Cedar shake roofing systems are one of the most temperamental roof coverings. Why? Wood needs to breathe. This means that the roof needs to be built with very specific ventilation. If the moisture in the face of a shake dries out ahead of the back side, you get cupping. Cupping weakens the shake and allows moisture a way to get under it. Thus, the shake must shed moisture from bottom to top by keeping the greater heat on top.
All untreated shakes age more rapidly on unvented sheathing. Ventilation should be provided on solid sheathing for the best results. The Cedar Shake and Shingle Bureau has advised this for decades. However, both extra steep and the shallowest pitched roofs on larger homes are both susceptible to extreme heat buildup. Releasing this heat is a must! It is necessary to have vents on a cedar roof.
Older cedar roofs in the Chicago area are prone to extreme heat build-up in the attic beneath them due to their extreme sizes and pitches. Nearly every roof we’ve replaced has clearly needed more ventilation added to give it a better chance at lasting 50 years.
Good ventilation is essential and can be achieved in a variety of ways:
- Applying felt under shakes
- Radiant barrier sheathing will reflect up to 97% of radiation and can keep your attic temperatures up to 30 degrees cooler
- Ridge vents under the ridge caps come in a variety of choice to fit the situation. the correct ridge vent products can facilitate the ease of installation of ridge units.
- Proper insulation
- Extra vents and some with fans
Cedar Shake Roof Ventilation on Older Installations
Older roofs (20+ years) are likely to have been installed with inadequate ventilation, primarily on the ridges and large step slope expanses. There may be ridge vents but they may not be large enough to move air flow well enough. In some cases, expanses that should have ridge vents are void of them. This is usually caused by an ill-informed installer construction a cedar shake roofing system as they would an asphalt/composite system.
Part of the reason poor ventilation is common with older roofs in antiquated logic. Cedar roofs will keep the attic up to 15-20 degrees cooler than asphalt shingles. This lead to the thinking that less ventilation was needed. In extreme summer heat the same insulation properties can work in reverse and a runaway heat build-up can occur in the attic.
The oldest cedar shake roofs may not have any decking. The shakes are nailed directly to tightly spaced rafters without anything in between. Though it’s rare to come across an “ancient roof” it’s not as problematic as coming across a shake roof that was applied on top of an asphalt/comp shingle. This has been done time and time again by those DIY hackers and the shadiest of “roofers”.
Sub-par ridge vents are still used to this day. It’s easy for a cedar shake specialist to spot a “hack job” by the type of vent product used, the way they’re attached, and the mess of caulking involved.