Structural

1 | FRAMING TYPES

COMMON RESIDENTIAL FRAMING TYPES

Here at ION we generally categorize roofs into two main types of residential roof framing. There are a few other types of framing you will occasionally see, but these are by far the most common:

Conventional framing

Prefabricated wood truss framing

Conventional Framing

WOOD TRUSS FRAMING

(The biggest thing to look out for is the metal gusset plates.)

CONVENTIONAL FRAMING

It is common in the PV industry to hear conventional framing being referred to as just "rafters." Conventional is much more common on older builds or newer custom builds. The exception to that is some markets in Texas and Virginia still use conventional framing in common new construction. New construction conventional framing in these markets should almost always pass structural calculations because they should be appropriately engineered to current design standards.

Older conventional is a little trickier because a lot of them were not engineered correctly to begin with or the codes they complied with are very outdated. Be more particular when dealing with older conventional framing.

Here's a quick rundown of conventional framing:

Built on site
Consists mainly of:
- Rafters
- Joists
- Ridge board
Sometimes has collar ties or kicker supports.

These supports help with load and can be used to break up the span in our calculations

Common Conventional Framing supports:

Knee wall:

Marked by perfectly vertical members connecting the bottom beam to the top purlin attached to the rafter.
Should have the same spacing as the rafters.

Kicker Supports:

Usually attach to a proper beam at the center of the ceiling joists.
These are usually spaced at 48"

Purlin & Strut:

This type of connection is similar to kicker supports but usually has additional flat members used to more securely attach the interior struts to the purlin.
These are usually spaced at 48"

Collar Ties:

This type of support is used to bridge between rafters opposite from each other.
These should match the spacing of the rafters.

Sister Rafters:

This type of support helps strengthen existing rafters by essentially doubling the total rafter thickness.
Members of the same size, or sometimes upsized from the size of the existing rafter, are fastened directly side by side with each existing rafter.

For auditing Sister Rafter upgrades we have to make sure that they match the structural upgrade detail in the approved city plans. (especially in Prince William County we need to make sure that the nail pattern is correct)

As you can see in this example that repair detail it states that the 2x4's are "Full Length", but in the repair photos they used 2 pieces on the front section.

PREFABRICATED WOOD TRUSS

When you see prefabricated trusses, it's good news for the design. This is the easiest framing type to engineer on. Look for a series of triangular shapes with webbed supports and metal gusset plates fastening members together.

This type of framing is most common in construction from the 1970s onwards. If a home was built before the 1970s, it is a lot less likely to be truss framing but not impossible.

Quick facts about truss framing:

Pre-engineered and manufactured specifically for local structural considerations.
Consist of top chords, bottom chord, web members all connected using gusset plates.
Easier to Engineer PV on.

Over Builds

It is common for dormers to be technically categorized as a type of overbuild on the main truss roof structure, but they are usually built at the same time.

In most cases normal overbuild on trusses bear directly down on the main trusses and can be counted as part of the truss framing
Look for overbuild that is built directly over sheathing and is of the same quality as the main home.

In order for installers to access the attic for these overbuilds, Installers are okay to cut a access point for the attic.

HORIZONTAL TRUSSES

Trussed hip roofs will most likely have horizontal box trusses with vertical flat rafters running over them.

This does not allow the 2.5” depth for the lag screws to land in.
This will cause the need for alternative mounting options on the Design & Engineering side.

In this example you can see

When looking at a truss roof, the roof sections on either side of the ridge will have vertical top chord members (green).
As the trusses continue, they will follow the same orientation even as they continue under the hip sections. This causes the truss top chord to now be horizontal on the hip sections (yellow).
Most often, the sheathing is attached to flat vertical rafters on top of the horizontal trusses which can run the full or partial span of this roof section (red).

HORIZONTAL TRUSSES

When installers call in (they should know how to install them) But we should inform them that we need the (8) Screws and that they HAVE to upload a clear photo showing the proper number of screws

LESS COMMON FRAMING TYPES

TJI
Steel
Horizontal Purlins

Any framing type that is not a typical truss or conventional framing should undergo a preliminary structural review. Flag accordingly if you see this.

