Building a Reciprocal Roof

A reciprocal roof is type of roof where the individual rafters mutually support each other. They originated in medieval Japan but probably date back further and aspects of reciprocity can be found in the roofs of many early human settlements. Reciprocal roofs are generally found on round, elliptical or polygonal buildings as the weight of the roof can be easily directed downwards via the post and beam henge on which it sits.

The first rafter is held in supported by the upright charley stick. This is removed when the last rafter is in place allowing the roof to settle into its pitch.

Osian scribing the beam onto the principle rafter to achieve a perfect fit.

 

 

 

 

 

 

 

 

 

 

We came across the idea and fell in love with this form of roof when we worked with Neil at Earthmovesdesign. When we were designing our house design we decided to incorporate a twin reciprocal roof over our open-plan kitchen and living room to divide the space and to draw light into it via the openings in the roof.

Checking the angle of the scallop. With 12 principle rafters the angle at which they cross each other is 30 degrees.

Using the tripod and winch to lower one of the principle rafters into place.

 

Our design did present a number of engineering problems. The first issue was how to build such a roof on a non-circular henge. We were advised by a specialist engineer, Chris Southgate, to add additional braces at each corner of the henge to deal with the weight of the roof.

Beautifully executed seat cuts.

Using a plum line to check that the opening maintains circularity. Each opening is 1 metre in diameter.

The other issue was how to set the roof pitch. Normally the rafters are laid on top of each other in the centre and the pitch is determined by their collective spiral height (= diameter of all rafters at their thinner end). Our spiral height would be between 2.5 and 3 metres which would produce a 45 degree pitch. However as our pitch was set for 30 degrees we had to scribe and scallop out the rafters to achieve this. Having a carpenter on board who was able to deal with the complexity of this design was essential.

View from the top of the roof.

View from the inside.

 

The task was very physically demanding as our rafters have a minimum diameter of 200mm and are up to 6m long. We used Tirfor winch and a 4 metre tripod which a local engineering firm, Coughlan Engineering, fabricated for us to lift the rafters into place. We built both roofs on the beams they will be affixed to on the ground, as assembling this at height would be very expensive. We aim to lift of each completed roof and lower it onto the henge when it is erected using a crane.

We were delighted with the finished result which is sculptural and beautiful.  We really look forward to gazing at the sky and the stars though our roof.

The next task is to lay-up the north and east post, beams and braces. We will then begin the process of erecting and fixing the frame and roof, which will take 4/5 weeks. We are hopeful of covering the structure before Winter so that we can complete some of the internal jobs before next Spring.

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Working with Roundwood

In the last month we have been working alongside Osian Denman – a heritage carpenter – who has taken on the task of helping us to construct the roundwood frame and roof. This part of our build requires great expertise and skills to realise our design and to guarantee an excellent finish.

Enjoying the weather and the woodwork

We are learning lots of new skills and are using an array of both traditional and modern tools. We’re enjoying learning about how the structure will all fit together.

We have been taking advantage of the long evenings where possible to pack in as much work as we can.

An essential characteristic of working with roundwood is to see both its roundness and the square inside at the same time. Roundwood carpentry is different from other woodwork in that the wood being used is in its natural state. Despite this it still relies on keeping things square and plum as in ‘normal’ carpentry.

Using a plane to make sure the area around the tenon – the shoulder – is perfectly level.

Checking that the flats (the planed/level part of the beam where braces will be attached) are at right angles to each other. You can see ink lines on the wood which are used for measurements.

 

An interesting aspect of carpentry is the use of inches rather than millimetres for measurements. Inches allow for easy subdivision – half, quarter, eighth, etc – are all useful and easily recognisable measurements to visualise rather than, say, 20 mils.

Some of the beams receiving the finishing touch on the framing bed.

