So you want to get an antenna or other instrumentation high into the air? But you need to do it on the cheap. Mounting a mast on a building can be a way to cheat a little and avoid having a structure connected all the way to the ground.
There are some caviots though. First, the building must have gables (flat sides on the ends). Second, there must be a very minimal overhang on the edge of the gable. Gables should not have gutters on them, but they may have some overhang of the roof material. As long as this is not much more than 1 inch this method will work.
Step 1: Tools and Materials
There are a couple stages. First you make the "mast attachment" blocks. Then you attach the blocks to the building. Lastly you attach the mast to the blocks.
Pressure treated 2x4 - Very important that the wood is pressure treated!
#12 x 2.5" self-tapping screws, preferable hex head
#12 x 1.2" screws (self tapping not necessary)
2-hole rigid conduit strap. Very important! - it must be sized to match the mast pole you intend to use
Cordless drill (to attach to the building)
Again, make sure you use pressure treated lumber. Otherwise the wood will deteriorate very fast from the outdoors and you risk the mast/antenna falling off the building. Also it is important to use a matching size of conduit strap (the "U" shaped thingy). A strap which is mismatched to the pipe it is holding will not hold as well and again you risk the mast sliding towards the ground.
Step 2: Cut Off Sections From the 2x4
Lay out your parts as shown. Give yourself some extra space if necessary. Mark and cut off a block from the 2x4. You will need to make at least two of these, so cut another block off of the same length. If you will be supporting a dish or anything that may have a heavy wind load, you should consider using a 3rd block for extra support.
Step 3: Drill the Block Attachment Holes - Part 1
It is important that you get the hole sizes correct to save yourself frustration while up on the ladder outside the building. Take a long screw (2.5") and hold it up to a drill bit. Find a drill bit that is a slightly larger diameter then the screw+threads. That's right, we're drilling a hole that the screw will not thread in to. These holes just support the screws loosely. The reason is that we just want these screws to tap into the building and not screw into the block. If we don't do it like this, while you are trying to tap the screw into the building the block will want to move up the body of the screw while you are operating it. This gets very messy once we add the 2nd and 3rd block with the mast already attached in a future step.
Ok. So now drill two holes on one side of the block (where the screws are sitting on the previous step).
Step 4: Drill the Block Attachment Holes - Part 2
Ok, now we drill the holes for the conduit strap. This is different! Hold up the short screw (1.5") behind a smaller drill bit. We want to select a drill that is sized so the screws bite into the wood. The threads should be a wider diameter than the drill we select (as shown in the top photo).
Put the conduit strap on the wood block and mark holes where the screw holes are in the strap. Now drill out those holes.
You can see how the block assembles in the last two photos.
Step 5: Attach One Mounting Block to Your Building, Attach the Pipe
Start by attaching one mounting block in the highest position you need using two long self-tapping screws (2.5"). Depending on the materials the building is made from, you may need to experiment to locate the gable wall studs you will be mounting to. I banged on the building with my hand until i found the wall studs, but the aluminum siding the building was made of made this easy. Your mileage may vary. Drill the long screws through the gable supports into the studs, try to keep the block level.
Next, carry your mast pipe up the ladder (without the antena attached). Screw the mast pipe down to the mounting block using the rigid conduit strap. One block should be enough to hold the unloaded-mast pipe until you can attach the additional mounting blocks.
Ok, so now you can adjust the pipe so it is perfectly vertical (and dangling down the side of your building). If needed you can use a plumb bob to verify if the pipe is aligned correctly.
Step 6: Add the 2nd And/or 3rd Mounting Block
Now that your mast pipe is at a 90 degree angle to the earth, you can mount the 2nd block. It will be slightly trickier now that there is a pipe in your way, but if you carefully followed my directions on drilling correctly sized holes then it should be manageable. Once again, drill the long screws through the mounting block holes into the gable ends. Make sure you hit solid wood and are not just hitting plywood or aluminum siding. Sometimes I had to retry (and silicone over the extra holes I made).
Step 7: Attach Antenna and Raise the Mast
Now you need loosen the conduit straps and lift the mast pole so the top is just above your building and clamp it back down. Attach your antenna or other equipment as needed. Attach cables and such. Now loosen the conduit straps again (carefully!) and lift the mast pole to it's full height. Tighten the conduit straps back down. That's most of it!
Step 8: Maintenance
Over time even pressure treated lumber will deteriorate. I recommend inspecting the mounting blocks every 6 months. The screws may need re-tightening also at these periods. If you find the wood has deteriorated to the point that the screws holding the conduit straps will no longer hold tight, then it is time to replace the block of wood.
I recommend lowering but not removing the mast pole before changing the wood blocks out. If you do them one at a time, with the mast lowered, it should not be any trouble for you.
Step 9: One Last Thing: Grounding
One last thing worth mentioning. There are a couple schools of thought when it comes to grounding antenna masts, here is my advice. Ground your antenna mast to an 8' grounding rod placed near the bottom of your mast. The reason being, this prevents static from building up on your antenna or mast pole which should reduce the likely hood of a lightning strike. If you don't ground the pole, it could become insulated from the building allowing it to potentially build up a high static charge during a storm which could help influence a lightning strike against it. This makes sense to me so this is the advice that I follow. Again, there are other schools of thought so your mileage may vary.
I use a standard bronze pipe grounding clamp. It should include a separate screw to tighten onto the grounding wire.
That's all folks!