How to Build an Insulated Cathedral Ceiling

Vaulted Ceiling Properties

Vaulting depends on balancing the lateral roof pressure against counter pressure created by the angle of the vault and the support of the walls. Remember balancing two upright playing cards against one another when you were a kid. If the pitch was too steep, the cards toppled to one side. If it was too shallow, the cards fell flat. When you got the vault angle just right, the cards held each other up.

The principle is the same for vaulted roofs, and there are many possible designs. In fact, Tim Carter of Ask the Builder notes that there are over 100 possible types of trusses, and many are appropriate for residential construction.

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What Are the Costs?

Costs vary according to size of your space and how much the framing and other components have to be changed and moved. Here’s a breakdown for a basic vaulting project:

  • planning, structural engineer, and permits: $1,000
  • demolition and disposal: $3,700
  • reinforce framing, remove ceiling joists: $3,600
  • moving, adding wiring: $2,900
  • insulation: $1,300
  • drywalling: $3,900
  • painting: $1,600

Total: $18,000

Building Basics

Incorporating a vaulted ceiling is best done during the original construction of the house, or, if desired, as a part of a new addition to the house. While retrofitting a vaulted ceiling is possible, it’s cost-prohibitive for most homeowners because it involves extensive structural engineering to modify existing ceiling joists or roof trusses to accommodate the new vault.

Vaulted ceilings can be constructed by either stick-framing, which means attaching each joist and rafter individually, or by setting roof trusses that come engineered from a truss manufacturer with the vaulted space already accounted for. Either scissor trusses or vaulted parallel chord trusses, both of which are constructed from multiple wood members to serve as the roof structure, are used to create vaulted ceilings. A representative from the truss manufacturer will consult with the contractor and have the trusses engineered to suit. Trusses are delivered to the job site on via trucks, and the builders set them in place, their undersides forming the desired vault shape.

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However they are constructed, installing a vaulted ceiling is a job for the professionals—not a DIY project—that requires adherence to local building codes and engineering specs. Any experienced contractor or builder should be able to construct a vaulted ceiling, but as you do your research, ask to see examples of their work in this regard.

Photo: Zillow Digs home in San Anselmo, CA

Photo: Zillow Digs home in San Anselmo, CA

How to Vault a Ceiling

  1. First, head to the attic. If you see a complicated framework of 2x4s held together with perforated metal plates, your roof is framed with trusses. You’ll have to remove the entire roof structure to vault the ceilings. But if it’s framed traditionally with big lumber rafters, the roof can stay in place.
  2. Second, measure the depth of the rafters. Anything less than 5½ inches isn’t deep enough to insulate sufficiently, unless you use spray-in foam. With batt insulation, you have to ­allow for a 1-inch air space to ventilate the ­underside of the roof. Spray-foam insulation doesn’t need an air space.
  3. Third, check for mechanical complications: ducts, plumbing vents, HVAC equipment, and wiring. Anything in the area to be vaulted will have to be relocated. You’ll need an HVAC contractor to determine whether your heating system can handle the increased volume of a room with a vaulted ceiling. ­Also, before you start the work, consult a—structural engineer to make sure the collar ties are in the right place to prevent the walls from spreading apart—after the ceiling joists are removed.
  4. The carpentry itself isn’t that difficult. Once the collar ties are fastened horizontally between opposite pairs of rafters, 1x3s are nailed to the underside of the ties to provide solid backing for the new ceiling surface.

Ventilation

Without the proper ventilation, your attic space can become a major problem.   The potential for mold increases where there are high levels of moisture building up in the space.  Premature degradation of roofing as well as framing members are another concern when ventilation is substandard.  So if you are planning on adding a vaulted ceiling, be sure to include a thorough review of the future ventilation of the space above.  Here are a couple items to make certain are included in the design…

  • Air Space – include a 2″ space between the insulation batts and the underside of the roof decking.  This will allow any moisture that gets in there to escape
  • Vents – make sure you have enough venting for the space…ridge and soffit vent systems are some of the most effective (and keep in mind, each rafter bay should be vented)
The only time you may be able to get rid of venting in a vaulted ceiling is if you use a closed cell spray foam applied directly under the roof decking.  This method of installation is expensive but effective.   The biggest concern with this setup is with a roof leak.  The spray foam insulation may conceal moisture that is getting through the roof…which could cause deterioration in the roof framing over time..

Spray Foam at Vaulted Ceiling

Types of Vaulted Ceilings

Arched vaults come in a variety of shapes, from the basic, semi-circular barrel that runs the length of the ceiling from one end to the other, to perpendicular intersecting barrel arches, known as “groin vaults.” Also popular are arched ceiling planes that narrow as they rise to meet at a single center point, forming a dome shape called a “domical vault.” A variety of narrow, wide, large, or small arches can be combined to create custom vaulted effects. In the case of a cathedral vault, where the interior ceiling is parallel to the exterior roof line, installing skylights is a simple process.

Photo: istockphoto.com

Photo: istockphoto.com

Vaulted Rafters

Stick framing a roof with rafters is the old-fashioned and still most common method of obtaining a vaulted ceiling. An engineer must determine rafter size, placement and pitch based on span and other considerations. Rafters used to frame a vaulted roof are typically larger than rafters used to frame a roof that relies on ceiling joists for support.

The first step is the positioning of a substantial ridge beam, which is supported at the ends of the roof. The builder attaches the vaulted rafters, one at a time, to the ridge beam and to the wall plates. A special “bird’s mouth” cut is made at the point where the rafter meets the wall plate. This transfers some of the lateral pressure of the rafter downward through the wall.

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