Building a custom Truck Deck

Custom Truck Entry Step
So what do you do when the height from sidewalk to running board to interior or road to running board to interior is so high as to make entry too difficult for the average elderly person to use?

A 2004 Ford F250 super duty 6.0L Diesel has an overall distance of 22 inches from road to Interior floor for the 4x4 off road model. Their standard stock running board sits at 14 inches from the road. Of course the type of tire also plays a factor as different tires can raise or lower this distance.

In my case, because I have no plans to go off-roading and choose my campsites carefully I was able to borrow from the RV lifestyle and modify to allow a much shorter step up distance. Firstly, I removed the original running board which as I found was so badly rusted through there was only one of 6 bolts still holding it. I left the original brackets in place. The thickness of the running board was 2.25 inches and it's depth was 3 inches. I replaced this with two 2x6 boards to be later coated with rubber top tread and rubberized undercoating. Beneath the front door I cut out the bottom 2x6 section to fit an RV hide-step.

The end result was a running board that:
1) running board to interior down to 7 inch from 8
2) running board to step at 8.5 inches
3) step to curb at 1.5 inches and
4) step to road 6.5 inches
** When the step is retracted it is flush with the running board which is also no further out than the original.


Custom Truck Development
My objective was to have a versatile rig that would consist of one of three basic plans
The needs of these three options are both diverse and demanding.
  1. Truck GVWR = Total vehicle weight including payload.   10,000 lbs
  2. Tow GVWR = Total weight of towed vehicle/trailer     13,000/17,000 lbs
  3. GCWR = Maximim total combined weight      23,000 lbs
  4. Curb weight = weight of truck with 0 payload.     6600 lbs
  5. Payload weight = weight of camper or box contents    2400 lbs
  6. hitch weight = Maximum point load weight at hitch
    (bumper hitch or fifth wheel hitch)   1000 /1400 lbs
Unit Curb Weight Payload Hitch Towable GVWR/GCWR
The truck & camper :
The truck & trailer :
The truck & 5th wheel :
6600 lbs
6600 lbs
6600 lbs
2400 lbs
2400 lbs
 
 
1000 lbs
1400 lbs
 
13000 lbs
15000 lbs
9000 lbs
23000 lbs
23000 lbs
So the fifth wheel method requires three points of interest. One is that besides the hitch, carrying anything in the box can only be done if it doesn't interfere with the hitch and only if the hitch weight is lighter than 1400 lbs. The second is that the box needs extra reinforcing as the 1/32" deck plate is NOT strong enough to support a point load even close to 1400 lbs.
where the box can withstand 2400 lbs distibuted it can only handle a point load of 300 lbs!. Lastly, to obtain the rated 17000 lb 5th wheel trailer limit, you will need no box, hitch mounted right to the frame, a very low hitch weight and probably not to be using a extended cab or crew cab truck!
1) Park model Trailer to live in year round & a camper for vacationing
  Curb weight truck
Camper weight payload
hitch weight
trailer weight
total
6600 lbs
2800 lbs typical
965 lbs
7900 lbs 1900 lbs load 35ft
20165 lbs
2) Travel Trailer for year round & a camper for vacationing
  Curb weight truck
Camper weight payload
hitch weight
trailer weight
total
6600 lbs
2800 lbs typical
605 lbs
5400 lbs 1900 lbs load 28ft
17305 lbs
3) Fifth Wheel trailer to serve as both.
  Curb weight truck
payload
hitch weight
trailer weight
total
6600 lbs
300 lbs typical
1350 lbs
9900 lbs 1900 lbs load 31ft
23050 lbs


Changing a Truck Box for a Custom Deck
Planning details:

Know your reasons for changing the box to a deck and what kind of deck you need to have.

My Reasons

1) lower box sides (possibly for custom Camper)
2) add under deck storage
3) keep about same weight as box
4) stronger deck for 5th wheel hitch

Step 1

Obtain design plans for your make of truck

Step 2

MeasurementsFord F250sd xlt Diesel
Ground ->box top 55" reduce to 45"
Ground ->deck height 37" increase to 39.5" due to need for frame blocking
Overall box length 105" reduced to 97.5"
interior box length 95.5" increased to 96"
box width of 79.5" increased to deck width of 93.5"
Space between wheel wells unchanged at 50"
Frame width 3"
Frame spacing 31.5" inside 37.5" outer
Ground box clearance 19" unchanged in new deck
Lower wheel well opening 37"
Foward compartments 37" long by depth 18" (driver side) & 24" to 18" (passenger side) by ~24" high
Rear comparments 22" long by 18" depth by 18.5 high

Using these details we end up with these two impressions
Change in weight will be calculated later

Truck Box removal
Removal of the box on this model is fairly straight forward. First thing you need to do is remove the liner (if so equipped. My liner was held on with an top edging with wedge fasteners the full length that snap into groves in the box top-ridge. From here you need to release the four corner hold-down rings that screw through the liner into the box. Lastly, you have to compress the sides and front of the liner so you can tilt it past the wheel wells and take the liner out.