 

The Lay-Up

All parts of the timber frame are assembled beforehand on the framing bed, labelled and put to one side. These are then put together in the form of lay-ups (which is essentially a practice run where if there are any inaccuracies they can be corrected before the large beams are suspended in the air. Correcting at height will be more difficult and potentially dangerous, so time spent making adjustments on the ground is time well spent.

Lay-up of the beams which form the henge for our twin reciprocal roof over the living room/kitchen

 

 

Concrete compromise

At the outset of our build we aimed not to use any concrete in its construction due to its large embodied energy and polluting production. Reducing the embodied energy in what we use is very important and this had led us to choose materials in their raw state so as to reduce our carbon footprint . However, we’ve had to compromise on this as our engineering specification for supporting the posts means we needed to use some concrete in order to achieve compliance and to ensure our house will be structurally sound.

We used 4 tons of concrete. I used an online calculator to calculate the amount we had to mix. Luckily our local hardware – Tullamore Hardware – allows returns of used bags of cement as it can be difficult to store on site.

Reinforced concrete pads (17 altogether) with rebar. The rebar will ensure the posts can’t move from side to side, as they will be drilled to house the reinforcement.

 

We have enjoyed several visitors to our site in the last few months. It’s a great experience to share our ideas and dreams with friends and acquaintances.

 

Natural Building

There are many definitions and interpretations of natural building; it is a collection of different approaches to building rather than a ‘one size fits all’ term. One definition I think which adequately sums up our approach to natural building is “a building philosophy that emphasises sustainability through using minimally processed, locally available, plentiful and renewable resources to create healthy living environments” (Crimmell and Thomson, 2014). I think this is a much clearer definition than ‘green building’ or ‘eco-building’ – this three letter prefix has almost become meaningless when used in relation to the building industry and is often an example of ‘green-washing’.

In our experience when you reduce the embodied energy in your building materials, e.g. straw bales and roundwood you do have to add considerable amount of physical energy to the building process in order to utilise the materials. For example our roundwood had to be debarked and will have to be further cut and jointed to become structurally supportive – all of this takes lots of time and energy and no doubt the finish will be beautiful, but it is definitely not a short cut.

To get most of the bark off we used both debarking spades and a small axe for tricky bits

The debarking of all our poles took four weeks (5/600 hours of labour) and involved brothers, sisters, nieces, nephews, fathers, uncles and friends. We had four debarking spades so that anyone who came could help out.

Luckily the weather in April was really dry so the moisture content in the wood has begun to reduce (some of the poles were over 50% moisture – they need to be below 20% when we close off the house).

It was great working outdoors during this period and we’ve been visited by a cock and hen pheasant, several rabbits, and, a startled hare. The cattle and the swallows have returned – along with other birds – there is a frenzy of nest-building in operation.

When completed we were left with a huge amount of bark which will be used as mulching around our site – for gardening and walkways.

The debarked poles were measured and marked with Roman numerals to aid identification in the construction phase – this task was quite complicated and took about two days to complete – we checked the lengths and diameters against our saw list and plans. The poles were then moved up to the site -we were able to move the small diameter poles (150mm) which will be used in the roof ourselves – but we had to hire machinery to move the larger diameter ones (300-350mm).

Moving the large diameter poles

Unfortunately due to some humid weather mold has begun to grow on the poles, this is a natural occurrence on debarked wood, and we will have to deal with it when the poles are being erected in the timber frame.

The mold spores travel freely through the air and by debarking the poles we have provided an ideal habitat for them! We have experimented with bicarbonate of soda which not only removed the greenish/black mold but also seems to prevent it from returning.

Large diameter poles on site. Next stage – wooden ring beam, frame and roof…

The next stage of our build is to begin work on the wooden ring beam which sits on top of the tyres and will support the straw bale walls. We will be working with a roundwood carpenter to help us with bracing the poles, and are awaiting the delivery of a tripod to aid lifting the large diameter poles.

