Building an Observatory

This article is of the construction of a roll off roof design observatory. I will try to discuss and show all the problems that I needed to solve over the course of construction. I started work in October 2008.

Planning and Excavation

Choosing the site for my observatory on my block was relatively easy. I had a flat turn around point which was far enough away from the house to give me a clear view of the south and south east. It is also far enough away from the house to prevent heat bloom issues. I could not really move further away from the house because power lines and a road would interfere with imaging. As you can see from the image below I basically marked out position of the pier support first and the outer corners of the observatory. I am allowed 10sqm before I had to submit plans to council. So I am keeping just below that figure to prevent the hassle of approval. I knew from the outset that there was limestone deposits on my block and that would mean a lot of tough work in the excavation department. Hence the jack hammer in the frame.

The pier support for this observatory was supposed to be 1m x 1m x 1m. My thinking was that 2.4 tonnes of mass would be more than enough to counteract any vibrations from the pier. This would place the center of balance somewhere around 300mm under the surface of the concrete.

We had assumed that there was only one layer of limestone to get through. The first hour was easy, we dug through the first layer of limestone only to discover that there was another layer a little below the first layer. This layer was very thick and very very hard. It is a good idea to have the right tools for the job and a buddy to help you out. A mate of mine helped me out with the excavation. He is seen here working the "pit". The only way we could get anywhere was drilling and chipping the limestone. It took several hours to go down 200mm. In the end I managed 700-800mm in depth and figured this was close enough to having enough mass. The overall dimensions made for a 0.9 cubic meters mass. This would be somewhere around 2.2 tonnes of weight.

Forming up and Concrete

Once the pier pit was as deep as I though prudent I formed the hole up. Try to remember if you are doing this that several things really need to be done to make life easier down the track. Make you form work exactly square. I am not meaning a square, I mean ensure your corners are at 90 degrees. Do this by checking the diagonals. They should be the same distance if your forming is square. The reason for doing this is that all your observatory measurements can come off the edges of the formed concrete.

Also of interest you should try to get the slab as level as possible. If your pier is placed near plumb and level then you will not need to work so hard to get the mount polar aligned down the track. In my case I am planning on placing the mount directly onto the pier without a leveling plate on the top. So everything being level and plumb from the start will make the polar alignment easy. I found that I had to raise one side significantly to gain level, this meant that I had to back fill quite a lot. Most of the big rocks that I had pulled out during the excavation were now used as a dam to prevent he concrete from flowing out during the pour. Some of the dirt that came out of the hole was then placed behind the rocks to provide a bit of stabilizing.

Now for the steel work. Steel is used to reinforce the concrete and prevent it from cracking or fracturing. In this case I used F72 mesh and 15mm re-enforcement bar. I used tie wire to secure it all off. It does not have to look flash just rigid. Ensure that the top of the steel is set well below the finished level of the concrete. The steel as seen below is set down 120mm below finished level.

In the image below you can see the pier hold down bolts and cage construction. I don't have welding skills but working in the construction industry I used a bit of lateral thinking. I am a plasterboard contractor so I used steel stud which I screwed together and fixed into position. I made sure that the hold down bolts were located in the center of the pier holes and then worked from there. Ensuring that all parts were square to the pier. This means that later when I install the pier it will simply slide over the bolts. The image below does not show the correct depth of the cage. It will be in fact a lot deeper into the concrete. However for setting up the cage I thought it best to go as close to the pier as possible to ensure everything was square and fit correctly. Remember you only get shot at this so you want it right from the outset. 

So with the cage built it was time to install this in pier support hole. as you can see from the image below the cage is installed below the top of the reinforcing mesh. To prevent the hold down bolts from moving under the conditions of the pour I fixed guide rails to the form work. As it turned out though I needed to have these guide rails on edge to prevent the timber from sinking. That is exactly what transpired when the concrete was poured. The weight of the concrete around the cage bent the timber downwards. As a further tip I made sure the hold down bolts threads were not exposed to the concrete. I did this by placing extra nuts on the threads. This made for a clean finish after the pour was over.

So when it came to the concrete itself, I wanted something to be maintained at all costs. That being the level of the concrete. For the design of pier I chose it was critical to get my concrete very level. Screeding the concrete level was a bit of a challenge but in the end it was very close to level. This will mean I only needed to shim for any minor level differences in the top plate of the pier. In the image below you can see the finished concrete with the hold down bolts in place.

