Below are lots of words and pictures attempting to show you how to build a reciprocal frame. I have finally got it together, with the indispensable partnership of Helen Iles of Undercurrents, to make a video that should explain things a bit better.
Click on here for the May 2012 Living in the Future video on Reciframes.
This is the basis of an article published in Permaculture Magazine issue no.59, 2009.
(But be warned ` the writing makes it sound much too complicated and we have simplified the marking on each rafter so this is obsolete really except for minor stuff. The above video is a better guide).
I didn't build my first low impact funky structure till I was over 40. The reason? I thought building dwellings is what experts do, and, specifically, I didn't know how to make a simple, cheap roof that looked good and kept the rain out. Walls are easy, you think, but roofs -- what if it falls in? So here is a way of making a good cheap roof that you can build as soon as you decide that the society we live in is not going to solve your housing needs for you, and get round to building your own shelter, or at least starting with one for the chickens or pet dog.
The term 'reciprocal frame' was coined by Graham Brown of Findhorn in about 1987, so it hasn't been around for very long. I will call them Reciframes to save syllables. The idea has been around for ages - Leonardo da Vinci designed a floor using reciframes where none of the floor joists was as wide as the floor itself. In Japan there are many ingenious structures using the principle, and those Morris dancers among us will remember the Sword dance where six swords are held out into the centre of a circle by the dancers and placed one on the other until they are locked together. The flash young handsome dancer (there's usually one) then leaps up onto the sword reciframe in the centre and is supported at shoulder height by the rest to the accompaniment of wild cheers and windblown scots bagpipe music. Yes children, try this at home. The first reciframes I heard of in Britain were the whisky barrel houses at Findhorn community, and Jack Everett's dojo near Stroud. Once grasped, the idea of a reciframe is very easy to make your own, and it is spreading very fast as people learn how easy they are to make, how efficient and strong they are, and how great they look from underneath (and above, until you put the roof on). I made one as soon as I heard of the idea in 1990 and have built about 14 reciframe roofs, always using roundwood. There are about six or seven devoted reciframe builders in this part of wild west Wales, and we develop our designs by taking a good look at each other's buildings, covertly asking each other how we did a particular new feature, then amending our design a tiny bit for our next project. One day we will all meet at the same time and then the whole thing will really take off, but at the moment there is a lovely sense of spiralling evolution. I have never built one exactly the same way as the last. The whole process is improving, week by week. Here I will go through the basics as I understand it with you, on the assumption that there will be some person somewhere who already has a much improved system. So don't treat this as gospel - use it as tips to save you time - maybe a couple of decades.
You can use a reciframe to roof over a structure of any shape. You don't need a central pole, and none of the rafters need be as wide as the building itself. You also get the option of a skylight in the centre, which is a big plus for a simple den or dwelling which wants to make the most of natural light. You can use from three rafters to sixteen or more. If your building is square and you put the rafters at the corners, it is going to look almost like an ordinary roof. If your building is shaped like an amoeba, and even with walls that rise and fall as they go around, you can use a reciprocal frame for that too. The rafters don't have to be the same distance apart.
What is a good idea, however, is not to try to fix the pitch of the roof. The pitch (slope) depends upon the diameter of your building, the thickness of the rafters where they meet, the number of rafters, and the shape of the building. But you are basically arranging rafters so that they support each other where they meet. We hold up the rafters with a temporary support, a Y-shaped post or two lashed poles called a Charlie, near the centre while we put the rafters up. When all are in place we take away the support and the roof finds its own level. A reciframe roof is usually of lower pitch than a 'normal' roof, and you are not trying to hold a particular pitch. So most of the weight will be going straight down the walls, rather than outwards. This means reciframe roofs are good for structures such as simple strawbale dens with loadbearing walls, although I try to build in a ring of some kind to spread any outward push from a roof. I will describe my default way of working, and will try to mention options where possible.
You need something strong to put your roof on. Reciframes are ideal for turf roof construction because if you have a large sheet of rubber (preferably) or plastic it will lie, within reason, on any kind of irregular shape. Reciframes are not the natural choice for a tiled roof, for example, because of the irregular and wacky angles that you get as you approach the centre. A rubber sheet forgives all that. But a turf roof means you have a lot of weight up there, so your roof needs to be well supported. I have used loadbearing straw bale, loadbearing cobwood, and a henge of wood that bears the load with sides filled in with various materials, usually straw or cobwood. I now have come down firmly in favour of the henge system, for several reasons.