Marking each pole at both ends is essential. II means diameter 150mm and length 3,500mm (3.5m)

Our timing is quite important and ideally we’d like to have the roof on by the end of July/early August so that we can get the straw walls up and plastered before winter.

Harvesting the roundwood

That’s a lot of wood and it all has to be de-barked too!

Using roundwood in construction is not widely practised in Ireland at the moment. Where roundwood is used it tends to be for small-scale structures. Its use for family homes is rare. This has led, in our experience, to a shortage of skilled roundwood carpenters and suppliers of specialist tools and other equipment. Hopefully in the future this situation will change, as people become more aware of roundwood’s natural beauty, strength and suitability as a building material.

Harvesting our roundwood was a detailed process.

Stage 1: The Saw List

This is a list of all the sizes and diameters of the wood which will form the post and beams for the walls and the rafters in the roof. Our saw list consisted of nearly 200 poles with 17 different lengths and four different diameters. Our engineer calculated the dimensions to meet specific building standards and to meet the design’s requirements for strength and load-bearing capacity.

Stage 2: Finding the forest

It was important for us to find a local, sustainable source for our wood. Ireland is one of the least afforested countries in Europe and we began the process of looking for our wood about six months beforehand, as we knew our lead-in time could be significant. I had spoken to another self-builder who used roundwood as part of the research for our home, which was vital in getting the timing right.

Our supplier was Paddy Purser (Purser Tarlton Russell Ltd) whose task it was to find a forest where all our wood could be harvested. This is more difficult than it sounds, as our wood would not be all the same age and therefor could not come from a forest which was clear felled. It had to come from a sustainably-managed forest, literally tree-by-tree.

The forest in Clora, Co Wicklow

The forest was located in Clora, County Wicklow, and forms part of the mysteriously named, Devil’s Glen. The foresters then took our saw list and began looking for suitable trees within the forest. Individual trees had to be measured with a calipers. The forest adheres to ProSilva ideas and methods, which emphasises a tree-by-tree approach.

When the trees were identified they were then cut and transported to a collection point.

Measuring the wood

We visited the forest to meet the foresters and to see where our wood came from. This was an important part of our building experience as we saw a very healthy forest environment and we made a connection with the origin of the main future supports of our home.

It was notable the much wider variety of wildlife in this forest compared to the dark, uniform forest plantations widely seen across Ireland.

Stage 3: Storing and debarking the roundwood

Storing all the wood on-site would have been difficult and luckily we were able to use the farm’s old silage pit, where the roundwood could be spread out, dry and be debarked. Storage of this amount of roundwood could be problematic on some sites, so it does need careful consideration.

Debarking in the morning

Debarking in the evening

Roundwood needs to be debarked as soon as possible after harvesting. Debarking removes the bark which is alive and moist, and a home for a variety of insects which can, if left un-debarked, slowly begin to eat your wood. We purchased several debarking spades for the Woodsmith website (I couldn’t find a supplier in Ireland).

Beginning the debarking process was pretty daunting! it was one of those moments when we thought “what have we let ourselves in for?” We have allowed ourselves a month to complete the task (working 5 and a half days a week). Log-by-log is the only way to proceed. Interestingly after delivering our wood, the driver was delivering his next load to the set of the Vikings for one of their sets.

Natural building methods can only be fully realised when you take your time and flow with the rhythm of your materials and tasks.

Spring planting and Living roof

In between ramming car-tyres with shingle and our wood arriving we’ve taken a few days to complete some gardening projects. One of our main priorities was to plant a shelter belt. Our site is exposed to south-westerly winds in particular, which is the predominant wind direction in Ireland.

Shelter Belt

Some of the 100 bare root trees which was planted for the shelter belt.