Next step was to fit the pier onto the pier support. Knowing concrete as I do, it is never level no matter how hard you try or how good a concrete finisher you may be. So to get around this issue I decided to shim the pier so that it is level at the very top. The pier construction would also have defects in it too. Shimming the pier was easy enough. I cute several tin squares roughly 1 inch by 1 inch with a larger piece folded over to prevent the smaller pieces moving all over the place. The image below shows the pier shims up close.

The image below shows the pier in position from a distant vantage point. As you can see the pier is only shimmed on one side. It is now very solid and when you bang your hand on the side of the pier the vibration dies down in under a second. That is by virtue of using just over 2 tonnes of concrete, a very thick walled pier and the gussets. Remember if you are building an observatory this is one of the most important things that you can do during the process. Don't under estimate how important the pier and pier support is in the equation.

Decking and Walls

The next part of the equation was setting out the deck support holes. I had 14 holes drilled by a local chap who was kind enough to work on a Sunday. If you are digging into limestone, take it from me you need heavy machinery. Digging one large hole was bad enough but digging 14 holes that are not real large was going to be more trouble that it is worth.

Once the holes were drilled the first order of business is to get the four corner posts set in a squared position. These posts will determine how well things work later on too, so it is really important for this to be all square. Put one post in first, ensuring it is plumb in two axis. Set the concrete (I used quick set and used two bags and at a depth of 600mm) and then move onto the next corner. Set that one in too. Now the important part is to ensure your next post is square (perpendicular to the first line) to the other two posts. Use Pythagoras theorem to establish that you have a square angle. ie use a2 + b2 = C2. In this case I used 900 + 1200 =1500mm. Once you have this in the final post is just a reflection of the relative distances involved. As you can see below the posts were set in well high enough to allow for some cut off when setting the height of the walls.

Now that the corner posts are in, the next thing that one has to take into consideration is how your deck will be supported. For me I chose to have two central beams supporting the decking and two outer beams taking the load of the walls. All the beams are 190 x 45 treated pine. The outer beams are cantilevered from the central beams by 600mm but supported by the outer posts. With any luck this prevents long term sagging of the outer beams. The image below shows the deck posts in but not cut yet. Two bags of quick set concrete in each hole set to a depth of 500mm for each post. It gives a really solid footing.

When housing out posts, most people make the mistake of housing out the beam to full depth of the beam. This is bad for a number of reasons, but the most important reason is that it weakens the post's strength. So therefore a general rule of thumb is that housing should never be any greater than one third of the posts thickness. So in the case of a 90mm post, 30mm is the maximum that you should house out. The housing is really there to provide a bit of support for the beam and help the bolts take the load. In this case I have housed my beams to 20mm; enough support to prevent shearing off the bolts but not too much to weaken the integrity of the posts. See the image below for a pictorial of what I am talking about. Be mindful of your deck height in relation to your pier height. I have allowed a 900mm high pier once the deck it completed. Take the time to work this out now as it will be too late once you build everything to change your mind.

The next step is to attach all the joists and bearers for the floor. You will want the floor to be very solid and have little if any bounce. The reason for this is that you may be carrying heavy equipment over it and have several of your friends in the observatory at one time or another. For my observatory I placed the main supports and bearers 700mm either side of the pier support center, with the outside of the joists being cantilevered onto some dummy bearers. Structurally this gives a lot of strength to the entire deck. You want to ensure that you triple grip (little metal triangle brackets seen below) each joist onto the bearer after you have nailed them off. This stops rolling of the joists and helps spread the load out.

Once you have it all in place it should look like the image below. You will note that I have inserted some extra joists around the pier so that the decking is supported correctly. This is in spite of the fact that Structaflor can support up to 600mm with 22mm decking. Joists for the deck were 90 x 45 treated pine and the bearers were 190 x 45 treated pine. Using treated pine is best for an exposed under floor, it will not rot readily and of course it prevents termites from causing issues later on.

Once you are satisfied that the deck structure is sound it is time to place your decking down. You could use a variety of materials to deck, either solid timber slats or floor decking is fine. In my case I used a product called Structaflor which is 900mm wide and come in lengths of 3.6 meters. It is water resistant for 3 months of continuous exposure to the elements and it perfect for sticking down a rubber floor mat later. It also has a tongue and groove system for ease of installation. You need to use a maxbond glue which you apply to the joists when dry and then either nail it down or screw it off. It is easy to work with so long as you have a jigsaw and a circular saw. The image below shows the deck completed and ready for installation of the the walls.

