The “A-frame” or tongue of your tiny house trailer is prime real estate. It is the perfect spot to house the ugly, heavy, and potentially dangerous systems that you don’t want inside your living space: Propane Tanks and Batteries.
However, you cannot simply throw a plywood box together and call it a day. Propane and batteries have very specific—and conflicting—safety requirements.
This guide covers how to design and build a custom tongue box that adds curb appeal while keeping your tiny house safe and legal.
The Golden Rule: Separation of Church and State
If you plan to store Propane and Batteries (or any electrical components like inverters) in your tongue box, you face a major safety conflict.
- Propane: Can leak. Propane is heavier than air, so it sinks.
- Batteries: Can spark (relays, loose connections) or off-gas hydrogen (if lead-acid).
The Rule: You MUST have a sealed physical barrier between your propane compartment and your electrical compartment.
- Do not put them in one big open box.
- Do not run electrical wires through the propane side without explosion-proof conduit.
- If a propane leak occurs, gas must not be able to migrate to where a spark could ignite it.
Designing the Propane Compartment
Your propane tanks (usually two 20lb or 30lb tanks) need to be accessible for refilling and, most importantly, vented.
1. Ventilation is Non-Negotiable
Since propane is heavier than air, if a leak occurs, the gas will pool at the bottom of the box like water.
- The Floor: The floor of the propane side should be open mesh or expanded metal, NOT solid wood. This allows leaking gas to fall harmlessly to the ground.
- Cross Ventilation: Add vents high and low on the sides to keep air moving and prevent heat buildup in summer.
2. Tank Securement
You cannot have tanks bouncing around. Use standard RV tank brackets or threaded rods with a cross-bar to bolt the tanks firmly to the trailer frame.
Designing the Battery Compartment
If you are off-grid, this is likely where your heavy battery bank lives.
1. Insulation is Key
Unlike propane tanks, batteries hate extreme temperatures.
- Cold: Reduces capacity.
- Heat: Degrades lifespan.
- The Fix: Line the battery side of the box with 1-2 inches of rigid foam insulation (XPS) to moderate temperature swings.
2. Venting (Battery Type Matters)
- Lead Acid / AGM: These release hydrogen gas when charging. You must vent this compartment to the outside (top vent, as hydrogen rises).
- Lithium (LiFePO4): These do not off-gas, so they can be in a sealed compartment. However, they cannot be charged below freezing, so insulation is doubly important.
DIY Build Materials: Weight vs. Durability
Option A: The “Cedar Chest” (Wood Frame)
- Pros: Easy to build; matches your house siding perfectly; can be custom shaped to fit weird angles.
- Cons: Heavy. Wood requires maintenance.
- Build Tip: Use pressure-treated lumber for the base that touches the metal trailer. Siding the box with the same material as your house makes the house look longer and more integrated.
Option B: The “Diamond Plate” (Metal)
- Pros: Lighter weight (if aluminum); extremely durable; road-ready look.
- Cons: Hard to build from scratch without welding skills.
- Hack: Buy a pre-made truck tool box (like for a pickup truck) and bolt it to the tongue. It’s not custom, but it’s waterproof and lockable instantly.
The “Tongue Weight” Equation
Everything you put in this box adds to your Tongue Weight.
- The Target: Your tongue weight should be 10-15% of your total tiny house weight for safe towing.
- The Math: If your house is 10,000 lbs, you want 1,000-1,500 lbs on the tongue.
- The Balance:
- Too Light? If your house is tail-heavy, adding heavy batteries to the tongue box is actually good for stability.
- Too Heavy? If you are already nose-heavy, consider moving batteries inside over the axles and keeping the tongue box strictly for light storage and propane.
Step-by-Step DIY Logic
- Weld/Bolt the Base: Create a steel angle-iron frame welded to your trailer tongue to support the box. Don’t rely on screwing wood directly to the trailer rails.
- Build the Divider: Install the central partition wall. Seal the edges with high-quality exterior caulk or expanding foam to make it airtight between sides.
- Install the Floor:
- Propane Side: Expanded metal mesh.
- Battery/Storage Side: 3/4″ Marine plywood (painted/sealed).
- Frame & Sheath: Build your walls. If using wood, apply house wrap just like the main house before siding.
- The Lid: This is the most common leak point. Build the lid with a significant overhang (1-2 inches) and install a high-quality rubber weatherstrip seal. Use a sloped lid to shed water away from the house.
Conclusion
A tongue box is more than just a junk drawer; it’s the mechanical heart of your tiny home’s exterior. By strictly separating fuel from spark and ensuring proper ventilation, you can turn that empty triangle of steel into a safe, functional powerhouse.
Safety Checklist
- [ ] Sealed Divider between propane and electric.
- [ ] Mesh Floor under propane tanks.
- [ ] Vents installed (High/Low for airflow).
- [ ] Locks installed (Propane theft is common).
- [ ] Drainage holes in the storage side (in case water gets in).