Howe Truss:
Using a Howe truss to support a ceiling from above is a common structural solution when you want to remove an internal load-bearing wall or fix a sagging ceiling without adding a visible beam in the room. In this setup, the Howe truss acts as a bridge across the span, and the ceiling is suspended from its bottom chord.
1. Understanding the Howe Truss Action
In a Howe truss, the diagonal members are in compression (pushing) and the vertical members are in tension (pulling).
Because the vertical posts are under tension, they "pull up" the weight of a ceiling.
- The Bottom Chord: long 2x8 or 2x10 laid on its side across the existing joists. This is what your ceiling joists will hang from. Since it is 33 feet long, you will likely need to "splice" two boards together. Never splice in the center. Splices should occur over a vertical member or at the 1/3 points of the span, using heavy plywood or steel "scab" plates.
- Suspending the Load: heavy-duty bolts or threaded rods connect the ceiling joists to the vertical tension members of Howe truss, using "Mending Plates" (gusset plate, metal connector plates, a critical component to prevent failure). Verticals (The "Hangers"): These must be perfectly vertical. If you use 1" steel threaded rods, they should pass through the center of the top and bottom chords with heavy-duty bearing plates and nuts on the outside.
- Diagonals: These should point upward and outward toward the ends (forming a "W" or "M" shape). In a Howe truss, these diagonals are in compression, meaning they are being squeezed. Standard 4x4 or doubled 2x6 lumber works well here.
- Support Points: The ends of your Howe truss rest on load-bearing exterior walls that carry all the weight all the way to the foundation.
To span 33 feet, a Howe truss needs significant depth to prevent the ceiling from bouncing or sagging in the middle. To estimate the Truss Dimensions for a span of this size (L=33′), you should aim for a depth-to-span ratio of roughly 1:6 to 1:10.
- Ideal Depth: Truss approximately 3.5 to 5 feet tall at the center.
For a 33-foot Howe truss supporting a standard residential ceiling (dead load of ~10-15 lbs per sq. ft.), an engineer would likely specify:
|
Component |
Likely Material |
Role |
|
Top Chord |
Double 2x8 or 2x10 |
Compression (Pushed together) |
|
Bottom Chord |
Double 2x10 or 2x12 |
Tension (Pulled apart) |
|
Verticals |
4x4 Timber or 1" Steel Rods |
Tension (Lifting the ceiling) |
|
Diagonals |
4x4 Timber |
Compression (Transferring load to ends) |
Interesting point: The "Camber" Requirement: Over a 33-foot span, the wood will naturally settle. To avoid a visible dip in your ceiling, the truss should be built with a camber. This means the bottom chord is built with a slight upward curve (perhaps 1/2" to 3/4" at the center). When the weight of the ceiling is applied, the truss "settles" into a perfectly flat line.
To calculate the point load, we need to determine the tributary area—essentially, the square footage of the ceiling that this specific truss is "responsible" for lifting.
1. Calculate the Tributary Area for 33 feet X 24 feet floor space:
The truss doesn't just carry its own weight; it carries half the distance to the next support on either side.
- Formula: Area=Span×Width
- Example: If your 33-foot truss is supporting ceiling joists that span 12 feet on one side and 12 feet on the other, it is "picking up" 6 feet from each side.
- Total Width: 12 feet.
- Total Square Footage: 33′×12′=396 sq. ft.
2. Determine the Total Load
Residential ceilings usually have a "Dead Load" (the weight of the materials).
- Drywall, Joists, & Insulation: ~10 lbs per sq. ft.
- Truss Self-Weight: ~2-5 lbs per sq. ft.
- Total Load: 396 sq. ft.×15 lbs/sq. ft.=5,940 lbs.
3. Calculate the "Point Load"
Since the truss is symmetrical, the weight is split between the two ends (the bearing points).
- 5,940 lbs/2=2,970 lbs per end.
Why this matters: A single 2x4 stud can typically only support about 1,000 to 1,500 lbs before it risks buckling under a heavy load. For a 2,970 lb load, you would need a "post" made of at least three 2x4s (one king stud and two jack studs) nailed together at each end of the truss.
4. Designing the Howe Tension Members
Since you are supporting the ceiling from above, the vertical members of your Howe truss are the most critical. You are essentially "hanging" 3,000 lbs from the top chord.
Summary Checklist for a 33-foot Span:
- Bearing: Ensure you have at least 4.5" of solid wood (3 studs) under each end.
- Depth: Ensure the truss is at least 4 feet tall at the peaks.
- Fasteners: Use bolts and plates, not nails or standard wood screws.
- Lateral Bracing: Once the truss is up, you must install "cross-bracing" to prevent the truss from tipping over sideways like a domino.
Step-by-Step "In-Place" Assembly:
- Lay the Bottom Chord: Place your long boards across the ceiling joists exactly where the truss will live. Block them up slightly so they aren't resting directly on the drywall.
- Build the "King Post" (Center): Install your center vertical member. If you are using a 4x4 or a steel rod, get this perfectly plumb (vertical) and secured to the peak of what will be your top chord.
- Install Top Chords: Run your diagonal top chords from the center peak down to the ends of the bottom chord.
- Add Internal Webs: Install the remaining Howe diagonals and verticals.
- Tensioning: If using steel rods for verticals, tighten the nuts slowly. You will actually see the truss start to "take the load" and lift slightly off the ceiling joists.
A Critical Warning on "Total Weight"
Remember, once you finish this, the entire weight of that 33-foot section of ceiling is no longer sitting on your interior walls; it is sitting only on the two ends of the truss. Before you pull out any interior walls, you must:
- Tighten the truss until it is carrying the weight.
- Check for "Deflection": Measure the distance from the floor to the ceiling. Wait 24 hours. If that measurement changes, your truss is stretching or the end-walls are sinking.
Interesting documents - Reference:
Design Guide for Timber Trusses, 2021, Available Online: https://www.carolinatimberworks.com/wp-content/uploads/2021/12/Design-Guide-For-Timber-Trusses.pdf