First, the weather. If you go flat out to build a nice wood henge, then put a reciframe roof on, you can have a waterproof covering over the whole building area as you start the tedious bit of building gorgeous walls of your chosen material.
You have much more leeway if the walls don't have to hold the roof up. If you need good hard solid load bearing walls before you get a roof on you have to mess around with big plastic sheets to keep the walls dry when it rains, then wait till they are strong enough to proceed, then worry that you didn't wait long enough as you pile the turves on.
Second, the aesthetics. A wood henge looks gorgeous, specially without black plastic all over it. So I'd go for a load bearing skeleton every time these days. My default design is round, with a level henge of good strong, plumb, posts and beams thick as your thigh in a circle, with feet buried firmly at least 70cm in the ground, charred and bitumened. The number of rafters is the same as the number of upright posts, so the weight will go straight down. The cross pieces of the henge are half lapped and pinned to give strength to the circle,
and there are several diagonal braces around the circle to hold the skeleton solid. The non henge option is, of course, possible. The walls in this option need not be level, and you can position the rafters over parts of wall that are the strongest i.e. not over windows and doors. This option requires a strong wall plate to spread the roof weight around the structure. On a straw bale wall I use, for preference, round oak branches that are curved to follow the line of the wall. If you use flat sawn wood for this, as you do on a cobwood wall, you can join the sections of wall-plate better, but you need to make a saddle of hardwood for each rafter to support its weight over the middle of the wall, rather than at the outside edge.
This shows a rafter being lifted a fraction while the saddle is put in place. Each saddle is made from a piece of hardwood as thick as your calf and as long as from hand to elbow, cleft down the middle, and a curve cut out, with adze or chainsaw, to take the rafter. Just keep the saddles on hand and fit them as required.
Preparation and measurement
Decide how wide your building is to be then cut your rafters (and cordwood for the walls, come to that) at least six months in advance. Ha! We are not doing the quick visit to B &Q here. We are building a funky roof of natural materials for almost nothing. Best way of obtaining roundwood to build a largish structure like a den, cabin or house is to borrow some money and buy a piece of forestry. 2 acres would be more than you need. Thin the piece of forestry, cutting out lots of lovely straight poles and snedding them at leisure. Sell the forestry again, for slightly more than you paid for it. Buy the rest of your building materials (mostly pond liner) with the profit. That's the way to get a free designer home. Anyway, I digress. However you do it, decide early and try to get your wood at source, ie on some forsaken boggy windswept piece of forestry in winter. Don't let me put you off - ask a friend or a specialist to get the wood for you, if you are clean out of welly boots and muscles. There is no best wood for reciframes - most wood is suitable, but bear in mind that the wood needs to carry weight across its length, so use wood with a good tensile strength that will retain those same properties after a few years. Sycamore, the poplar family and alder, for example, don't score very well in this category (although they are fine for cobwood walls), but ash, oak, chestnut, or spruce, larch and douglas fir do.
How wide can a reciframe like this be? Well, it depends on the number and strength of your rafters and of your assembly team. A reciframe is surprisingly strong, but it is best be safe than sorry. Don't expect more than a three metre span of a length of calf- or biceps-size roundwood without a support. This assumes a manual, fairly funky way of working. The building we made in Portugal in 2007 was of eucalyptus, which is a dense hardwood. All the rafters were as thick as your thigh and probably weighed at least 100 kilos each.
The building is 10 metres wide so we figured that we needed three strong internal supports at the halfway mark along the three strongest rafters to help the reciframe
.We also needed a team of at least eight strong young people to lift and manipulate them. For a small rural bedsit den, 5-6 metres diameter is a good size. I would not go for a building wider than 6 metres unless I used some internal uprights as well, although this is easy to do. What feels handlable for a group of people working to build a reasonable sized den is 12-13 rafters on a building of 5-6 metres internal diameter. Cut each rafter way too long, and cut too many as well, so you have a good choice. For my default structure of 13 rafters I usually cut about fifteen rafters of 3m (radius of building) plus 1 metre over in the centre plus 1-2 metres over at the eaves. So we have fifteen 5-6 metre long poles, seasoned for at least a few months to reduce risk of hernias. You'll also need long poles for the Charlie and central tripod, so cut a round 20 poles while you are at it.