Horizontal Purlins

TJI

Steel

2 | ROOF SHEATHING TYPES

BASIC SHEATHING TYPES

OSB
Plywood
Lumber
Tongue & Groove

ORIENTED STRAND BOARD (OSB)

OSB is made from wood strands 8 to 15 centimeters long. It uses the whole tree and makes use of crooked, knotty and deformed trees that would otherwise go unused. As strands, they are mixed with waterproof resin and interleaved together in thick mats, which are then bonded together under heat and pressure.

Quick facts about OSB:

Most common decking (sheathing) type in newer construction.
With the right resins it can be water-resistant and also can be chemically treated to improve resistance to decay or fire.
Often seen in similar applications as Plywood and has similar strength

OSB with foil insulation

In warmer markets like AZ, NV, NM, and TX it is fairly common to see foil insulation on the roof sheathing. Most commonly, this is premanufactured on OSB.

The callout should always be OSB if you see this.

Quick facts

The foil layer of the OSB lowers the Delta-T by reflecting the thermal energy due to radiation (sun’s heat) away from the insulation layer, which keeps the top layer of the insulation just several degrees warmer than the outside air temperature. However, foil-backed OSB often requires additional thermal insulation to prevent the transfer of thermal energy by conduction and convection.
Foil-backed OSB acts as a vapor barrier, particularly when it is cold outside. If water vapor from the interior of the building moves into the attic, and the foil-backed OSB is cooler than the dew point, the moisture may condense on the underside of the barrier. When installing foil-backed OSB, builders must pick a location where the barrier is unlikely to reach the dew point temperature.

Condensation on the underside of foil-backed OSB can cause several problems:
If the insulation becomes damp, it will lose some of its insulating values
Water spots may appear on the ceilings of the living spaces (IMPORTANT)
Moisture can lead to mold which is both unhealthy to the occupants of the building and can rot the ceiling framing or wall assembly

For foil-backed OSB to perform correctly, builders must make sure there is an open air space adjacent to the foil layer not less than ¾ -inch. If the foil-backed OSB comes in direct contact with other materials, like rafters and wall framing, heat will transfer through conduction from the foil-backed OSB to the adjacent materials. One reason contractors avoid using foil-backed OSB in wall systems is because it is hard to create the air gap needed for foil-backed OSB to work properly.
The Oak Ridge National Laboratory found that roof mounted foil-backed OSBs may increase roof shingle temperatures by 2 to 10 °F. Similarly, foil-backed OSBs installed on the attic floor may increase roof shingle temperatures by 2°F or less. Unfortunately, higher roof temperatures may reduce the roof’s service life.

PLYWOOD

Plywood is the next most common type of roof sheathing. it is used in very similar applications as OSB. Look for what appear to be large sheets with a consistent wood grain.

Quick facts about plywood:

Made from soft wood veneers glued together with alternating grain direction
Cross laminated sheet grain gives it its strength
All layers are fused together with water resistant resin in a similar fashion to OSB
Comes in several different thicknesses (⅜”, ½”, ⅝”, and ¾”) depending on the span of the rafters.

LUMBER

Lumber will often look a lot like it is part of the framing, however, these horizontal members are not structural. It was common in older constructions to have this type of sheathing as it promoted air circulation around wood shake and shingle-type roofs.

Quick facts about lumber sheathing:

Also called spaced or skipped sheathing.
Composed of lumber spaced evenly over the rafters.
Used to be a standard for shingle and shake type roofs as it allows for the shake/shingles to have air circulation.

LUMBER & PLYWOOD

It is most common to see newer roofs retrofitted with new plywood or OSB sheathing on top of the existing lumber sheathing. This can affect the design due to the mounting depth of the lag screw threading into the structural members given that lumber + plywood is considerably thicker than lumber would be alone.

When you see a plywood or OSB layer peaking out between skipped lumber sheathing, the callout will need to be "Lumber & Plywood."

