Timber frame details
23 / 09 / 2015
h2>About framing choices
So far I’ve encountered two viable options for framing my sweet home. The traditional wood frame and a sandwich element (structural insulated panel, SIP). I recommend to read this article on Tiny House Talk, in which you find basic pointers comparing different framing options. The reason I’m not even considering a steel frame is that I’ve never worked with metal – and I think I already have quite enough new stuff on my plate with this project. In this post I’m adding up the weight and cost of the traditional wood frame based on the off grid design.
The frame is made of 2/4 joists with a standard 550 mm modular width (600 mm center). The door frames are 185 mm deep and there’s a 50 mm widening for plumbing on two wall fragments. The second picture shows a 100 mm thick polyurethane sheet insulation.
You might wonder where’s the flooring frame. There is none. XPS-sheets have an incredible compressive strength – they easily support furniture and people if the weight is evenly distributed. The long term compressive strength of a polyurethane sheet of choice is 130 kPa (13 ton kgf/m2) so there is absolutely no reason to add weight with floor framing. You can basically throw surface flooring right on top of the insulation.
Since the door and wall frames aren’t of equal depth, the frame has to be extended either inward or outward. The design requires space in the walls for plumbing anyway, so there’s an easy 50 mm for the extension inside and 35 mm is left outside. To keep the trailer symmetrical I pulled the back and front walls 35 mm inward as well, allowing an all around outside layer of extra insulation, paneling or cladding.
Although the option is open, the extra layer isn’t taken into consideration in the calculations since it’s not mandatory. However now that I’m at it, I would like to point out that some of the indoor decoration panels imitating natural, heavy building materials, are made of polyurethane. With an outdoor paint coating, they can give the exterior of the home quite an interesting look. But then again – as they are super lightweight and insulating, they’re also quite expensive.
To support snow on the roof and furniture on the floor, the frame needs some boarding. Because of the XPS insulation, it’s not necessary to add a heavy base board to support the floor. What I’m concerned about is that if I choose a thin flooring with a big plate size, the board reaction to humidity might make it wobbly. Just the other week I was in an old house and the parquet must have stooped centimeters under my step (no exaggeration) and needless to say, it was awful. I was wearing heels and almost tripped over every time I walked over the up-and-down part of the floor. Even though, I don’t think it’s a viable risk with a flooring that consists of small pieces and is correctly installed, but to be sure I’ve made the calculations based on a quite thick and heavy board.
Even though the insulation sheets have a strong compressive strength, they are not load bearing elements on their own. The trailer body takes care of that with the floor, but the roof requires reinforcement on top of the joists. For that I’ve chosen a 15 mm plywood substructure for bitumen roofing.
Table of details
The weights in the table are calculated from each manufacturer’s density reports and converted to the unit of the object. I haven’t done much of price comparisons yet so the prices are only directive, although based on real selling prices. Also, keep in mind that fixings weight and cost too – not long ago I read a tiny house blog that mentioned screws and angle irons eating up 500 dollars alone.
|PRODUCT||NET WEIGHT||NET PRICE||LENGTH OR AREA||TOTAL WEIGHT||TOTAL PRICE|
|Spruce joist 50*185mm||4.3475 kg/m||3.9 e/m||19.91 m||86.56 kg||77.65 e|
|Spruce joist 50*100mm||2.35 kg/m||1.75 e/m||71.85 m||168.85 kg||125.75 e|
|Spruce joist 48*48mm||1.175 kg/m||0.89 e/m||20.80 m||24.44 kg||18.51 e|
|Finnfoam FI-300 100mm||3.5 kg/m2||14.42 e/m2||45.81 m2||160.34 kg||660.58 e|
|Flooring chipboard||15.8 kg/m2||8.92 e/m2||12.25 m2||193.55 kg||231.77 e|
|Roofing plywood||6.9 kg/m2||13.06 e/m2||14.40 m2||99.36 kg||188.06 e|
And here comes the interesting part of the post. How much does it really weigh and cost.
|PART||TOTAL WEIGHT||TOTAL PRICE|
|Framing||279.85 kg||221.91 e|
|Insulation||160.34 kg||660.58 e|
|Boarding||292.91 kg||419.83 e|
|Total||733.10 kg||1302.32 e|
I have mixed feelings about the results. The cost of the insulation surprised me pleasantly, I seriously thought it was going to cost thousands of euros. At first I reckoned that the weight was quite well also, not even near what I expected. But given it some thought, the frame needs to be even lighter. If I’m to carry furniture and normal appliances inside the home or actually waterproof the roof, I need to pinch on everything. There are other (possibly lighter) framing options in which I will return later, but I think we can make this one work as well.
The first thing to do is to throw the thick flooring to the garbage right away. Comparing to a cork or laminate, the weight would lower for a hundred kilograms straight off the bat. The price of a laminate or cork flooring would either double or halve the flooring expenses, depending on the product selected. Anyway, low enough weight is a priority over the price.
Even though the traditional frame seems doable, I’m still going to check out at least the sandwich option. I think some if not all of the structural elements need to be custom ordered to dimensions, which would speed up the building process tremendously. If I’m guessing right, the costs might increase astronomically, but we’ll see.
A quick tip for measuring areas
Besides a measuring tape, SketchUp has a really convenient area calculator. It lets you select multiple faces on the object and calculates the combined area for you in precision of the unit you have chosen for your template. Quite handy!