We wanted to include a mixture of native trees and shrubs so that it would look more like a typical country ditch rather than a homogeneous row of equally spaced trees. We purchased 100 bare root trees from Premier Nurseries, they included; Beech, Rowan, Laurel, Chestnut, Hornbeam, Birch, and Alder. In addition we also planted some Oak, Ash, Willow which we were growing in pots. Most of the trees were 40-60cm and at this height they don’t need support and will find it easier to establish a root system and become stable than 1-2m trees. We also purchased some shrubs from a local supplier Gardenshop.ie, including; Wild Rose, Elder, Hazel, Whitethorn, Blackthorn, Dogwood and Cherry.

When we finished planting the trees we added a 40cm x 40cm square of root barrier with a few shovelfuls of shingle or loose stones on top to allow the trees to outpace the grass and to maintain good moisture.

I found Mary Reynold’s book – The Garden Awakening: Designs To Nurture Our Land and Ourselves – and, Forest Gardening: Rediscovering Nature and Community in a Post-industrial Age by Robert Hart, both very useful reading in developing my thinking about planting and how to develop our garden.

Living Roof

Adding clay to the living roof.

The Studio we built, which is being used as a shed at the moment, needed more clay on the living roof and to be planted with wildflowers. It also needed to be treated so that the beautiful Douglas Fir colours would be given new life.

We planted the roof with wildflower seeds which we saved from out own back garden during last Autumn. We also collected a variety of grass seeds from grass which is not commercially grown. During the summer the roof should be full of colours and attractive to wildlife, especially the bees.

The studio was treated with Osmo Oil – a vegetable oil based treatment for wood from Pat McDonnell. It gives a superb finish and revitalised the wood as well as helping to fully water proof the building.

Mulch Path and Mini Herb Garden

Mini car-tyre herb garden

Adopting a permaculture approach we laid down a mulch path to prevent the top soil being worn away in the most walked-on places. We removed the grass and soil and used it to cover some of the ‘bald patches’ caused by excavations. We cut up the large ton sack the mulch cam in to act as a weed barrier.

For the mini herb garden, we filled several tyres with the clay we excavated for the foundation of the compost toilet. We used root barrier off-cuts and old sacks in the bottom to prevent grass and weeds growing (Growing in tyres for root crops has some critics but I think for a herb garden it doesn’t pose and issues). Car tyres are useful in preventing certain types of herbs, e.g. mint from colonising.

 

 

Car-tyre foundation

I first came across the idea of using car tyres as a building material watching Garbage Warrior, a documentary about Michael Reynolds and Earthship Biotecture. In Earthships the tyre is rammed with earth and forms the north wall of the building. Here it absorbs the sun’s heat during summer and it slowly releases it during Autumn and Winter. It also recycles or up-cycles waste material.

Levelling off the lowest point of the site.

For our house we are using car-tyre foundations to support our straw bale walls. The tyres are rammed with pea shingle (10mm shingle) and they not only support the walls, but also act as a self draining foundation and a damp proof course at the same time. The metal within the tyre means that it will act like a gabion. And they’re free to collect!  The self draining foundation is vital as it means water will never be able to wick upwards and affect the straw. Straworks have produced a very useful technical drawing of this type of foundation, which we are following.

Keeping the tyres level and more than 200mm from the centre points of the roundwood poles

We will need around 300 for our foundation and they will all have to be levelled and rammed…a considerable task. We have collected over 300 tyres (205s and 195s) –  with ramming each tyre will swell by 20-30mm. On our best date so far we filled 20 but on most days we will average around 14. This is quite heavy work and after two weeks of constant work we rewarded ourselves with a Saturday off.

There was some very difficult weather during this period which saw temperatures as low as -4ºC and included Storm Doris and Ewan. This site has become more muddy and some areas are difficult to manoeuvre in.

However tyre by tyre the foundations are taking place and we’re expecting our roundwood next week so we’re rolling with lots to do. Looking forward to finishing the tyres and starting on the wood.

Reinforcing the lowest point of the site with additional tyres.

