How to Build a Patio Cover with a Corrugated Metal Roof

DIY Step one: Figure out exactly why the project, in this case a patio cover, is needed. This makes it easier to decide where to build it and just how large to make it. My choice was simple. Facing the patio, I had a bay window and a back door on the northwest corner of the house. The bay window would leak during thunderstorms and the back door would freeze shut during ice storms. A patio cover was needed to keep foul weather away from the bay window and the back door, working as a structural solution to problems that otherwise would have been much more expensive to solve.

So I decided to make a twenty foot wide, ten foot deep patio cover. It had to reach from the edge of the house’s roof to the edge of the chimney, to provide enough overhang to stop wind-blown rain from reaching the bay window and the back door. The type of roofing was an easy choice for me. With a low slope planned for the roof, shingles were ruled out completely. A tar roof was possible, but it would be heavy and would smell like a BP oil spill. I chose to use corrugated metal roofing because of its resistance to high winds, light weight and its own structural strength. By choosing corrugated metal, I reduced the degree of slope, or roof pitch, needed to a mere 10%, or a 1 to 10 pitch. This meant the patio cover could start at the edge of the house’s roof at a height of eight feet and slope down to seven feet, leaving a clearance of more than six and a half feet between the lowest part of the frame's headers and the ground at its outer edge.

Another consideration was the amount of shade provided. By extending the patio cover well beyond the eastern and western edges of the patio slab, I ensured there would be more shade for the patio in the summer. I chose to go with a post and beam sort of look, kind of like a pole-barn, to reflect a Midwestern farmland feel. Perhaps sitting under the patio cover on a summer afternoon after a frustrating day at the office, drinking sweet tea or eating watermelon, would remind me of relaxing in the shade of a loafing shed after a long day of pitching hay…

Although local building codes wouldn’t let me use 4”x4” beams and joists, substituting them with 10” 2x headers and 6” 2x joists worked out just fine. I spent a day measuring and drawing up the plans, using a ruler and a few sheets of plain paper to sketch the plan. Then I used that plan to come up with a list of materials and then went to the lumber store to get prices. I changed my plan just a bit, to further reduce costs. One example was choosing drywall screws instead of nails because, for some reason, they were less expensive, and they won’t try to work themselves back out after a few years. Another substitution I made was choosing three and four inch deck screws instead of framing nails. Although more costly, they hold much better than framing nails in outdoor applications.

Some people said a building permit was unnecessarey, others said it was impossible to get without a contractor's license, said it would raise my property tax, etc. So I called the permit and licensing office and got the correct answers. Yes, a permit was required. No, it wouldn't raise my taxes, and as long as the property owner didn't care if I wasn't a licensed contractor, they didn't either. Getting the building permit was easier than I thought. I took the plans to the town’s permit and licensing office, filled out a one-page questionnaire, sat down with a clerk who asked a couple of questions and made notes on the questionnaire, and said they’d call when the permit was approved. The very next morning I got the call and went back to the permit office to pick up the permit. And, of course, pay for the permit, which cost about $85. There were a couple of changes to the plan, clearly noted in red ink, and completely understandable. The changes to the plan were very easy to accommodate and actually made the job easier and reduced the material costs. I took pictures of each critical step to show the building inspector, which made the final inspection a lot shorter. And the pictures came in real handy for making this article a lot better.

Shopping around for the best prices for materials was absolutely necessary. In the same town, in an area of less than 10 miles, the difference in prices was enormous. Total material costs ranged from $2,200 down to $800. Beware the store associates. Some are aggressive and want to push customers into an insane buying frenzy, bringing up a storm of confusing nonsense that somehow equates spending more to saving more. The lumber store I bought from had an associate who simply made sure I bought the correct materials for the job by looking over my plan and suggesting shorter screws for the joist hangars and a lower grade of lumber for the areas of the patio cover that would not be exposed to the weather. The associate also arranged for delivery, which cost $25.

Early the next morning the materials arrived at my house. The delivery truck driver slid the bundle of lumber onto my driveway and, after breaking the bands and counting the materials one last time, got my signature on the delivery ticket and drove off. I fell to the task of carrying the ten foot long boards and corrugated metal sheets to the back yard, laying them out in a pattern to roughly match how they would be assembled. I also separated each corrugated metal sheet and wiped it down with a light coat of vegetable oil, and re-stacked the metal sheets. This was something I had seen a neighbor do a long time ago, when I was a child. I don’t know if it served any purpose other than making me feel better. It’s a step I probably could have skipped, but who was I to doubt the wisdom of the ancients?

