Let's face it - every house needs a giant spider at Halloween. Not the "Oooo a scary spider just dropped on my head as I went to the front door" kind of spider - I'm talking about the "That furry black beast just ate my dog" kind of spider.

Well, this one isn't quite that big (see last photo for one that is) but the design can easily be scaled up with larger materials to a prop that is very large.

This design is based on a spider we made a couple years ago, with modifications to minimize the number of pipe fittings, and flexibility to add your own body shapes and sizes. We used 1/2 inch PVC pipe because we had lots of off-cuts left over from a gardening project - I would probably choose 3/4 inch if starting from scratch.

Given the number of joints in the legs you can also pose the spider in different ways (like front legs off the ground ready to grab a passing child) or omit the last segment of each leg so the spider sits flat on the ground. We found this configuration better for use suspended in a web.

Step 1: Materials and Tools

MATERIALS (all PVC pipe is 1/2 inch, you can scale up as needed). Approximate prices shown from small hardware store - you may be able to find better pricing at big box stores. Prices are for all components, not each component.

60 feet x PVC pipe [$12.00]
1 x PVC "T" fitting [$0.40]
3 x PVC 4-way "Cross" fittings [$3.00]
16 x PVC 90 degree "Elbow" fittings [$4.00]
16 x PVC 45-degree "Elbow" fittings [$13.00]
8 x pins to hold legs to body (16d nails work fine)

2-3 x cans spray foam insulation [$15.00]

Gloves (latex/vinyl) - spray foam is incredibly sticky and can only be removed with acetone
2 x cans black spray paint [?]
PVC pipe primer and glue [$9.00]
Duct tape [?]

TOOLS
PVC pipe cutter
Drill with 3/16 inch bit
Sandpaper
Drop sheet
Ruler

Step 2: Build the Spine

Cut 8 x 5-inch pipe segments - these will be the struts from the central spine to each leg.
Cut 3 x 3-inch pipe segments - these will join each of the vertebrae
Cut 1 x 12-inch pipe segment - this is the "tail" and supports the abdomen of the spider

Using the PVC primer/glue, glue 1 x 5-inch strut into each side of the PVC "T" fitting, and 1 x 3-inch segment into the center of the "T"

Now repeat with each of the 2 x PVC "Cross" fittings - 1 x 5-inch strut into opposite sides of the cross, and 1 x 3-inch segment into a center hole

Repeat for the last Cross fitting, this time using the 12-inch tail instead of a 3-inch segment

You should now have 4 x vertebrae ready to be joined. It is critical that all the leg struts are parallel - I raised then on two blocks while being glued to ensure they were all in the same plane. Apply primer and glue to the middle of a "T" fitting and the corresponding hole in the next fitting. Insert the fittings together, twist 90-degrees to get a good joint, then hold together on the raised blocks of wood so the struts set in the same plane.

Repeat for all 4 x vertebrae.

The final step is gluing one 90-degree "elbow" to the end of each leg strut. It is important that each elbow is perpendicular to the spine, I found the easiest way was to insert a longer piece of pipe dry into the side of the elbow not being glued, glue the elbow on, and while wet rotate the piece of pipe until it is at 90-degrees to the spine

Step 3: Build the Legs (8)

Cut 24 x 2-foot pipe segments - these will be the segments for the legs (3 per leg)
Cut 8 x 4-inch pipe segments - these will be the pieces that join the legs to the spine

Using PVC primer/glue, glue 2 x 2-foot leg segments to each side of a 90-degree elbow fitting. Repeat 8 times.

Glue 1 x 2-foot leg segment to a 45-degree elbow fitting. Repeat 8 times.

Now glue the segments joined with the 90-degree elbow to a segment with the 45-degree elbow at each end. Make sure that all pipe segments are parallel in the same plane, and that the joint with the 45-degree elbow points inward. Repeat 8 times.

Glue a 4-inch segment to one side of a 45-degree elbow. Now glue this to the other end of a leg assemble, making sure that the 4-inch segment is parallel to and in the same plane as the 2-foot segment on the opposite side of the leg. Repeat 8 times.