The eight foot box of an off road 4x4 3/4 ton has 8 carriage bolts with torx #50 fitting but don't expect them to remove easy if the vehicle has been in service for a time. Rust is a powerful welding agent! 3 came out with a little coaxing with WD-40 #4 snapped the first torx bit, and #5 stripped the impact driver! So 5 - 8 were finally removed by grinding an 'X' into the bolt tops and then grinding each quarter off in sections.

Fuel filler came next. Unscrew the 3 1/4inch screws holding the filler into place (Fuel cap must be removed before undoing and make sure to replace it right after the box is removed to prevent dirt getting into the tank.

Remove and fully disconnect the tailight wiring and make sure you have planned how to supply running lights without the box!. In my case I mounted simple trailer lights into the last two box mount holes. contact 1 is Right signal and brake, contact2 is Left signal and brake, contact 3 is back-up, contact 4 is marker light, five is ? and contact 6 is ground

Finally you can release the 2 brace arms that go between the frame and the box wheel well flange. You can now lift the 400 lb box off and place it on blocks (4 person method) or through tie-straps around the box and lift with a forklift (2 person method).
As you can see, the Frame is not the same height from front to back. This means that you must level the playing field with blocking. You will also note that between the frame rails are brackets and structures which will have to be worked around. While you could do a point blocking like the original box used to rise above obstructions, you will get a much stronger deck using a series of four cross beams.

Adding the deck support
The Truck box normally inclines slightly towards the back and levels as the load increases. So back when we took the ground to box top, we actually measured both at the front and back. This resulted in a 3" difference. After the box was removed however we needed to pick a common spot which actually is the the rear bumper height. It's height changed by 0.5 inches higher without the 400 lb box.
Using a straight 2x6 and the angle of the incline, our first task was to measure from the 2x6 to each of the four mount points on a frame rail to determine blocking heights.
Using these measurements it was determined heights of 1.5 inches to 4 inches would be needed.
The lengths of the cross beams were established at 63.5 inches for the first and last ones but the center ones need to be only 53 inches to accomodate the wheel well area. 2x6 boards were shaped to fit around obsticles as they spanned accross the frame rails and thickness was adjusted by notching bottom boards and adding top boards. Each group of boards were then bolted through the frame and treated with undercoating to prevent deterioration.
Once the deck supports are in place, it's time to install the deck boards. The 50" width uses 9 2x6's centered on the supports. This really only provides 49.5" of 8 foot decking. To accomodate over the wheel wells, 2 additional 8 foot boards are notched out from the wheel well forward such that the 2x6 board is narrowed to 1.75" for the first 74". A 74" 2x6 is then mounted perpendicular flush with the notch resulting in the needed .25" per side to make up the 50" between wheel wells. Finally, a 93.5" 2x6 is mounted at the front and back of the deck.
 
The box frames over the wheel wells is then completed. A simple stucture that consists of a second 74" 2x6 board and a 18.5" 2x6 completing the box shape. A pair of 2x10's trimmed to fit cover the top.
Time to stop the fuel filler from dangling. The outter 2x6 and 2x10 top are opened framed and the fuel filler is mounted into it's new home.
Each of the under deck compartments are quite simple. 2x2's frame the inner backing board and provide for securing the backing board to the under deck surface. The front and rear panels mount to the 2x6 framing surfaces and to the 2x2 on backing board. This makes the structure very solid. A base is then framed on 2x2's and secured to the bottom of the compartment. Notching of the panels was done to allow for flush fitting.
The forward compartments are more involved, being two sections with multiple depths. On the drivers side, both sections are 18" deep but on the passenger side the larger forward section is 24" deep and the smaller one returns to 18" deep. The custom deck is finished with sheathing which will be covered over by a skin of 1/32" thick steel. Actually, rather than pay some $2.49 per sqft for the steel, I have an old shed made of steel panels that outlived it's use, so I savenged it for the metal. A ball-peen hammer and a primer/paint does wonders!