The first item of real work was installing the ledger board. First I removed the ¼” plywood dressing right below the drip edge and saw a very solid framing 2”x4” underneath, perfect for attaching the ledger board. The local building code required a 2”x6” ledger board, which meant I had to saw off two inches of wood from the top of the bay window’s box. Also, the internet, cable and phone lines were in the way so I yanked them down and out of the way. Next I pre-drilled 1/8” holes, six inches apart, in the ledger board, because it was treated lumber and the deck screws would need a little help getting started. After attaching the ledger board with 4” decking screws, I used a large staple gun to staple the phone, internet and cable lines to it, on the inside toward the house, to better protect them from the weather.

            The next item of real work was digging post holes. Determining exactly where they should be dug seemed problematic, as I wanted them exactly nine and a half feet from the drip edge of the house’s roof. The house’s foundation was recessed two feet from the roof’s drip edge, and under the bay window the foundation stuck out 26” from there. All sorts of neat tools I saw people use on TV shows and formulas from various math classes flashed through my mind. Then I decided to simply stand up a couple of posts at the drip edge and used a bubble level to make sure they were perfectly upright. Then I measured and marked nine and a half feet on a couple of joist boards and laid them on the ground, from the upright posts to where the hole should be dug. I did this at both edges of the ledger board, and laid two more ten foot joist boards between them. I then used my square, or angle iron, to make the corners square. Then I marked the spots for the three post holes, the middle post hole going where the two 10 foot joist boards met. I then measured corner to corner, one way and then the other, and made slight adjustments until both measurements came out the same. With the exact centers for the post holes marked, I marked 18” circles around them because I planned to dig the holes 18” deep and wanted them to be as wide as they were deep, in accordance with local building codes.

            I started digging the holes with a simple shovel, all the while thinking about those post-hole diggers, the ones with motors and big augers, that take two people to operate, like I saw used on various home improvement TV shows. I thought about their cost and availability at equipment rental stores, and ran through a list of friends I could invite to help me operate such a tool. Before I could formulate a plan for renting a hole digger, I had already finished digging all three holes with just a plain old shovel. Then I used my garden hoe to tamp down and compact the dirt at the bottom of the holes.

Next I scratched my head as I thought about how I’d get the tops of the three posts even, and the problem of trying to nail or screw the rest of the frame to posts that were standing upright, without a helper there to hold things in place. What I came up with worked, although I never saw it done on TV. I laid a board on the patio slab, and set a bubble level on the end of that board. Then I took that end of the board to the hole and stood the 4” by 4” post up straight, holding it by hand, and lifted the board with the bubble level up until it was level and then made a mark on the 4” x 4” post. I then laid down the post and measured seven feet from its top and marked it. The difference between the two marks was how much taller I wanted the post to be, so I attached a length of treated 2” x 4” at the end, adding enough length to the post so that its height would be perfect. I did this for all three posts.

The rest of the frame I assembled on solid ground, not wanting to perform the precise job of frame assembly under the stress of holding lumber in place with one hand while trying to install decking screws with the other. I needed to cut four braces with precise 45 degree angles, and I had a single 10 foot long 2” x 10” board to make them from. After contemplating the need for a try square, and searching my memory of how to get the angles drawn with my angle iron, I resorted to a much simpler method. I measured the width of the board with a ruler, then marked that length from the end of the board and cut from there to the corner to make a triangle of wood that was exactly the right shape. I measured the short sides of that wooden triangle-shaped block and they were exactly the same so I knew it was cut at a 45 degree angle. I used that to mark the lines I would cut to make the braces. I made the two outside braces three feet long at their outsides, and the two inside braces two feet long at their outsides.

With the braces cut, I laid out the frame on the ground and squared the pieces for assembly. I pre-drilled the screw holes with a 1/8” bit to prevent cracking, and on the braces I pre-drilled half way into the wood with a ¼” bit so that the screws could sink deeper. After attaching each piece, I checked for square at each joint to make sure I hadn’t knocked things out of place. Surprisingly, the pieces stayed where I wanted them and I had to make no further adjustments. After assembling the frame I marked 24” on center all along it and on the ledger board and attached eleven joist hangars to each. Then it was time to prepare to stand up the frame, something that weighed about three hundred pounds at this point.

Then I prepared the joists, eleven boards, ten foot long 2” x 6”s. I had to cut an angle and a ½” notch at the top ends so the roof of the patio cover would come in below the house’s drip edge, and then cut off seven inches from the other end at an angle so it would mach up to the frames’ header at the far end. For this I made a template from a piece of cereal box cardboard, cut out the pattern and drew it on the ends of each board. Then I cut with a circular saw and used a bow saw to take out the last little bit of the notches. If I had one, I would have used a jig saw. For the far ends, I made another template, one without a notch, to ensure all the angles and lengths were the same. A real pro might have made measurements for each cut instead of making a template, but I didn’t want several repetitions of human error screwing up my patio cover.

