FHB Logo Facebook LinkedIn Email Pinterest Twitter Instagram Tiktok YouTube Plus Icon Close Icon Navigation Search Icon Navigation Search Icon Arrow Down Icon Video Guide Icon Article Guide Icon Modal Close Icon Guide Search Icon Skip to content
Subscribe
Log In
  • How-To
  • Design
  • Tools & Materials
  • Videos
  • Blogs
  • Forum
  • Magazine
  • Members
  • FHB House
  • Podcast
Log In
How-To

Venting the Roof

With today's construction methods, the minimum code requirements may not be enough.

By Kevin Ireton Issue 61

Synopsis: Few topics in residential construction have caused as many arguments as how, why, and even whether to ventilate a roof. This staff-written article explains the basics on this issue. It also includes a look at manufactured ridge vent systems and shows how a site-built ridge vent could be constructed.

On the subject of roof ventilation, the Canadians are fond of quoting Gus Handegord, one of their premiere building scientists. Handegord says, “If you go into an attic in the winter, find frost on the underside of the sheathing and there are no vents, you should put some in. But on the other hand, if you go into an attic in the winter, see frost on the the underside of sheathing and there are vents, close them up.”

Although his advice sounds contradictory, Handegord simply recognizes that attic ventilation is a relative issue. How, where, when and even whether you should vent your roof depends on the type of construction, level of insulation, climate, site and probably a dozen other variables. The truth is, very little research has been done on roof ventilation, and the experts don’t always agree on what’s best.

In this article the term “roof ventilation” refers to a vented air space over the insulation in an attic or a cathedral ceiling. Roof ventilation should not be confused with house ventilation. In a tightly built house, mechanical ventilation is needed to expel stale air and to bring in fresh air for breathing and combustion.

Codes and warranties

All the building codes require roof ventilation. Most of them enforce 1:150 and 1:300 ratios. According to these ratios, you need a minimum of 1 sq. ft. of net free vent area (NFVA) for every 150 sq.ft. of ceiling area below the roof. (The NFVA is the total area of the vent opening and takes into account the area blocked by any screening or louvers.) If you use a vapor barrier on the ceiling, however, or if you divide the ventilation evenly between high and low vents, then the minimum is 1 sq. ft. of NFVA for every 300 sq. ft. of ceiling. The vapor barrier reduces the amount of moist air infiltrating the attic, hence reducing the need for ventilation. Dividing the ventilation between high and low vents increases its efficiency, which also reduces the amount of ventilation needed.

Manufacturers of asphalt and fiberglass shingles also agree that ventilation is a good idea, at least for the longevity of their products. Ventilation under the roof deck (to reduce heat buildup) is a condition of shingle warranties. Some manufacturers (Georgia-Pacific, for instance) even go so far as to stipulate the 1:150 ratio.

A little history

Prior to the 1930s, people didn’t vent their roofs. They didn’t have to. Roofed with wooden shingles over open sheathing, and with little or no insulation in the attic, houses weren’t tight enough to trap heat or moisture. But the next 60 years saw the advent of building papers, asphalt shingles, plywood, insulation and vapor barriers, which had the same effect as putting a lid on a pot of boiling potatoes. As a result, moisture — from breathing, cooking, bathing, plants — began to accumulate inside houses. Households weren’t generating more moisture, but tighter houses were retaining more of it.

For more photos, drawings, and details, click the View PDF button below:

Sign up for eletters today and get the latest how-to from Fine Homebuilding, plus special offers.

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters
×

Get home building tips, offers, and expert advice in your inbox

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters
View PDF

New Feature

Fine Homebuilding Forums

Ask questions, offer advice, and share your work

Log in or create an account to post a comment.

Sign up Log in

Become a member and get full access to FineHomebuilding.com

Up Next

Video Shorts

Featured Story

Avoid Silicone-Caulk Failures

Silicone caulk has its advantages, especially in certain applications, but other sealants are often better choices.

Featured Video

A Modern California Home Wrapped in Rockwool Insulation for Energy Efficiency and Fire Resistance

The designer and builder of the 2018 Fine Homebuilding House detail why they chose mineral-wool batts and high-density boards for all of their insulation needs.

Related Stories

  • Control Layers for a Remodel
  • Replace Rotten Boards in a Roof Overhang
  • Do I Need a Gutter Apron?
  • Podcast 598: Members-only Aftershow — Long-Lasting Paint Jobs

Discussion Forum

Recent Posts and Replies

  • |
  • |
  • |
  • |
  • |
  • |
View More Create Post

Highlights

Fine Homebuilding All Access
Fine Homebuilding Podcast
Tool Tech
Plus, get an extra 20% off with code GIFT20

"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.