The next part of construction involves the walls. I have chosen a wall height of 1.8m which is steel framed using stud and track system from Studco. The studs and track are 1.15BMT with noggin track in the center of the wall. Each stud is screwed off on each side at each junction. This makes for a very rigid wall and of course is impervious to rotting or termite destruction. The image below shows the stud work in place. It took me about 5 hours to erect the walls. You will need a cut off saw and heavy duty screw guns for this work. You should also ensure that the stud and track is well fixed to your posts and deck. I used 12 gauge course threaded screws for the job and I put them in at 400 centers. If you live in a windy location this should be fine for a fixing dimension. You don't want your observatory to blow away. You will also notice that I have installed the door jamb. Use an exterior jamb set which are around 140mm. Ensure that you have it well fixed into position. I plan on using further anchorage from the inside of the jamb set into the wall frame behind. I have temporary installed it with 12 gauge screws, but for security purposes you will want much larger screws placed in behind the door stops. This will prevent most break an enters. Only someone with heavy duty equipment will be able to get in with this system in place.

Now that you have the walls up, you will want to get the external cladding done. This is the most time consuming aspect of the entire project. It takes hours to clad these small walls. The cladding I used was Hardies weather board. It is wood grained 10mm thick and very weather resistant. It is easy enough to cut with a dry saw or even using the score and snap method. You will need to use screws or nails that have a reach of 30mm. The reason being is that each layer up over laps each of the previous layers. To finish off the corners you will need the aluminum corner beads. They are easy to install, just ensure that you leave the stud closest to the corner unscrewed so that you can slip the corner bead into position. Then simply place the holding screw into the top of the corner bead and screw off the board to the corner stud.

To prevent water penetration it is a good idea to use sarking. The type of product you use is largely unimportant, just ensure that you have good coverage. What sarking does (its the blue material seen through the door below) is to act as a vapor seal. It is not insulation. You need sarking in place to keep the insulation in good condition for the next 20 years. The image below shows the walls finished and clad.

Roof Structure

Now that the walls are completed it is time to start thinking about the roll off roof. This has to be constructed first so that you know exactly how the fixed roof will be placed over the top of it. The fixed roof is facing almost due south and that is the direction in which the prevailing weather emanates So when building your own observatory think about the weather, you don't want your observatory to leak.

The roof is constructed from 1.15 BMT steel stud and top hat sections. Each stud is formed to make the truss which has a center pitch of 300mm from the bottom chord. These are placed at 750mm centers and then the top hat sections for the purlins. The purlins are fixed at 400mm centers. You should ensure that you use heavy duty screws with a breaking strain of 600lbs per square inch or more. I found it convenient to build everything in situ rather than building it on the ground and then trying to lift it into place. The image below shows how this is set out.

So now for the slider mechanism. I decided to run the slider mechanism internally within the observatory for security and for looks. It is comprised of a 100mm C purlin with 15mm lips. These are placed lying on their sides and then 5 wheels are placed inside this with the 180 timber beam bolted into position and an axle of M8 threaded bolts. A length of aluminum extrusion is fixed to the bottom of the beam to keep it straight and to fix the wheel studs into. The final element is to fix this slider to roof. Some important lessons I have learnt here is that the slider really needs 5 wheels per each side. The bolts should also be made from hardened steel and finally the connection to the roof should be nice thick plate steel or aluminum extrusion. You also need to ensure that the whole assembly is parallel with each sliding assembly and also with the channels. I worked on a tolerance of 5mm and have found that this should really be around 1-2mm. The image below shows the slider mechanism in place and fixed to the roof.

Once you have it all in place you will need to test it the way it slides out. I found this useful to show where certain defects were in the design and those could be corrected now rather than later. Unfortunately one cannot find all the defects but you get the idea. The image below shows the roof sliding all the way out on the out riggers. The temporary bracing can be seen holding the outriggers in position.

The next step I took was to sheet the roof. Once again I used heavy duty sarking to prevent heat and condensation from entering the roof cavity. I then screwed down zinc coated corrugated tin. Placing plenty of screws on each purlin. The observatory is built in a day time windy area. Night time is fine, it is nearly always the reverse of the day. Anabatic winds blow in from the Southern Ocean which is only 5 km's from the site. With that sort of environment it pays to be cautious. The image below shows the sheets going on. I found it good to stand on a tressel and screw the sheets down. I don't think that the structure would support my weight, so this was the best method of fixing.