Sand the last inch of the 4-inch segment on each leg to allow a looser dry fit into the "hip" elbow attached to the spine.

Step 4: Attach the Legs to the Spine

Insert the 4-inch segment at the end of each leg into the "hip" elbow fitting attached to the end of each strut of the spine. Rotate the leg as you insert it to ensure the leg seats all the way into the fitting (approx 1-inch). Position the legs into the final shape you intend to pose the spider, usually four legs forward of the body and four legs rearward.

Once satisfied with the position of the legs, drill and drill a 3/16 inch hole all the way through the hip leg fitting and leg segment inside. Drill the hole approximately 3/8 inch below the end of the fitting.

Step 5: Build the Body, Attach to the Spine

I decided to use old packaging material I had laying around - you can use almost anything as long as it is light and waterproof/durable (the spider will probably be outside in the elements).

Make the abdomen for the spider. This can be made from anything as long as it is big, round and light. A beach ball or balloon is ideal. Other spider builders have made fiberglass balls - I think a beach ball is easier :)

Our spider uses a plastic garbage bag filled with foam packing peanuts, and 'shaped' into a spherical shape with duct tape.

Attach the abdomen to the tail of the spine with duct tape or similar. Try and fill major crevices with duct tape.

Make the thorax for the spider - this can be made from almost anything as long as it is reasonably firm, and long enough to overlap the front of the spider. I used some blocks of polystyrene foam, you could also use a plastic bag filled with crumpled paper, cardboard box inside a plastic bag, etc. Attach the thorax to the spine with duct tape or similar.

Continue taping the thorax, abdomen and spine together.

Step 6: Foam the Outside of the Spider

Start spraying sealant foam on the outside of the spider. Make sure you use random patterns as it will continue to expand and set into the shape you spray. The more random your pattern, the more 'organic' the final skin will look. Don't try to spread the foam after spraying, just spray and let it plump up.

As each side dries, you can rotate and keep spraying. Slide a nail into the end of the spray tube to prevent it setting in the tube between applications.

Be careful you do not get the sealant foam on your clothes or hair. It is incredibly sticky and only removed with acetone - not good for either surface.

Step 7: Paint the Spider

Once the foam is dry, spray the spider body and legs. You may need to go over the body twice to reach all the nooks and crannies. Place masking tape over the last inch of the legs before spraying so that the fit to the body does not become tighter (remember you sanded this in step 3). Remove the tape prior to assembly.

Step 8: Final Assembly of the Spider

Check the spider to see if any foam is blocking the "hips" attached to the body. Cut away any foam with an Exacto knife or similar.

Insert the legs, the pins, then respray any pieces missed.

Voila! Biggest spider in the neighborhood.

Step 9: Final Thoughts and Alternate Designs

The spider can be scaled up with larger diameter PVC pipe and longer leg segments.

Pictures on this page are from our "Big Brother" spider - he has 4-foot leg segments, a 4-foot abdomen and is as big as a small car. This gives you an idea of how the basic design in this Instructable can be modified by removing the last leg segment so it sits flat on a roof, web, etc.

Some other thoughts and options:
- Mold a face and add scary eyes and teeth
- Buy pipe insulation sleeves and slide over the leg segments to make them thicker
- Rotate the front legs back to make the spider 'rear'
- Successively shorten the last segment of each leg front to back to make the body angle upward, ready to pounce
- Stick marbles or glass beads in the front of the 'face' for eyes
- If you intend to suspend the spider, attach an eye-bolt or similar to the center of the spine, extending down through the foam to the outside of the body, so that a rope can be attached to take the weight

Step 10: Last Minute Change...

Found an old skull prop that I had and decided to go for a "human skull bursting through the front of the spider" effect. Steps were easy:
- Carve out space for the skull
- Hot glue it on
- More spray foam - make sure it runs back over the body
- Mask the skull with painter's tape
- Spray the new foam red
- Go back over the overspray with black