge vent is closed to simulate conventional installations in the western states."
- Closed-off eaves. At the bottom edge, the tile courses often start with a bird stop. The TRI manual notes the "Bird stop may be either solid or vented."
When the ridge is closed, the attic's exhaust usually comes from individual vents set into the tile field. Those are often dormer vents (the curved "eyebrow" vents) or O'Hagin-style vents shaped to match "S" tile. They're passive, so they depend on wind and rising warm air to move anything.
ORNL showed what happens when the top of the roof can breathe. With the ridge open, "the bulk air temperature near the underside of the slate tile was 10°F cooler than that observed for the same slate tile with the ridge vent closed," and about 5°F cooler for the S-Mission clay tile. The researchers concluded "that opening the ridge caused more heat to be exhausted out the ridge."
Start with intake
Just like any attic, a tile roof attic can only push out as much air as it pulls in. Building scientist Joe Lstiburek puts it simply in Lstiburek's Rules for Venting Roofs: "Put more vents down low than up high." He also says, "Attics should be ventilated with air from the outside, not the inside."
FSEC's results back that up: "The effects of ridge venting (on this roof with a simple geometry) seem to be nominal when compared to the effect of soffit venting."
So check the eaves first: find the intake vents, keep insulation clear of them and count your net free area. Our guide How Many Soffit Vents Do I Need? walks through the math.
Bigger is not better on tile either
It's tempting to cut a big powered fan into a tile roof. But a high-CFM fan in an attic that's short on intake will pull air from wherever it can: other roof vents, gables, chimneys or the house below. We explain the problem in Why Solar Blaster Bets on Balance, Not Brute Force.
Solar-assist the vents you already have
Instead of cutting new holes in tile, Solar Blaster adds a small solar fan to the passive vents already on the roof:
- The Solar TileBlaster fits dormer ("eyebrow") vents on tile roofs. It has three solar-powered fan units that turn on one at a time as the sun gets stronger, and it still acts as a passive vent when the sun isn't out.
- The Solar S-TileBlaster is made for O'Hagin "S" style tile vents, using a 4.5W solar panel and one fan unit.
The fans run hardest when the sun is strongest, and they assist the passive system, intake low and exhaust high, instead of overpowering it. To see how hot Arizona attics get in the first place, read How Hot Does an Attic Get in Arizona?
Being honest about the limits
- A fan doesn't fix a roof. Cracked or slipped tile and worn-out underlayment let water in. That's a job for a tile roofer.
- No intake, no airflow. If your eaves are closed off, add intake before adding any exhaust.
- Every tile roof is different. Your building official has the final say on vent area.
- Research isn't a promise. The FSEC and ORNL numbers came from test buildings in Florida and Tennessee, not from your house.
- Air leaks come first. If house air leaks into the attic around lights, pipes or the hatch, seal those gaps.
The bottom line
Tile gives your roof a head start, but the attic underneath still needs balanced ventilation: plenty of intake down low and working exhaust up high. Where the ridge is closed and the vents are passive, a solar fan matched to those vents can help on the hottest days.
As Daniel Rheaume of Solar Blaster puts it, "Our products move air, which is a natural cleanser. Clothesline fresh."
Questions about your attic? Call us at (480) 747-7097 or email sales@solarblasterfans.com.
