Category: Roof

A word on the importance of drips

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Rafter extensions

There has not been a lot to report of any real interest of late, hence an article about drips. However, before getting into the drips, I should record what I have been up to. This almost exclusively concerns the roof: finishing of the roof plate (cutting holes and driving down hazel pins, inserting noggins and insulation, glueing and nailing down the top plate), and putting in place the remaining roof frame elements (the purlins on the ridged element, securing the rafters on the lean-to element, exending the ridge rafters on the side where they have to overhang the walls). This has all been rather boring work.

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The roof arriving

The roof itself has arrived, although not without problems as some of it fell off the forklift when being taken from the lorry to site. I am using basic pre-insulated box profile steel sheeting – so it is very simple to install, albeit not a one-man job hence why I am waiting for Builder Tim to have a couple of spare days. I am very much looking foward to the security of having a permanent roof on.

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Ooops!

This brings me onto drips. For the last few months I have had a temporary roof in place – i.e. tarpaulins fixed onto the basic roof frame. I had to do this, rather than install the roof sheeting earlier beause of the need to access the top of the walls to drive pins down through the bales. Having tarps is a bit risky because they are always prone to being torn-off by heavy winds and when there are winds there is usually also rain. It is easy to get to a point where you have fixed the tarps and then stand beneath them on a rainy day and feel comfortable with the fact that no rain appears to hitting the straw. However, this can cause you to overlook the issue of drips. It is almost certain that there will be some small holes somewhere in your tarpaulin, possibly where you may have hammered some battoning onto it to hold it down. Water will probably be getting through these holes – maybe only a very small amount, but as you know from what happens if you put a bowl under a dripping tap or leaking pipe, this can very quickly mount up. You therefore have the potential to be channeling a very large amount of water into a very concentrated area, yet not be able to see this happening.

I fact, I found three areas where this had been happening which I only noticed because grass was starting to sprout from the straw. In these places a narrow strip of straw, only two or three inches wide had become saturated to a depth of about 5 or 6 inches into the straw, down to a depth of around five bales. In all instances I was able to track this back to a small hole in the tarp. Fortunately I don’t think this is a major problem because the dampness is very localised, should dry out now the leak has been plugged and I could probably pull-out and replace the straw if necessary.

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The ‘office’

However, this is something that it is worth paying attention to if you are likely to have a temporary roof up for any period of time. Also bear in mind that a tarp will channel water and if you are not careful in how you secure it (or if it breaks loose from its ties) it does have the potential in a heavy shower or storm to direct a lot of water into a place where it can cause real damage in a very short time.

I have also had some issues where windblown rain has got into the face of a wall, but this hasn’t got to any great depth. However, I will make sure that before I put render on I have checked to make sure that walls are completely dry.

Here endeth the precautionary lesson about drips.

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Weather-tight (ish)

Have now put some tarps over the roof frame using lashings of rather fetching pink baler twine. Baler twine is officially the best value product known to man – 10,000 feet of it for £19.74. No wonder farmers use it for everything. You can’t dig around here for more than a couple of minutes without coming across a piece of it. In much the same way that the early Celts were know as the beaker people, on account of the different types of clay beaker they left behind them, when future humanity unearths the remnants of our civilization from the time before the Trump apocalypse we shall probably be known as the twine folk.

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Raising the roof

This weekend we raised the roof, despite the forecast of gales. It took seven of us, Rachel and myself, plus two ‘DIY handy’ brothers-in-law and three very capable neighbours (all of whom were sailors, one also an engineer and another a former Navy officer – thus very familiar with ropes, spars and scrambling around at altitude).

It was actually quite straightforward, once we had worked out a system. This system was:

  • To place the apex of each roof truss onto a pile of straw bales – to both get it a little higher of the ground and give a platfrom to pivot them on
  • Flip the truss up so that one corner is at the level of the roof beam
  • Rest this corner on the beam and secure with a rope and then lift-up the other corner with a combination of rope from above and pushing from below and also swing it onto the roof beam
  • Repeat so that all the trusses are hanging upside down between the roof beam
  • Raise-up the end truss horizontally (with a rope and pushing from below) and slide its apex onto the end roof beam
  • Repeat for all the others so they rest on the base of truss ‘in front’ them – so that all are laid like a pile of toppled dominoes
  • Then winch-up the topmost truss, using a pulley from a tree conveniently located at one end of the site
  • Repeat for all the others (we abandoned using the pulley once it was apparent that having a rope on the apex of each truss allowed us to control the top, and using a length of 4×2 to push the truss up).

All finished by lunchtime.  Then just time to put in some floor platforms to make it easy to work on the purlins and add some more bracing in order to withstand the gales.  It is still standing two days later, despite everything Storm Isobel could throw at it.  Thanks everyone.

