February 26, 2013 § 19 Comments
How do you calculate the weight of your tiny house and how do you know if your trailer can handle it? There are several ways these things can be determined, it all depends on your situation, as does everything… Here is my logic and some of the tools I used to get there.
The first thing to decide would be if you already have a trailer you’re going to build on, if so then you have to decide how much it can carry and design within this. I am not going to go through the details of how to find out how much your trailer can already carry because Andrew already did a stellar job of demystifying that over at Tiny r(E)volutions HERE. So, if you already have a trailer, figure out how much you can load it with and design your house using the basic principles below. Note: If you buy plans from Tumbleweed or the like they generally come with a gross build-out weight to them, that is handy to use so long as you know you’re going to use those plans with no/minor modifications, if however you are going to change some materials around (say you want granite counters instead of plastic laminate counters) you can use some basic calculations with the tools below and add that to their weight to verify that you’re in the right range still.
What if you design your own tiny house, how can you guesstimate your weight so you know how much your trailer will have to hold so you can buy the correct trailer? You COULD find a similar-ish Tumbleweed and err on the side of caution guessing a little heavy but that isn’t really a solid method and I don’t suggest it… For those of you who have followed my project you know that I pretty much had the house designed before I got the trailer and that the trailer I found needed to be modified to hold the weight I wanted it to… Because I had these known modifications I was able to more accurately plan my axles, both the location and the maximum weight. I ended up buying a duel 5600 pound (each) capacity axle flat-bet goose-neck trailer, but initially I was planning on a concrete floor (6000 # in itself) so I ended up installing a third axle just to be safe. If you are going to go this way and customize your trailer be sure to calculate in the weight of your trailer, mine is just shy of 3000 pounds for all the steel. If you add 6000 for the concrete and 3000 for the trailer (it is home made and a little beefier than a trailer shop would make a trailer, so it is heavier than most) it brings you to 9000 pounds before you even add the house! So two 5600# axles would only leave me 1600 pounds for the rest of the house in that example, hence I welded on a third axle allowing me to go up to 16,800# total, allowing me 7,800 for the rest of the building materials. Now, through the design process I have since nixed the concrete in favor of much lighter weight tiles so my trailer is overkill, that was a calculated decision though.
Through the planning phase you will most likely be doing a sketchup model or somehow actually drawing plans, tiny houses are small enough that you can pretty easily take a volume or square foot count of materials fairly painlessly, you have to do this to buy your materials anyway. Once you have those calculations you can apply weights to things and get a pretty decent idea of where you’ll stand.
I have collected a list of common building materials that you can use. I like to add a contingency onto the end weight to account for things that are harder to pin down, like the faucets, hardware, furniture etc., I suggest about a 20% contingency, that may seem high but it’s better to be over than under! If you make a material list and add things up and it’s too heavy you can start to massage your finishes to make things work out better. This is a pretty important part of the process so spend some time on it. I keep saying that this project is weird for me because there are three main design factors that went into my project and the least important is aesthetics (weird for someone with a design background). The MOST important has been weight and number two is cost since I set a pretty tight budget. THEN I get to worry about looks :).
Common Material Weights:
Wood Stud, 2×4 (pine): 1.31#/linear foot
Wood Stud 2×6: 2.05#/linear foot
Metal Studs: 1#/linear foot
SIPs Panels, 6″: 3.47#/square foot
Insulation, rigid – 1″: 1.5#/square foot
Insulation, batt 1″: .04#/square foot
Insulation, spray-in 1″: .5#/square foot
OSB Sheathing, 3/8″: 1.22#/square foot
OSB Sheathing, 1/2″: 1.63#/square foot
OSB Sheathing, 5/8″: 2.03#/square foot
Plywood, 3/8″: 1.08#/square foot
Plywood, 1/2″: 1.44#/square foot
Plywood, 5/8″: 1.8#/square foot
Gypsum (drywall), 1/4″: 1.1#/square foot
Gypsum (drywall), 3/8″: 1.65#/square foot
Gypsum (drywall), 1/2″: 2.2#/square foot
Gypsum (drywall), 5/8″: 2.75#/square foot
Metal Roofing: 2.5#/square foot
Asphalt Roofing: 2#/square foot
TPO Roofing: .7#/square foot
Carpet: .3-.7#/square foot
Porcelain tile: 4.5#/square foot
Hardwood Floor (birch), 1/2″: 2.4#/square foot
Hardwood Floor (oak), 1/2″: 2.05#/square foot
Hardwood Floor (poplar), 1/2″: 1.45#/square foot
Wood Planks (pine), 1/2″: 1.46#/square foot
Cement, 1″: 12#/square foot
Granite, 1/4″: 3.6#/square foot
Hardie Board Siding, 1/2″: 3#/square foot
Linoleum: .75#/square foot
For a more comprehensive list click HERE.
For a more comprehensive list of various woods click HERE (a board foot is equal to a 1′x1′x1″ section of wood, you can do simple math from there to get to your thickness/width).
You’ll notice some of these are assembly weights, if that is the make-up of your wall skip adding up the individuals and just use that weight per /square foot of wall.
In addition to your building material weights you will need to account for your appliances, the best way to do that is, if you order them on amazon there is an exact weight. If not you can certainly get an idea on how much a stove similar to the one you’re getting will weigh, again, lean to the cautious side and round up… If you are going to have a water tank on board or a water heater use it’s ‘when full’ weight (water weighs 8.34# per gallon).
Once you have all of that added up figure out your contingency, I use 20% to account for the faucets, the furniture and all those little pieces that it wouldn’t be efficient to add up… Then you have your weight or at least a healthy stab at it to get you started!
Hopefully that is a semi helpful bit of info! Anyone else? How did you determine this? Did you figure it out beforehand or just build?