            To stand up the frame without help, it was necessary to attach prop sticks; that is, I used a single framing nail to attach a 10 foot long 2” x 6” board (I used my joists) to the outside of the frame, its length away from the bottom of the frame, on each side. These two boards would rotate on their nail, their end holding up the frame as I lifted, their other end on the ground, preventing the frame from going back down. Then in the middle I attached a 10 foot board to the center post and lifted it from the very end, the leverage cutting the strength needed to lift the frame in half. As I lifted the frame I walked in toward it, and had the frame stood up in its post holes without having to strain myself. Next I attached the joists to the left and right sides, and with the corners secure I was able to remove the prop sticks and center lifting board and install the rest of the joists.

            At this point I had the frame attached to the joists, and the joists attached to the ledger board on the house. To make the frame perfectly level across the top, I lifted the posts just a little and tossed in just a bit of dry concrete ready mix to raise its height just enough to make my bubble read perfect. Then to make the posts exactly straight up and down, I lifted and moved the bottom ends in the post holes, measuring with the level after each adjustment. Then I took a half-hour lunch break, to let things settle a bit. I re-checked everything with the bubble level again. It was still plumb and square, so that meant it was time to pour some concrete.

            I had watched neighbors build a little fish pond in their back yard. I helped them dig, but then left because I disagreed with their choice of concrete. They ordered up a load of concrete, brought to their home by a ready-mix truck that parked on the curb and dispensed concrete into their wheel barrow, which they trucked back and forth to their pond. Then they hurriedly tried to form the liquid into the bottom and sides of their pond. A miscalculation meant they ordered enough concrete to fill the pond solid, and they also miscalculated how quickly they could wheel-barrow the stuff from the curb to their back yard. All in all it worked out. The truck left at 4:30 pm, quitting time for the driver. The pond itself ended up with a two foot thick bottom and foot thick walls. The lack of enough re-bar for the thickness of concrete meant the concrete eventually cracked and the pond had to be sealed with roofing cement before it could hold water. But it did turn out as a decent little fish pond.

            For my project I chose six 80lb. bags of post-set concrete. I mixed one bag at a time in a large metal washtub, not wanting to buy a wheel barrow. I used my garden hoe to mix in water until it became a near-liquid, something resembling sloppy mud. Then I used my shovel to put equal amounts into each post hole, repeating this process until I got to the last bag. For the last batch of concrete I added less water and mixed it to the consistency of raw cookie dough, and wearing work gloves I grabbed gobs by hand and formed it into a dome at the base of each post, to prevent water from pooling. Then I used a block of scrap wood as a trowel to tap and smooth the surface of the concrete. The results looked OK to me so I took a one hour dinner break, to let the concrete set.

After that it was simple but repetitive work. I attached 1” x 6” boards to the top of the joists, and then screwed the corrugated metal roof to them. The most difficult part was getting the upper end of the metal sheets to go under the drip edge and roof decking of the house, butt up against a bead of clear-drying roof cement I had put in there with a caulking gun. Also, putting a bead of sealer along the edge of the metal sheets before overlapping them was a bit demanding, but that is a step I could not skip.

The job was done at that point. As for the corrugated metal roofing, I chose rubber washer metal-into-wood screws with ¼” bolt heads and used a ¼” magnetized drill adapted socket to put them in. That type of screw goes into the valleys, into every valley at the top and bottom ends of the panels and every other valley throughout the rest of the roof, into the 1” x 6” boards placed 24” on center.

As with so many projects, I deliberately over-bought on lumber. I did that because occasionally a board shows up so warped or knotted or cracked it is unusable. Sure, the supplier would gladly replace it for free, but I don’t like to put jobs on hold, waiting for a problem to get resolved. I used the couple of “left-over” boards to replace the rotted seat of a patio bench and construct a tall, narrow table suitable for holding a single hotplate. The patio is much more enjoyable with the cover on it, and the ravages of foul weather no longer threaten the back door or bay window on the northwest corner of the house.

Materials used:

Wood:

11  2”x6” x 10’ boards (joists)

3   2”x8” x 10’ boards (beams)

10  1”x6” x 10’ boards (metal roof nailers)

3   4”x4” x 8’ treated (patio cover posts)

Other:

6 bags of ready mix concrete

22    2”x6” joist hangars

10   10’ corrugated steel roof panels

Misc:

Framing nails, 10 penny

1¼” washered metal to wood screws.

3” deck screws

4” deck screws

1¼” drywall screws

1/8” drill bit (wood)

1/4” drill bit (wood)

#2 Phillips screwdriver drill bit

¼” drill socket