If you are building a den as one of a group, or if you have not built one before, it is important that everyone involved understands the principle of what is going to happen, and the things to look out for in the operation. I would therefore recommend that you always carry out a simulation on the ground before you start balancing heavy beams around above people's heads. What I tend to do these days is a two stage simulation, using the actual rafters.
1. Lay all the rafters out on the site, preferably before you have built anything, dug any holes or even marked anything out, but after clearing the site of topsoil etc, so you have a clean flat area to work in. Bring in each rafter and lay each one across the last one about 50cms from its thinner end, making up a simple spiral on the ground.
Reject any poles that are not sound in any way at this point. Arrange the rafters where you think you will use them, and mark on the ground where your walls could go allowing for well over a metre of spare rafter at the eaves. This allows you to see whether your ideas about how much space you have, how large you want it to be, etc are realistic, bearing in mind what rafters you have available. Obviously you want to use the strongest and straightest parts of the poles that you have, and can even cut the thinnest pieces off your rafters if you are clear they will not be used. When you have your spiral on the ground, measure its height.
2. Now simulate an actual reciframe with the rafters you have selected, still on the ground, but using a small Y-shaped pole or tripod Charlie maybe 1.5 metres high.
This was for a henge, so stakes mark the posts. For a non- henge structure you can mark out the line of the wall with some other method eg rope laid on the ground where the wall will be. The small Charlie will need to be still strong enough to hold up all the rafters.) In the picture the Charlie is a tripod.
Since we are now about to simulate the erecting of our reciframe with real big rafters, it is necessary to explain the positioning of the Charlie, and the relation of each rafter to the next.
What we are aiming at is a central circle that is over the centre of our building. You can do what you like of course, but this is the method I know.To have a regular central hole we need each rafter to miss the centre by the same amount. We will have either a henge or a wall to guide us, and we can decide how big our hole should be. Say we want it a metre wide. Then each rafter will miss the centre of the hole by half a metre, right? So if we put a stake in the centre of the circle, place a rafter on the henge point where rafter no 1 will go (it matters not where we start) and place it across the circle, to the right of the central stake by half a metre. Where does it point to across the circle? Place something brightly coloured at that point. Our Charlie must support this pole and must not get in the way of other rafters, so the best place to locate the fork of Charlie is along the line of this first rafter, about 60cm towards the outer circle. Place Charlie in this spot, lay the first rafter on him with a metre or so to spare beyond the fork, and tie the rafter to Charlie with string. Now take rafter no.2, go around to the next point clockwise where you want a rafter (if you have a henge, it will be over the next upright), and lay no 2 onto no.1, allowing the same distance from the central stake. Put a 6 inch nail into no.1 to stop no.2 rolling down no.1.Don't bang it too well - this is still only simulation - better to reuse the nails . You can tie it to no 1 as well. Note where the rafter points across the circle. Usually I find that the Sacred Red Jumper (SRJ) has to be something like two-thirds of a post span to the right of the post opposite. If you move the SRJ to the same equivalent place for each rafter as you put each rafter on, you get a regular central hole.
At this point we must introduce a new word. Imagine that your rafters, laid one over the next, look rather like a hand of cards. You want to see the picture or value of each card in your hand, so you spread them out evenly with the same amount of each card showing. Similarly, we want a regular distance betwen one rafter and the next around the circle so that all are taking roughly the same amount of weight and each is resting on its neighbour in a regular way. We therefore use the word 'Rummy' to describe the distance between the point at which pole A rests on pole B and B rest on C, as seen directly from above. If the rummy is small, say less than a handspan, your central hole will be rather small - maybe less than 60cm across. If it is two handspands or more the central hole may be quite large - over a metre - depending on the number of rafters you use. The hole shown here in reci12 and 13, is of thirteen rafters, with a rummy of one generous handspan. It is about 90cm wide, thus accommodating two fine coach windscreens, one on top of the other. It is regular because each time we placed a new rafter on the preceding one, we drove a long 6", 15cm nail into the rafter below where the new rafter rests against it, and the nails are a handspan apart. Dear reader, I fear I have lost you. Recipes are hard work, are they not? But if you simulate your reciframe with a small Charlie as described, you will, magically, see what I mean. Using the SRJ around the perimeter, and having a rummy of the same distance between nails, you will acheive an awesomely satisfying central hole. This is kind of important, because you may well find your eye drawn to this roof on many occasions in the future. I could show you photos of satisfying and less satisfying circles, but hey, life is short. Here's a nice one, using these principles.
from above and below.
