Planning a Deck or Porch for the Modular Cabin

Planning a Deck or Porch for the Modular Cabin

The plans are approved and stamped, so we meet with a contractor today for an estimate on grading, the foundation and tie-in work, and the utility hookups, sewer, water, and electric, for our new modular cabin. He can also give us a price for the steps, decks or porches or anything else we want to add.

All customers will need a licensed contractor to put in the foundation and prep water/power/sewer hookups, and ready for the cabin to be set come delivery time. When picking a spot for your small prefab cabin, make sure that a block foundation can be placed there. This block foundation should create the minimum 18” clearance needed between the cabin subfloor and ground for plumbing hook-up purposes.

Site Preparation for Small Prefab Cabins

So we can’t delay deciding what kind of porch or deck we are going to have.

12×40 Log Cabin Porch

The porch on our log cabin was 12-feet wide and the entire length of the cabin. It made up nearly one third of the footprint of the cabin.

The back porch with the ceiling, vinyl siding, decking, railing and balusters -The Back Porch - Building Our Schumacher Home - Project Small House
Our back porch with the ceiling, vinyl siding, decking, railing and balusters during construction

We built our new home with a porch nearly as big. This time the porch is 12-feet deep and 28-feet long.

Elevation and Foundation Plan for our 12×28 porch

This is the plan for it with the foundation and grading.

Rear Elevation Plan for our 12×28 porch

The porch runs from the dining room wall to the living room wall.

Plan for our 12×28 porch

We can walk out to the porch from our bedroom or the dining room.

Foundation Plan for our 12×28 porch

This is the foundation plan for the porch we have now. The porch for the new cabin won’t be a lot different.

26-feet long and 12-feet deep porch
The living room and kitchen windows will open to the porch

We want to do something similar with the new cabin.

If we start with the living room wall and go to the kitchen, we will have a porch that is 26-feet long. We would like it to be able to be 12-feet deep.

Deck on a Westwood Modular Cabin

We looked at some of the photos on Westwood Homes website to get some ideas. Most of the cabins add a deck.

The deck looks really nice. An advantage is, more light can come in the windows if there is no roof over them. A deck should cost less, too.

Covered Porch on a Westwood Modular Cabin

I really would like to have a covered porch, something like this, but without the stairs coming up in the middle.

Covered Porch on a Westwood Modular Cabin

It looks like this porch is 12-feet deep, like the porch we have now.

Rocking Chair Porch
Photo by SA, Virginia State Parks staff

Can we have a porch 12-feet deep? The side wall is only 8-feet high. Will the roof be too flat?

We could do a smaller porch, like this one. This porch is not as deep, closer to 8-feet deep.

Roof Pitch

The sidewall of the cabin is 8-feet high. I did some research and found they recommend the lowest ceiling height for a porch to be no less than six feet, six (or 8) inches from the floor.

I did a lot of math I didn’t need to do.

How to Calculate Roof Pitch

  • Measure the horizontal distance from the edge of the roof to the centerline.
  • Measure the vertical distance from the roof surface to the centerline.
  • Divide the vertical distance by the horizontal distance.
  • Multiply the result by 12 to get the roof pitch in inches.

Roof Pitch Calculator

Rise is height. Run is width.
Rise is not the height of the wall, it is the difference between the two walls (or wall and post, you know). If one side is 8-feet high and the other side is 6 1/2 feet high, the rise is 18″.
Run is the width between the two walls.

The smallest pitch of a roof is 1/4:12, which translates to 1/4 inch rise to 12 inches of run or 1/4 inch per foot.

So if the porch is going to be 12-feet wide, 8-feet high against the cabin, 6.5-feet high on the other side, will the roof be too flat?

There was also a lot of information about snow loads that I don’t think we need to worry about here.

Then I found a website that calculates everything for you, EASY Roof Pitch Calculator. Just put in the rise and run in inches. (Rise is height. Run is width.)

Rise 18”
Run 144”

Pitch 1.5/12
Angle 7.1 degrees
Rafter Length 145 1/8”

Metal roofing can be installed on pitches as low as 1:12 (5 degrees). Our roof will be metal and will be 1.5/12 or 7.1 degrees. So, yes. We can have the porch we want with the roof we want.

Pressure treated lumber comes in 14-foot lengths, which is perfect.

Covered Porch on a Westwood Modular Cabin

Attaching a Porch Roof to a House

There are three ways we could attach the roof of the porch to the roof of the cabin.

We can have the porch come straight out from the existing roof, like this. (I think this looks best.)

Porch on Main Arts and Crafts Lodge, Prince William Forest Park, Triangle, Prince William County, VA
Built by the National Park Service with Civilian Conservation Corps
Documenting Plan by Daniel De Sousa

The porch roof can attach to the wall below the roof like this. This makes the roof of the porch much lower than the roof of the cabin.

Porch on Cabin, Prince William Forest Park, Triangle, Prince William County, VA
Built by the National Park Service with Civilian Conservation Corps
Documenting Plan by Jason W. McNatt

Or the roof can attach above the edge of the roof like this. This is the way the porch on the house we have now attaches.

Deck on a Westwood Modular Cabin

Turn a Deck into a Covered Porch

Can we build a deck now and add a roof to it later?

Yes. And building a deck is much less expensive than building a covered porch.

A deck foundation only has to 55 PSF (lbs per square foot). A porch, with the roof, is heavier and the foundation has to support 80 PSF.

How much money will it really save?

A roof over a deck, made of transparent polycarbonate roof panels supported by lumber.
Photo by Beeblebrox

This is an idea… Lots of light gets in, it weighs hardly anything… and it still keeps the rain off the deck. It could be finished better…