The foundation

Straw bale houses need “a good pair of boots and a good hat” – foundation and roof. February has been all about the foundations. Our foundation has two components – the roundwood post and beam frame which will support the roof, and the car-tyre/gabion foundation which will be the base for the straw bale walls. The former will sit inside the latter. This design reduces the weight on the walls – which by definition are not load-bearing – but does add some complexity to the construction phase.

Prior to beginning the foundation we tested the base – which is made up of aggregate (see previous post) which we’ve added to the site – to estimate its bearing capacity.

Testing the Foundation base – this test measures the bearing capacity of our site, essentially how much weight/load can it safely support. As we are not basing our house on a concrete slab we need to determine if its base is suitable. The test we conducted was a plate load test. This was carried out by Testconsult.

Plate load test using a 13 tonne digger

The test involves loading a weight (a 13 tonne digger) on to a 300mm diameter steel plate which is placed on the soil to be tested. The reaction to the loading by the soil is then calculated by three very sensitive dials.

The engineer noted the recording and works out the ultimate score in KN/m² (for more details follow this link) for each test site.

 

 

Test plate centred over the location of one of the main roundwood posts

We tested the four places on our foundation which will be bearing the greatest load from the roof in the roundwood frame. We also tested a filled car-tyre – rows of which will support the straw bale walls.

13 tonne digger to test the base

 

 

 

 

 

The groundwork begins

January has been a busy month – site preparation and ground work. This initial phase of our build begins to change the character of the site. It becomes noisier, more mechanised; the soil is dug up for foundations and trenches; large lorries arrive and dump tonnes of aggregate; whacker plates; rollers; elephant’s foot etc. In short it begins to look like a building site and includes the following parts;

1: Remove the topsoil – all organic material (top soil) from the house’s footprint was removed. This also means we now have a massive heap of clay (100-150 tonnes) to store somewhere.

Top soil removal

It’s a good idea to consider this carefully, as if it’s stored in the wrong place, it will cause delays later on.

Your JCB driver also needs to separate top from sub soil/gravel so that it can be later added to your garden or used for raised beds.

2: Excavate holes for the poles – each round pole which will support our roof structure needs a cubic meter of structural aggregate (T0) as a footing.

Setting out the location of the poles

23 separate holes were marked out and filled, ramming at intervals to achieve maximum compaction.

3: Add additional layers of aggregate – over the entire footprint an additional layer of 300mm of permeable aggregate (T2 or 804) was added. And lastly 50mm of blinding (T3) was added. All rolled with a vibrating roller.

Another 20 tonnes

All aggregate has to be certified and the receipt must show this for building regulation. Aggregate was from a local supplier (http://www.kilmurrayprecast.ie/).

In total we added 200 tonnes of aggregate onto the site. The weather has remained quite dry which makes large, heavy groundwork much easier to undertake. In fact on one of the recent stormy days dust on the site was a problem – which is almost unheard of in Ireland…in January! What I learned here about project management was the importance of booking the aggregate lorries in advance so that the JCB is working constantly and there is a steady stream of work.

The next part of groundwork to be completed is a plate-load test to get accurate information about the bearing of the soil.

In addition to the groundwork we also erected a fence to mark the sites southern perimeter and lay ducting for our electrical connection and put in piping to move a water trough for the cows (who’ll be returning when the weather get warmer and the grass gets sweeter).

Who needs the gym in January when you can swing a sledgehammer?

We also began some planting for our shelter belt moving some of the small white thorn and black thorn bushes which are beginning to grow on the site because the cows are not there to graze them back.

It makes me realise about how true it is that a forest is the most stable ecosystem and one which land will return to if left untouched.

Building a pump house

Domestic pump houses tend to be quite dull buildings. They are generally windowless and featureless. We decided to try to do something different with ours, as the location of the building was quite prominent, and we had lots of off-cuts which we wanted to use. We wanted it to be a feature as well as having a practical purpose. We wanted to build something interesting.