Here's the rummy principle in essence.
STOP PRESS SIMPLER SYSTEM SHOCK!
In Sept 2011 in Cornwall, at the build at Planit-Earth in Sancreed, near Penzance, we came up with a much simplified version of this Rummy stuff. If you mark out your rafters in advance you can save a lot of time and get a nice round hole. 1. Measure down from the top end of each rafter and put a X in black marker at the 70 cm point on the top of the rafter. 2. Divide the circumference of the hole you require by the number of rafters. In this case it gave us 29cm. Put another mark 29 cm down the rafter from the first mark and, with an adze, cut a shallow groove at an acute angle to correspond with the angle of the next rafter. If you have 12 rafters, the angle will be 30%, for example, but you can guess it and refine it when fitting. The cross will marry up with the groove on the rafter below. If you follow this religiously, and fix each rafter with its cross directly above the groove, you can tie the rafters together as you are progressing, and at the end put a very long bolt through each join. Often the bolt will be almost exactly where the cross is.
Here are some pics from the Cornwall build:
Going for Real
OK you've done your simulations. Choose a clear day and go for it. Take care lashing a good central tripod together so that you can climb up and have your head and shoulders sticking up above the circle, ie the top of your tripod must be high - maybe 3 or 3.5 metres up. Use plenty of rope. Lash your Charlie to this. If Charlie is a pole, the fork of the Y must hold the first rafter high enough for all the other rafters to pile on each other, forming the central circle, and for there to be enough room left for the last one to fit in under the first one. I use a formula:
When all is ready, go for it. Have all children kept well clear. Allow no-one within the circle who is not totally focussed and in full possession of their senses. Cut out chat. One person speak at a time. Put rafter no.1 on, as you did in the simulation. Tie it to Charlie. Tie it loosely to the henge. Place number 2 rafter on number 1, with its heavy end clockwise around the outer wall, and its top end pointing to the Sacred Red Jumper. Make a shallow indentation or groove with a small adze or axe in rafter 1 for no. two to sit in, and lay the rafter in it. From your commanding yet apparently suicidal position at the top of the central tripod, whack a good strong 6" or so nail into No.1 to stop no.2 rolling out of the groove and away. Tie 2 to 1. Move SRJ around clockwise to its next alignment place on the henge or wall. Repeat until you have put the last rafter in under the first and everything looks wonderful. With luck you will have a nicely shaped central hole and a small gap between first and last rafter. At this point, take a ratchet fixing strap and tie it around first and last. Check that all rafters are tied to their neighbours with baler twine. Untie Charlie from rafter no.1. Make a groove and put a nail in the last rafter for where the first one should go. Then get trusty and brave helpers bit by bit to hammer away the foot of Charlie or to knock gently his two feet apart.
. (Have a look at the earlier picture of the hole seen from above and note the fixing strap holding each rafter to its neighbour.)
Then make good around the walls, using metal fixing strap, long nails, rebar or oak pegs. Put the kettle on.
Finishing the Roof
All I have just written here takes only a couple of days to do. One for simulation and one to do it. The rest of the roof might take you two or three weeks, but here is the process in brief, and pictures. Usually we use some kind of hardwood struts, fixed as radials, to hold the weight of the turf and to bind the whole thing together.
. Fix turf retainer, or fascia , of cleft chestnut at eaves. Cover with white canvas. Cover with 30cm of straw. Cover with rubber pond liner
. Fix spiral drainage pipe onto rubber with gaffer tape. Cover with Polyfelt geotextile. Turfin'.
Cut hole and place windscreen skylights over turf. Weight down skylight at corners with turves.
2. Mark out, saw, drill and screw planks, sawmill slabs etc onto the rafters to give a solid wood ceiling.
. Fix slab fascia to eaves. Cover with agricultural fleece, then 30cms straw, then Polyfelt, then rubber pond liner then Polyfelt then turf. Add skylight to taste.
Making a simple roof like this is very cheap, it can be lots of fun to do, and they look so natural. Keep thinking along safe lines as you're working up there, but don't be afraid. Reciframe roofs have a lot of failsafe built in, and they are much easier to make than you'd think. Good luck.