TONGUE & GROOVE

Tongue & Groove has a similar look to lumber but there's no "skipping" between pieces of lumber in this sheathing type.

Quick facts about tongue & groove sheathing:

Tongue & groove refers to the type of joint used for fitting lumber together in this sheathing process
Pieces of lumber are cut with “tongue” and “groove” pieces that interlock with one another

Light Reach Disqualified Structure Types:

Car Ports
Awnings
Multi Family Homes
Condominiums
Townhomes

3 | MEASUREMENTS & CALCULATIONS

BASIC CRITERIA

Structural calculations completed by engineering consist of a number of jurisdictional specifications such as: wind speed, exposure category, snow load, etc. These will be controlled by local standards that can differ from jurisdiction to jurisdiction.

It is critical that we use accurate data for general structural inputs used in these calculation that can only be obtained on site:

Framing Type
Size
Spacing
Max Span

SIZE

Size refers to the dimensions of the top chord/rafter:

Rafter:

Commonly 2x4 up to 2x10 but can be larger

Trusses:

Commonly 2x4 up to 2x12

SPACING

Spacing is the space between each vertical rafter/top chord member:

For trusses and rafters, 24” o.c. is most common
Can be 16”, 24”, 36”, 48”

SPAN

Span is a measurement that is typically derived measuring from the exterior wall to the ridge board. This span can be reduced when there are consistent structural supports which would reduce the amount of loads bearing directly down on the exterior load bearing walls.

For trusses, this factor is pre-engineered and our input can be adjusted to pass calculations. Typically, it can be estimated to be somewhere in the neighborhood of 72"
It is critical to factor in the full max span for conventionally framed roofs.

**For conventional framing, We can use structural supports such as knee walls and collar ties to reduce the span. This will help with structural calculation**

MINIMUM REQUIREMENTS FOR SUPPORT TYPES

Use the following guidelines to make your judgement regarding structural supports. If the conventional framing supports do not meet this criteria, you cannot use it to reduce the max span and it may need Preliminary Structural Review:

Knee Walls:

- Vertical supports on knee walls can have a max spacing of 24" (most of the time, this means they need to line up with each rafter when rafters have a typical 24" spacing).
- Attic supports appear to be newer than 1950
  - If the wood looks old, look up the build date. Use your discretion.
- Knee Walls can typically reduce the max span by about 33% when qualified.

Kickers:

- Vertical struts, or kickers, can have a max spacing of 48" (most of the time, this means they need to line up with every other rafter when rafters have a typical 24" spacing).
- Attic supports appear to be newer than 1950
  - If the wood looks old, look up the build date. Use your discretion.
- Kickers can typically reduce the max span by about 33% when qualified.

Collar Ties

- Collar ties must attach to every rafter to be considered consistent.
- Attic supports appear to be newer than 1950
  - If the wood looks old, look up the build date. Use your discretion.
- Collar ties can typically reduce the max span by about 33% when qualified.
  - When an attic has both Collar Ties and Knee Walls/Kickers, you can reduce the span by 50%

If we need the install crews get measurements of the rafter while on site we have alternative methods of measuring rafter size which would include:

Removing a recessed light to complete a normal measuring tape measurement.
Poke Hole test.
Measuring the size of the vaulted members from the exterior at the eave if they are exposed.

BONUS/PORTLAND STYLE FRAMING SPAN

The term “Bonus Style Framing” refers to a room or space framed into the attic between collar ties and knee walls. These spaces are often finished but sometimes are only framed in with windows and outlets/lighting much like a common unfinished basement.

Count knee walls unless the AHJ is the city of Portland, OR
Can be trusses or conventional.

Portland style of framing can be and if the crew has any questions about it, We should grab EIT or Engineer if available.

Interesting Facts!

Here is the typical life expectancy for different roof type and attics.

InterNACHI's Standard Estimated Life Expectancy Chart for Homes

Quiz Time!!!!!!!!!

Mounts

<------ Time to move on to mounts

Page updated

Report abuse