Day 1: Two sides complete. The teak door was salvaged from a skip.

The skills we gained from building the much larger site office were employed here to design and build the pump house. We assembled the four sides in the farmyard and moved them to their final location, where they were bolted and nailed together. It took about 3 days to make the four sides.

Day 3: We assembled all four sided

 

 

 

 

 

 

 

 

 

 

 

On Day 3 when we had made all four sides, we braced them to keep their shape and drilled the holes for the bolts. It was important to do this on a level surface so that it would fit together properly. The structure was extremely solid and plum. We now moved all the sides up to our site to be put together.

On Day 4 we moved all the pieces up to the site and began cladding them with the off-cuts from the site office. The triangular windows are thick perspex salvaged from a skip. We wanted to allow natural light into the pump house as it will be used as a nursery for plants/small shrubs, maximising the south facing aspect.

We had some beautiful, crisp, dry, sunny days in mid-December which were ideal for this type of construction. The only problem was fading light which curtailed the working day around 4 pm. Any later would lead to loosing tools and making mistakes.

On Day 5 we began work on the living roof. We used the same method as before – layers of geotextile, insulation, pond liner and soil. The roof will be planted with wildflowers and grass in March/April. In the future we aim to plant shrubs or trees in this area, and in time what is now quite a prominent small building will ‘grow’ into its surroundings.

Living roof completed – just waiting for Spring to arrive to get planting

Insulating the walls with straw which will be covered with 18mm smart ply to protect the water pump from the frost.

Drilling a Well

A century ago, my grand Uncle Tom commenced a project which was both dangerous and potentially beneficial…luckily it was the latter. My grand uncle began digging, by hand, a well to supply water to the family home. Up to that point my ancestors were reliant on water from a spring in the fields behind their house (the spring – which I remember producing crystal clear water – has since then dried up and calcified due to bog drainage).

Well drilling 30 November 2016

Despite the intensely hard work, in some ways my grand uncle was lucky, lucky for two main reasons;

1) he struck water flowing in an underground stream about 10 metres down,

2) the soil he was digging down into was fairly compact, unlike the loose gavel strata evident in other parts of the farm.

He used bags of straw to ‘scaffold’ the sides of the well. The well was then walled with rounded limestone rocks which exist in abundance under, and sometimes within the top soil. It is still an impressive site to see and we used to love gazing into it as children, dropping in stones to produce that eerily beautiful sound.

The well remained in use until the 1970s when a deeper, more reliable water source was required for the family farm as it increased the production of milk, and thus required a much greater volume of water. This time the well was drilled to about 24metres and still serves the farm today.

Building a house in the countryside often necessitates drilling a well. Unfortunately there is no group water scheme locally, so we needed to find our own source. We did explore rainwater harvesting but opted instead for a well.

We contacted Leo at  http://www.leodempseywaterwelldrilling.com/ who has a lot of experience drilling in the Midlands area. Coincidentally his relatives also dug the upgraded farm well in the 1970s – a process which took over a week. Technology and speed have increased since my grand uncle’s day but the principle is still the same – keep drilling until you get a steady supply!

The well was successfully dug, after a spot had be divined – amazing to see this performed. We hit a good supply about 100 metres down, which was similar with other houses locally, but much deeper than the farm house, only a field away. The well drilling took 6-8 hours and when completed the well head was capped to avoid any contamination.

A capped well head. The grey material is the grindings from the drill and will be mixed to form base for the pump house. It’s a rough type of cement.

The costs are not inconsiderable; a) about €30 a metre, b) More expensive piping may be needed if there is crumbling rock, c) a water pump will also be required (€500+), d) water softener system – very important in a limestone area, e) cost of pump house construction, €5-10,ooo in total.

For the pump house we are going to construct it from the off-cuts from building our site office/workspace, and some additional bits we picked up for free. Once the pump house is completed, we will have water, and, later. electricity, as ESB Networks will have a connection point.