Once I had the roof sheets on, I set about fixing the barges and fascia's. To fix the fascia I used small blocks of timber that I screwed directly to the roof trusses. I then screwed the fascia onto the timber blocks. A couple of screws per block which penetrate about 20mm should be enough to hold it into position. For the barges, these were screwed directly to the truss. Ensure that your barges are at the same height as the top of the corrugation of the sheet. This will allow the barge caps to fit neatly and prevent water finding its way into the roof structure.

Once you get the barges fitted, you then need to think about the posts for the outriggers. These have to positioned nearly perfectly to the line of the fixed running rail inside the observatory. Deep anchorage is vital to prevent movement of the posts. 300mm of concrete will give you enough anchorage. Lots of bracing, the use of a plumb bob and string lines will help you to get the alignment and positioning of the posts correct. While you are doing outrigger posts you also need to fit the cross bracing that it needed to keep the whole structure stable. Ensure that you house out the posts about 20% of the bracing material and use good quality bolts. The image below illustrates what I am talking about.

Once the sliding roof is working and completed it is time to consider the fixed roof. My thinking on this would be that the fixed roof should be 50mm higher than the sliding roof and it should extend 100mm further over the sliding roof once it was in the locked position. You need to use the same pitch of 300mm from the bottom chord (or what ever you choose), but just raise the roof a little higher. For this I selected a 90 x 90 timber stud off-cut on each side. I fixed this to the top of the wall via several 150mm coach bolts. Strapping will also prevent the roof from flying away.

Once again I used top hat section for my purlins. This time I set them to 500mm centers. Once again ensure that these are well screwed down. I used two screws at each connection. Meaning that each purlin has 4 screws fixing it to the two trusses. The image below illustrates this point.

Once you have sheeted this part of the roof, placed the barges and fascia's into position the next thing you need to do is the roof plumbing. If you do not know how to do this get an expert in. I have worked in the building trade a long time and have worked with several people who are roof plumbers. However, this was my first single handed roof plumbing exercise. It took quite a lot of thinking to get this right. The barges must go on before the ridge cap. Each barge must be end sealed for not only looks but to help prevent water entering the roof and the building. Each barge must be as level to the roof as possible so that the lip of the barge sits inside lower corrugation. The image below shows what the barge caps should look like.

The image below shows how I went about folding the barge cap around the barge. The basic idea is that I cut the barge cap 120mm longer than the barge board. Then I cut a small section for the end to fold down and one on the inside to fold down. The 120mm long part then wraps around the two folded pieces to form a nice neat finish. A little bit of silicone here will prevent water penetration. Make sure you use paintable silicone though, as you want to paint the barges.

When this has been completed on all four barges you can install the ridge cap. The ridge must be installed about 130mm longer on each end. This is so you can cut and fold the ridge to form a weather seal. To pretty this all up you can cut and a blocking end and fix it uniformly so that the end looks like it is creating the seal. The image below shows the finished roof (all bar the gutters) but with the sliding roof open.

Now that you have the roof plumbed, you will want to install the door and locks. Hanging a door is not very hard but I can say I had not hung a door for 15 years and made a few mistakes. I used wide blocks (hinges) for the observatory door. Wide blocks are designed to be very heavy duty and will prevent most thieves from breaking the door at the hinges. Make sure also that the door is a solid core door. You want the door to present a huge obstacle for any would be thief and the door will be resistant to weather conditions.

For door locks I used Lockwood external fitted dead locks. The locks are easy to install and are what the insurance company likes to hear. Additionally, these locks are better than using an internal barrel lock. The image below shows the door installed and the first lock in position. You can see that I have installed some temporary door stops too. You will need to install these before you can install the lock. Note also how the fixed roof and sliding roof comes together. I used two different size fascia to make the roof look like it is one piece at the underside of the roof.

The final steps to lock up are the installation of any further locks; remember that you have to lock the sliding roof. I used hasp and staples with heavy locks to secure the sliding roof. Also installation of the door stops and finishing the gable ends. Security is your main issue here. Besides doing all this will keep out the weather. The image below shows the observatory at full lock up.

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