Get home building tips, offers, and expert advice in your inbox

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Video

View All Videos
  • Replacing Old Shingles with a New Metal Roof
  • Architectural Strategy for a Unique Skylight
  • Podcast 571: Pooling on Flat Roofs, Irrigation Timers, and Single-Pane Windows
  • Combat Extreme Weather With a FORTIFIED Roof
View All

Roofs

View All Roofs Articles
  • A Flat Roof in a Cold Climate
  • Reduce Sheathing Waste
  • Architectural Strategy for a Unique Skylight
  • Podcast 571: Pooling on Flat Roofs, Irrigation Timers, and Single-Pane Windows
View All Roofs Articles

BOOKS, DVDs, & MERCH

Shop the Store
  • Pretty Good House
    Buy Now
  • Code Check Complete 3rd Edition
    Buy Now
  • 2023 Tool Guide
    Buy Now
Shop the Store

Fine Homebuilding Magazine

  • Issue 318 - October 2023
    • Make Mudsills Square and Level
    • Turn Up the Heat With Induction Cooktops
    • The Fine Homebuilding Interview: Lloyd Alter
  • Issue 317 - Aug/Sept 2023
    • Finishing Drywall With Seamless Results
    • A Flat Roof in a Cold Climate
    • Compact Cordless Shop Vacs
  • Issue 316 - July 2023
    • Timber-Framed Solar Canopies
    • Build a Transom Above a Stock Door
    • Understanding Toilet Design and Efficiency
  • Issue 315 - June 2023
    • How to Craft a Copper Roof
    • 5 Lessons from Building in a Cold Climate
    • The Advantages of Cellular PVC Siding
  • Issue 314 - April/May 2023
    • 7 Options for Countertops
    • Tool Test: Wood-Boring Bits
    • Critical Details for Ductless Heat Pumps

Fine Homebuilding

Follow

  • YouTube
  • instagram
  • facebook
  • pinterest
  • Tiktok
  • twitter

Newsletter

Get home building tips, offers, and expert advice in your inbox

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Membership & Magazine

  • Online Archive
  • Start Free Trial
  • Magazine Subscription
  • Magazine Renewal
  • Gift a Subscription
  • Customer Support
  • Privacy Preferences

Taunton Network

  • Green Building Advisor
  • Fine Woodworking
  • Fine Gardening
  • Threads
  • About
  • Contact
  • Advertise
  • Careers
  • Copyright
  • Terms of Use
  • Site Map
  • Do not sell or share my information
  • Privacy Policy
  • Accessibility
  • California Privacy Rights

© 2023 The Taunton Press, Inc. All rights reserved.

X
X
This is a dialog window which overlays the main content of the page. The modal window is a 'site map' of the most critical areas of the site. Pressing the Escape (ESC) button will close the modal and bring you back to where you were on the page.

Main Menu

  • How-To
  • Design
  • Tools & Materials
  • Video
  • Blogs
  • Forum
  • Reader Projects
  • Magazine
  • Members
  • FHB House

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Popular Topics

  • Kitchens
  • Business
  • Bedrooms
  • Roofs
  • Architecture and Design
  • Green Building
  • Decks
  • Framing
  • Safety
  • Remodeling
  • Bathrooms
  • Windows
  • Tilework
  • Ceilings
  • HVAC

Magazine

  • Current Issue
  • Past Issues
  • Magazine Index
  • Subscribe
  • Online Archive
  • Author Guidelines

All Access

  • Member Home
  • Start Free Trial
  • Gift Membership

Shop the Store

  • Books
  • DVDs
  • Taunton Workshops

More

  • FHB Ambassadors
  • Reader Projects
  • Podcast
  • FHB House
  • Customer Support

Account

  • Log In
  • Join

Newsletter

Get home building tips, offers, and expert advice in your inbox

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Follow

  • YouTube
  • instagram
  • facebook
  • pinterest
  • Tiktok
  • twitter

Join All Access

Become a member and get instant access to thousands of videos, how-tos, tool reviews, and design features.

Start Your Free Trial

Subscribe

FHB Magazine

Start your subscription today and save up to 70%

Subscribe

We hope you’ve enjoyed your free articles. To keep reading, become a member today.

Get complete site access to expert advice, how-to videos, Code Check, and more, plus the print magazine.

Start your FREE trial

Already a member? Log in