Lord of the Ring Beam

One of my main worries has been the logistics / feasibility of constructing the roof ring beam up on the scaffold platfrom.  I am not sure why, but issues such as supporting it while it is made, nailing it from underneath, positioning it correctly over the base plate areissues you can work out in theory, but can’t really be resolved until you can try it out in practice.  Now that the scaffold is up, and given the lack of rain, I have set aside this week for ‘cracking-on’ with the ring beam.  The good news is that everything seemed to work.

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Laying out the OSB on the temporary ‘table

The scaffold was built so that a series of screw jacks, with ‘U head’ tops could be set-up either side of where the walls are going to be. Timbers sit between these which the ring beam and roof can sit on.  There are four of these lateral supports for each wall.  The first job therefore was to put in some timbers and level them up at seven bales height above the baseplate.  This was a bit of a fiddle because some of the scaffold poles had to have 5cm or so cut-off the tops to ensure there was enough space to wind them down onto the straw and slot the timbers out.

I then put some temporary lengths of timber between these, along the length of where the ring beam will run.  These are just to provide a form of table on which I can build the beam and hopefully it will be straightforward to pull them out once the beam is complete.  Then it was simply a case of laying out the pre-cut pieces of OSB that will make the bottom plate of the beam and using some long lengths of 4×2 as uprights, together with a spirit level to make sure the OSB pieces sit exactly above the baseplate.  The next job was to cut the holes into the OSB where the window and door supports will go.  I decided to do this at this point, rather than when the OSB pieces were sat on top of the baseplate template – just so that these holes would reflect the actual positioning once raised up.

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The OSB base fully laid out

Having got the OSB base positioned  I put the pre-cut timbers in place to check eveything fitted together and then mixed up the Cascamite wood glue, removed selected timbers, put the glue down and then replaced and clamped the timbers.  Next was the tricky business of nailing the timbers in place from underneath.  This was easy for the side of the beam alongside the scaffold platfrom, but a real pain on the other side, where I had to stand on a ladder and nail above my head – not much fun when you have 50 or 60 nails to put in (and no nail gun).  Anyway, this was uncomfortable rather than impossible.

My one outstanding worry is the setting time of the glue.  This is around 6-12 hours at 15 degrees centigrade – but I have been doing it in temperatures close to zero.  Nothing says than you can’t use the glue in freezing temperatures, but rather that setting time is temperature dependant.  I therefore think it will take some days.

Anyway, four days in and I have completed three, of the four sides of the beam.

This means I can schedule in a roof-raising weekend for the start of February. I will need as many people as possible prepared to help raise-up the trusses and put at least some of the purlins in place. Let me know if you want to come along.

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Gluing, clamping, nailing the timbers in place

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Another view – where you can see the screw jacks, U heads and timber supports

The roof

Now the baseplate is complete, it is time for the roof.  As per latest thinking on straw bale builds, the idea is put the roof up (with a tarpaulin covering) on temporary scaffold supports, and then build the walls underneath.  This gives weather protection for the straw – especially important given that we will be putting the walls up in a wet winter.

The main components of the roof will be the roofplate ringbeam which will sit on top of the straw walls, four roof trusses which will sit on top of this and a series of purlins running the length of the roof.  I am using a ‘purlin heavy’ approach because the final roof covering will be sheets of box profile metal – and these need support laterally, rather than down the length of the roof.

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Ringbeam pieces on the baseplate

The biggest design consideration is how to construct the ringbeam, because even if made in relatively small sections it will be very heavy – thus something tricky to raise-up to wall height level after construction.  I have therefore decided to construct the ringbeam in-situ on top of the scaffold supports and to this end have pre-fabricated all the components by laying them out on top of the baseplate, using this as a template.  There are a lot of pieces because the ring beam is using doubled-up timbers in order to limit how deep the beam has to be with trippled-up timbers in key areas such as window spans or places where the trusses will rest.

I have also spent the time between Christmas and New Year making the roof trusses.  These are a very simple design, given my limited carpentry skills, which are bolted together, rather than jointed.  I also think a more industrial bolted approach suits the agricultural style of the building, given that two of the trusses and the purlins that sit on them will remain exposed within a vaulted roof.  I have used rough-sawn, untreated timber which looks much more natural / organic than the the planed and bevelled stuff.

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First truss laid-out

I asked my ‘consulting builder’, Tim Moss, to help me lay-out the the first truss given that this requires a bit of technique and trigonometry.  Once we had made a template ‘jig’ and cut the first pieces it was then relatively easy for me to cut all the rest and then bolt them together.

The plan is to move the trusses within the footprint before the scaffold goes up (because once the scaffold wall is in place it there will be no way in other than hoisting them over the top).  Then it should be a case of suspending them upside down from the ring beam, once this is built, and then pulling them upright (the opposite of the process I used to lower the trusses in the building I demolished).  Setting up the trusses and then secruring them with the purlins is a form of barn-raising job the could easily be done over a weekend with a few helpers – so I may well be putting the call out to volunteers in the not too distant future.

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Trusses complete

The scaffold arrives on Monday – which will be exciting since it will actually feel like a building is going up and it will also be the first time to really get a feel for the best way of making the ringbeam.