Structural Framing Types

STRUCTURAL FRAMING TYPES

Sector 2

This training reviews the basics of framing types and structural input for the purposes of Design & Engineering.

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

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trickier because a lot of them were not engineered correctly to begin with or the codes they complied with are very outdated.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 run down 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:

If the supports are attached only to the rafters with no purlin and are spaced 48" o.c. or closer, then we can add new supports in between them. If the supports are greater than 48" o.c. then we can't add new supports because it'll interrupt the load path of the structure too much and add too much weight to the ceiling and other components of the roof that haven't seen that load before.

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 & Purlins:

Consist of a diagonal member (kicker) that connects to a perpendicular horizontal member (purlin) that is directly up against the rafters
Kickers must be at most 48" apart if the purlin is a 2x4.
Kickers may be 72" apart if the purlin is a 2x6 as long as the roof snow load is 20 psf or less.
If neither of these apply you can't count the kicker & purlin system as a support and you will ignore it in your calculations and sideview sketches, but if an upsized sister upgrade is required, you can't use the roof section as that upgrade won't fit.

Collar Ties:

This type of support is used to pull the rafters together
We cannot count it as a support in our calculations, even if the collar ties are consistent
Ignore collar ties in sideviews, upgrades, and calculations

Sister Rafters:

This type of support helps strengthen existing rafters by essentially doubling the total rafters.
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.

Site Survey: Make sure to document the whole attic so we can see what supports are present and how consistent they are.

Design & Engineering: Make sure to make note of structural supports you can use to reduce your span during your data collection process.

VAULTED CONVENTIONAL FRAMING

It is important to distinguish any roof areas that are vaulted. This has an affect on the structural calculations that will be used in the Design & Engineering stage of the project.

Site Survey: Make sure to document any roof areas that are vaulted with no attic access and indicate where these areas are on the reference map.

Design & Engineering: Make sure to call out the roof framing as vaulted to ensure the correct calculations are used.

VAULTED CONVENTIONAL FRAMING MEASURMENTS - SITE SURVEY

Vaulted roofs will require a poke hole test to verify the framing info.

Use a stud finder to get the spacing.
Use a solid piece of wood or steal to complete the "poke hole" measurement.

Make sure your photos clearly indicate the methods used for measurements and clearly show the final measurements without any room for doubts. Context is everything.

make sure to include the surveyed member size from the poke hole test in your survey notes.

Alternative methods of measuring rafter size would include:

Removing a recessed light to complete a normal measuring tape measurement.

Measuring the size of the vaulted members from the exterior at the eave if they are exposed.

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.
Truss Construction POST-1995: For buildings constructed in 1995 or earlier, it is generally safe to assume that trusses are used throughout the structure, as this was a common practice during that time period.
Consist of top chords, bottom chord, web members all connected using gusset plates.
Easier to Engineer PV on.

Trusses come in a variety of shapes and sizes, but are easily identifiable by the use of gusset plates and webbed members.

VAULTED TRUSSES

Trusses can be used for vaulted ceilings as well:

If trusses are in one area, trusses are probably in all others.
- This is not a hard and fast rule. There are exceptions such as sections of overbuild that were framed after the principle home structure was built.
The key to identifying this is to look for an interior pitch that appears to be less steep than that of the exterior roof.
Vaulted Truss Identification: Compare the interior pitch (the slope of the ceiling) with the exterior pitch (the slope of the roof) to determine if scissor trusses are present. If the interior pitch is less steep than the exterior pitch, scissor trusses are likely used, allowing for a vaulted ceiling inside the structure.

This is an example of scissor trusses that do have a small attic access. Always try to access the framing directly if you can to get good photos.

In some cases, trusses might be fastened to another type of framing that spans over vaulted sections. This could be:

TJI (as seen to the right)
Typical Rafters

Make sure to document ANY change in framing over vaulted sections.

Site Survey: try to differentiate between “vaulted” scissor style trusses and vaulted conventional framing.

Design & Engineering: Use your best judgement about vaulted roof sections. If the whole home looks to be built at the same time and other attic sections are trusses, most likely it is trusses. It get's trickier when you start to consider overbuild sections built at a diferent time.

OVERBUILD

In some situations, it is possible to have both trusses and conventional framing or framing of two different qualities if part of the framing was built over at a later time.

Indicators that a section of overbuild was not part of the original home blue print would be:

- Visible shingles underneath the overbuild section
- Mixed framing types and sizes
- Wood framing in one roof area looks significantly newer than the rest.

Site Survey: this is another reason why it is important to survey under every roof section of the home.

