Wren Technical Design Guide
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This is a brief introduction to the latest WikiHouse structural system, and how to get started using it. Use this in coordination with the CNC Manufacturing Guide and each project Assembly Guide
As always – all information is shared under the licences & terms of the WikiHouse constitution. All information is shared ‘as is’, and without any kind of liability or guarantee, and will not be appropriate for all uses & contexts without modification and/or testing. Make sure you consult an engineer and/or other relevant experts, and comply with all local legislation - as you usually would.
The WikiHouse chassis system is designed to use any structural panel material (usually structural plywood), of size 1220mm x 2440mm x 18mm (or 1200mm x 2400 x 18mm in Europe). However there are some other factors to consider:
Tolerance Even though the stated thickness may be 18mm, within the pallet thicknesses can vary. Parts need to fall within +/- 0.5mm - in other words, 17.5mm-18.5mm . Any sheet outside this range should only be used for external panels.
Keep dry! Do not let the parts be exposed to prolonged moisture, humidity or hot direct sunlight. This causes them to expand or shrink and can make them unusable.
Structural performance There are various existing standards for panel products, but they may be tested to varying degrees. Your sheet material should meet the following technical criteria (if in doubt, consult an engineer):
- Compliant with BS EN 13986, suitable for use as structural floor decking on joists
- Compliant with BS EN 12871
- Preservative treatment as Wood Protection Association Commodity Specification C11.
- To be sourced from Europe or America, no Far Eastern wood-based panels to be used.
Sustainably-sourced Always use FSC (or equivalent) certified timber. Obviously.
Off-gassing Adhesives used in wood products can off-gas chemicals, which don’t make for healthy environments. While the industry is improving on this, always look for plywood or OSB which is formaldehyde-free.
Begin by deciding on the form of your structure. This form can vary over the length of the house, providing all the connection points line-up. The rules are:
Max room span - this will vary with load (eg wind, snow etc), size, frame depth, materials. Room spans limits of up to 3.6m on single storey structures are recommended (although longer spans may be achieved). Be sure to consult a qualified structural engineer to work out solutions for possible spans and heights.
Overall width - Theoretically you can have an unlimited number of spans (within reason), provided you have a practical method for raising the completed frames upright.
Overall height - Maximum number of storeys in completed projects so far is 2. Ultimately the target is 3 storeys, but more development work is needed to achieve this. Depending on your site conditions, you may need to be mindful of the implications of tall, steep designs increasing wind loads on the structure.
Roof Profile can be more or less any shape in section, but cannot include curves (currently) and cannot slope along the length of the house, only the width. Where very unusual shapes of roof are used, you may need to introduce an extra supporting wall or horizontal section as a tie (consult an engineer).
Also called a Superbox, this a moment-resisting portal frame, repeated along the length (y-axis) of the structure. It is made up from two primary elements: Fins and Spacers, with joints staggered to ensure structural continuity.
The default frame box depth is 250mm and width is 150mm, though it can be lower for smaller structures or higher for larger ones.
The full Superbox Frames are assembled from the following components:
###3.1 Fins (F)
Fins are made up from a series of repeated Grips, 18mm high stubs that connect the assembled frames to the sheathing panels. By default these are 180mm long, and arrayed around the whole fin on a repetitive grammar of 300mm modules. A slot is included at each grip location where the spacers will fit into.
Dovetail (D) joints split fins into parts so no piece is larger than 1200 x 2400 overall, otherwise it won’t fit on a sheet. Where these D-joints are located needs specific engineering consideration, and should always be a minimum of 2 grips away from any corner or junction. To mistake-proof your design, try to orientate your ‘male to female’ joints in such a way that it is impossible to put parts in the wrong way
Fins are duplicated and spaced apart to form the sides of the Superbox Frame. Be mindful to keep slots and holes in the fins to a minimum size (we suggest smaller than 18 x 50mm) so as not to compromise the structural integrity of the frames.
Notches into the depth of the Fins should be avoided, particularly at the corners, as this will reduce its structural effectiveness.
###3.2 Reinforcers (R)
Reinforcers are normally required around corners, where the Spacers are discontinuous.
The Reinforcers can be screwed (in a repeating grid pattern) or glue-bonded to the inside of the corner fins.
This doubles the material thickness around the corners where the spacers are discontinuous and don’t carry any load. As a general rule the reinforcers should cover a minimum of two grips out from the corner junctions.
###3.3 Spacers (S) These form the top and bottom flanges of the Superbox, so there is internal row (IN) and an external row (EX) all around the Frame.
They mallet into slots in the Fins, located at every grip point. You’ll notice that these slots don’t have ‘dog bones’ - instead they crush in to form a tight fit.
If a span exceeds 2.4m (sheet length) you’ll need to join two or more lengths of Spacers together using a dovetail joint. These joints should be located away from D-joints in the Fins to keep structural continuity in the Frame. At occasional intervals in-between the two rows of Spacers sit the Space-Invaders (so-named by the WikiHouseNZ team because of their shape), which keep the Frame in a box shape during assembly.
