How Many Roof Vents Does a Roof Need?

The code default, in the residential code as adopted at OAC 748:20, is one square foot of net free ventilating area for every 150 square feet of the vented space. That can be halved to 1 in 300, but only if a specific condition is met, and one of the two available conditions does not apply in Oklahoma at all. On a 1,500 square foot attic that means 10 square feet of net free area at the default, or 5 square feet if you qualify for the reduction. Arrowhead Roofing has been ventilating northeast Oklahoma attics since 1987, and this is the calculation almost nobody runs.

Note: this is a general guide only. Ventilation requirements depend on the code edition in force, the design of the attic, and the products used. Your installer should calculate it for your specific roof rather than working from an example.

The arithmetic on a 1,500 square foot attic, both ways.

The Short Answer

Two numbers get quoted and they are not alternatives you pick between.

1 in 150 is the requirement. 1 in 300 is an exception you have to earn by meeting a condition. A great many pages state it the other way round, presenting 1 in 300 as “the code” with no conditions attached, which understates what a roof needs by half.

The Condition That Does Not Work in Oklahoma

This is the part worth knowing, and we have not seen it anywhere else.

The code offers more than one route to the 1 in 300 reduction, and they are alternatives rather than a checklist. One of them is a vapour retarder installed on the warm side of the ceiling. The other is a specified split of the ventilation between high and low.

The vapour retarder route is written for Climate Zones 6, 7 and 8. Oklahoma is not in any of them. So in Tulsa that route is simply unavailable, and the only way to qualify for 1 in 300 is the second one:

Not less than 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the attic or rafter space. Upper ventilators shall be located not more than 3 feet below the ridge or highest point of the space, measured vertically, with the balance of the required ventilation provided by eave or cornice vents.

Read that carefully, because it is doing three things at once. It sets the split. It requires the upper vents to sit within three feet of the ridge. And it requires the rest to come from eave or cornice vents, which means a roof with no intake at the eaves cannot qualify at all.

Net Free Area Is Not the Size of the Hole

The single most common mistake in this calculation.

Net free ventilating area is the unobstructed area air can actually pass through. Every vent has a screen, a louvre, a baffle and a frame, and all of them obstruct. The code requires openings to be protected against rain and snow, and specifies mesh between one sixteenth and one quarter of an inch, which is exactly what does the obstructing.

So you do not calculate net free area from the dimensions of the opening. You take it from the manufacturer’s published rating for that vent, add up the ratings, and compare the total to what the roof needs. A box vent might be rated at 50 square inches. Getting to 720 square inches takes rather a lot of them, which is the point at which most people discover their roof is short.

How Attic Ventilation Works

Watch this video to learn more about how attic ventilation helps to keep your roof performing its best.

Intake and Exhaust, and a Rule That Is Not a Rule

You will read that code requires a 50/50 split between intake and exhaust. It does not, in two separate ways.

At the default 1 in 150 ratio the code specifies no split at all. The 40 to 50 percent upper figure exists only as a condition of the reduction, and it describes the upper portion rather than a half and half arrangement.

And the industry recommendation is not 50/50 either. The asphalt roofing manufacturers’ association puts intake at 50 to 60 percent of the open area and exhaust at 40 to 50, and states plainly that the intake amount should always exceed the exhaust.

The practical version: intake should equal or exceed exhaust, never the reverse. An attic with plenty of ridge vent and blocked or absent soffit vents is the common failure, and adding more exhaust to it makes matters worse rather than better.

The Part That Costs Money Later

Ventilation is not only a code question. It is a warranty question, and this is where an abstract calculation turns into a number on an invoice.

A house with inadequate ventilation does not qualify for the top manufacturer warranty tiers. Those houses drop to a lesser tier, which covers the product but not the labour. So a ventilation shortfall that costs a few hundred pounds of vent to fix can quietly cost the difference between a lifetime labour warranty and none at all.

It is worth asking about before the roof goes on rather than after, because the assessment happens at installation.

What Changes the Answer

Three things change the calculation for a specific house.

Whether the attic is vented at all

An unvented assembly, typically one insulated with foam at the roof deck, runs under an entirely different set of provisions with its own conditions. None of the arithmetic above applies to it.

Whether the eaves can take intake

Some houses have very little soffit, or have soffit vents that were painted over or blocked by insulation decades ago. Without intake at the eaves the reduced ratio is unavailable, and the ventilation will not work well regardless of what is at the ridge.

What is already up there

Existing vents count. The calculation starts with what the roof has, and quite often the answer is that it needs a few more rather than a new system.

Related Questions

Does ventilation affect how long the roof lasts?

Yes, and it is the half of the equation nobody looks at. An attic that cannot breathe cooks the deck and the shingles from underneath. How long does a roof last? covers it alongside the material ranges.

What else does the Oklahoma code require?

Four state amendments to the roofing chapter, plus the edition changeover. What building code applies to roofs in Oklahoma? sets them out.

Should I expect this to come up on a replacement?

It should, because the assessment happens at installation and it affects the warranty. How do you prepare for a roof replacement? covers what else to sort out before the day.

See Also

The two answers closest to this one are what actually wears a roof out, and what else the code requires here.