Design & Engineering: make any note of overbuild sections in your data collection as the overbuild framing quality/criteria may differ from the main roof framing underneath.

COMMON DORMER OVERBUILD

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.

Porch Overhang Qualifiers:

1. The rafters are supported at the edge by a beam of decent size (larger than a regular 2x member)

2. The posts are 4x4 wood or steel (not those fancy lattice posts)

3. The posts land on a foundation

4. There doesn't appear to be any sagging or any other funny business going on with the framing

If all are true, then run the span as if the rafters were 2x4s @ 24" o.c. and you can use the porch if that passes

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.

This is an example of a truss roof where horizontal trusses appear under the hip section.

It can be a little confusing, so let's look at an overhead diagram of the framing so we can see exactly what we’re talking about here:

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).
- Occasionally, there will be no flat rafters over top of the horizontal trusses at all.

Site Survey: It is critical to survey the structure underneath each roof section to document horizontal trusses.

Design & Engineering: When choosing mounting planes over horizontal trusses, not that either roof sheathing mounts or structural blocking will be required in those areas.

Note that this will not be the case for hip roofs using conventional framing.

All rafter members will be vertical on a conventionally framed roof.

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.

2 | ROOF SHEATHING TYPES

BASIC SHEATHING TYPES

OSB
Plywood
Step/Lumber
Tongue & Groove

OREINTED STRAND BOARD (OSB)

OSB is most common on newer construction. Sometimes it can be referred to as particle board, though that's not the technically correct term. That is, however, the key to identifying this type of sheathing. Look for particles of wood with random grain orientation.

Quick facts about OSB:

Made up of small strands of wood scraps and lumber that otherwise would not be used
Fused together with a water resistant resin under heat and pressure to create boards.
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.

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

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

MEASUREMENTS THAT ARE ALWAYS NEEDED AT SURVEY

In some cases, we will need an extensive survey of the attic including a variety of measurements but most of the time, it can be fairly simplified:

We always need a measurement of the rafter size.
We always need a measurement of the rafter spacing.
We always need 360 of the attic showing what supports are present.

These three measurements will be the only measurements you need to get for a majority of conventionally framed projects.

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.

MEASURING IN THE ATTIC

The full span will be from the eave all the way to the ridge measured along the ceiling joist. It is not the measurement of the hypotenuse (rafter) to the ridge.

RAFTER SPAN

We can use structural supports such as knee walls and collar ties to reduce the span. This will help with structural calculations.

Site Survey: Make sure to document the whole attic so we can see what supports are present and how consistent they are.

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.

This Knee Wall example has a spacing over 24". This would need to be sent for preliminary structural review so that the structural team can call out the appropriate Knee Wall upgrade.

This Knee Wall example has a spacing of 24". We could use this Knee Wall to reduce the max span in our structural calculations for this roof section.

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.

This Kicker example has a spacing over 48". This would need to be sent for preliminary structural review.

This Kicker example has a spacing of 48". We could use this Knee Wall to reduce the max span in our structural calculations for this roof section.

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%

This Collar Tie example has an appropriate spacing we could use to reduce the max span for structural calculations as there is a Collar Tie attached to each set of rafters.

This Collar Tie example could not be used to reduce the max span for structural calculations as the Collar Ties are not consistant and not on every set of rafters.

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.

Site Survey: Make sure to document any bonus rooms in the home and indicate where they are located on the reference map.

HORIZONTAL PURLIN SPAN & SPACING

With Horizontal Purlins, span and spacing are reversed. This is an uncommon framing type and is mainly seen on uninhabited barn structures:

Span: space between trusses
Spacing: distance between each horizontal purlin

Design: if you are ever in doubt, ask for a structural review to ensure the structural integrity of a structure consisting of horizontal purlins.

3 | WALK THROUGH EXAMPLES

In this exercise, we will review all of the structural info we have reviewed so far. This will include framing type, sheathing type, size and spacing.

EXAMPLE 1

This is an example of conventional framing:

Notice there are no webbed members or gusset plates.
Also notice, there are no other supports other than the rafter.
- Extra care should be given in cases where there are rafters with no additional supports

What type of roof sheathing do we see here?

looks like lumber and plywood.

Now, let's look at the rafter size:

appears to be 2x4.

And what about the spacing?

This appears to be 32" spacing.

Make sure to always pay close attention as you never know when you'll see an uncommon spacing like 32".

The next thing we need to know is if we'll be able to reduce the max span and by how much. Do you see any consistent supports here that would be used to break up the max span?

It would be wise for the designer to complete structural calculations at this point to rule out the need for upgrades/structural review.

EXAMPLE 2

EXAMPLE 3

4 | QUIZ

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