Frames are assembled on a flat surface first, then raised. At some point during the process insulation needs to be added to the inside of the box frame. This will change depending on what insulation you use.
Connectors are generic, and relative to the depth of the Frame, by default 250mm. Generally there are only two types:
-‘Primary’ connectors (C1) slot in between frames during the barn-raising. In the floor these act as joists to support floor panels etc. at the corner points are pegged into place.
-‘Secondary’ connectors (C2) just slot into place in the roof after the frames have been raised.
The sheathing panels are an integral part of the structure, providing rigidity to the frame. They slot onto the ‘grips’ around the frame and line the bays between frames, both internally and externally.
As rule, try and keep these panels as large ( near 1200 x 2400mm) as possible. If a wall, floor or roof span exceeds 2.4m then a dovetail ‘zip’ provides the connection and continuity between two panels, transferring forces along the skin of the structure. In the floor it’s best to locate this zip joint on a floor connector to prevent sagging over time.
The panels must be screwed directly onto the Spacers in the superbox frames at regular intervals (we suggest every 150mm). This prevents the panels being pulled off the frames by wind suction, and stabilises the frames by providing racking resistance.
Openings for windows and doors can be made in any bays in the side walls and roof. Any side wall openings can be as wide as the gap between two frames (nominally 1050mm) and as long as the wall or roof span.
The openings are ‘boxed out’ with plywood trimmers, which form the sill, lintel and jambs so windows and doors can be easily installed.
Be sure to keep enough bays ‘solid’ without openings along the length of the structure for bracing.
While the moment-resisting Frames do most of the work, the End Walls need to be self-supporting and not blow out under excessive loads. Earlier versions of Wren proposed a plywood stud wall system, but this relied on screwing panels into the end-grain of the plywood. The structural strength of this cannot be currently verified as there isn’t any available data on the resistance/pull-out loads of a screw into the end-grain.
The current system employs a series of structural boxes (or cassettes), with an internal framing which creates a fixing face around the edges of each box for screwing the facing panels to.
There is a possibility for the End Wall boxes to act a shear walls in the future, and therefore reduce the lateral loading the frames and sheathing. More development and analysis is still needed to verify this.
Once you have a complete assembled chassis, internal walls are separate ‘stud’ wall box kits which are simply screwed into the frame at the appropriate location. Generally these are not load-bearing so they can be positioned anywhere, and changed during the life of the building.
The Wren chassis system can be used on more or less any site, including sloping sites, however the site must provide 2 or more ‘rails’ (usually timber joists) which are parallel and level.
These ‘rails’ can be supported by almost any kind of foundation type (eg screw piles, concrete trench foundations, adjustable pads etc.) They can also ‘step’ down a hill if required. The chassis can then be assembled onto those rails, and bolted to them using steel brackets.
Your foundation design should be always be checked and approved by a certified structural engineer.
Parts can be laid out onto cutting files manually in sketchup (you will need sketchup pro to export a dxf file) or use CNC software such as V carve. Free tools to make this process easier are currently in development.
Parts need to be ‘offset’, to give the structure some tolerance. Without these tolerances, the frame will be almost impossible to assemble. As a general rule, outer edges of parts should be offset inwards by 0.25mm (to make the parts smaller). Slots in parts should be offset outwards by 0.5mm (to make the slots larger). However, this does not apply to the ‘crush’ holes for spacers, which need to be tight.
Check the guide cutting file (.dxf) in one of the project repositories (ie. Microhouse), to view the layer names etc. In fact you might want to use it as a template.
Always cut a test piece first with the material and the machine you are going to use, and adjust the tolerances accordingly. It should be reasonably easy to assemble, but tight when assembled.
See the CNC Manufacturing Guide here
Include several Wiki-mallets (‘persuaders’) and ‘Stepups’ in your cutting files. They’re very, very useful for all kinds of things, from making a level surface, to safe working above your head, to getting in and out of the house during the construction process, or simply for sitting on when you have a tea break!
The system is designed to be as ‘product agnostic as possible’, allowing you to use almost any kind of cladding, internal lining, services, insulation, footings, doors, windows etc as possible. However, some solutions work better than others. It’s worth investigating other projects as they emerge to learn lessons.
Particularly important elements include:
Breather membrane The chassis should be wrapped in a breather membrane (such as Tyvek Supro or similar).
Insulation Solutions include soft-fill insulation (we suggest sheep’s wool or other recycled products, fibreglass insulation is horribly itchy and volunteers will hate it!), expanded polystyrene (must be pre-cut) or blow-in cellulose (a key element of this is the cost of blowing in. Can anyone invent or source a low-cost / hireable blower that will allow self-builders to do this for themselves?).
Please do share lessons, design files, photos, and costings from your project. There are number of ways you can share them.
Slack The WikiHouse Developer Community Slack channel. Request an invitation here
Twitter Tweet to @WikiHouse .