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I’m building a 18.2 feet (555cm) by 15 feet (460cm) shed, and due to local regulations it can’t be higher than 8.2 feet (250cm).

Because I have a 20-inch (50cm) thick foundation structure above ground, and because I want at least a 6 feet (184cm) high doorframe, I’m way above 8.2 feet.

To reduce overall height, I’ve reduced the roof pitch from 15 degrees (3/12) down to 4 degrees. That’s less than a 1/12 pitch (4.76 deg). My concern is that this is bad for water evacuation and structurally inadequate in general. Does such a low angle increase the pressure on the side walls?

Another option I’ve looked at is to have a flat slanted roof with a 1 deg pitch, but because of the width of the shed, the span would be huge (4.6m, 15 feet), and I would need really thick beams (19.5cm 7.6 inches) which would eat away at my ceiling height. Since this is only carrying roof load, could I go with less thick beams?

Project with 4 deg roof pitch (less than 1/12)

Project with 1 deg flat slanted roof

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  • 2
    What are your roof loads? Does it snow where you are?
    – Ecnerwal
    Apr 19 at 10:52
  • 2
    The 8.2 is above ground level? Can either lower the building or raise the ground.
    – crip659
    Apr 19 at 11:20
  • 4
    So consider putting some / all that foundation in the ground...
    – Solar Mike
    Apr 19 at 11:21
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    Can I suggest you write a new question? It would go something like this: ‘I have tall piers and I need to frame a floor low to the ground’. Give technical details and drawings of the existing piers. (Above and below ground.) Explain why they got that way in the first place. (Flooding?) Be sure to detail any limits on the footprint of the shed. (Could framing be set low on the outside of the piers, for instance?) Apr 19 at 14:16
  • roof pitch isn't a huge deal on something this small. Are there large trees around? If it snows can you remove the snow? I mean if you put up a metal roof of almost any kind - and there are lots of choices now - the pitch won't mean much.
    – DMoore
    Apr 19 at 17:14

3 Answers 3

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There are two issues with a very flat roof - one is using a suitable method to waterproof it (EDPM rubber in a single sheet is a good choice, on that) and the other is ponding, causing deflection, causing deeper ponding, causing more deflection, ultimately leading to collapse. Choosing undersized rafters will end this way on a low-slope roof. Typical recommendation for a so-called flat roof is 1/4" per foot minimum slope.

Your "peaked roof" example is structurally naive, lacking the bottom of the triangle, or alternatively a central beam or load-bearing wall. It's also failing to take advantage of the reality that people can easily tolerate a lower roof near the walls, and the door location aggravates that.

Given that you have planned poorly for local restrictions, the door framing you show is unrealistic - you should likely be using a rafter beam as the top frame of the doorway, given you only left yourself 16cm. More likely you should rent a concrete saw and cut your piers down significantly, or mount your support beams to the side of the piers, not on top of them.

@crip659 & @Solar Mike's suggestion in comments to raise the ground level under the building is another solution to the "too-tall foundation" problem. Get a load of sand, rock or gravel delivered and spread it around.

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  • Thanks so much for your help @Ecnerwal. I have updated my original question with a visual showing a 45x145mm beam to support the roof rafters. Would this make the structure more reliable? Regarding the flat roof, it does have a 1 degree angle in the hope of evacuating at least some water, but I agree it's not the best.
    – Moyk
    Apr 19 at 11:27
  • Raising the ground around the foundation is an option but my neighbor already came to inspect everything so he probably wouldn't be a fan. What a bout a 5 degree roof angle using trusses 40cm (16 inch) appart and made of 2x4 rafters with strong 18mm OSB collar ties?
    – Moyk
    Apr 19 at 12:18
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    While keeping relations with the neighbors is important, meeting code and providing usability trumps the neighbor's opinion. Unless, of course, the neighbor is paying for the construction.
    – FreeMan
    Apr 19 at 12:23
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    @FreeMan - don't discount the neighbor. In many small towns an inspector will allow things or not based on anyone complaining. I am not sure where this guy is but let me tell you if I am doing anything "gray" I chuckle it up with neighbors, ask them what they think... I've been in towns where literally the inspector says yes or no and code and rules don't apply... Chocolate chip cookies have gotten many of my plans approved. Moreso than my detailed plans.
    – DMoore
    Apr 19 at 17:18
  • @Moyk Is your neighbour perhaps worried about you setting up noisy machines in the shed? Apr 20 at 17:39
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In my opinion you need to use a ridge beam and keep the pitched roof. You need to find what kind of live loads you roof will need to tolerate, particularly snow and ice loads.

Once the roof live load is considered, that gives the info on what is needed for the building of the ridge beam.

A ridge beam is different than a ridge pole.

A ridge pole is a single member where the roof rafters make their attachment at the peak. It can be of 1X material or 2X material. It helps the framing maintain a straight line and adds some structure, but collar ties or ceiling joists are still required to reach potential strength, along with proper nailing.

A ridge beam is made up of numerous 2x material an eliminates the need for ceiling joists or collar ties. Whether the beam is made up of 2, 3 or even 4 pieces of 2X or perhaps a lesser number of LVL or PSL members is figured by design loads, the amount of weight the beam needs to carry over its span. Also the area where the beam rests on the walls will need additional studs under them for there will be tremendous pressure in those 2 places where they bear. If there is a pier under there, it needs no more. If there is no pier there, then a beam needs to be placed in the wall either high at the top plate or low at the bottom plate to span over the closets piers.

About your door. The door is in a non-bearing wall. The header you have can be much smaller since there is nothing but the possible sag of the roof rafters along their span that may cause an issue. Using a continuous piece of plywood over the door to act as a diaphragm will minimize that potential. Plywood on both sides is great assurance.

About the pitch of your roof, again depending on the snow and ice loads a 1 degree pitch is not enough. By the time the roof sags under the weight, I fear the water will puddle in the sag. A 5 degree pitch should remedy this problem. The more the better.

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  • Thanks so much for your help @Jack. I have updated my original question with a picture that I believe shows your suggestion. You can see I added a ridge beam in the 3rd picture (in yellow), along with a doubled structural column (in red) in both facing walls. Does this make the structure strong enough? Also, is it correct to say this would turn the wall where the door is into a bearing wall, and so the door frame would need to be reinforced? With this solution, there is also no more need for the center vertical column in yellow in the image, in the middle of the shed, correct?
    – Moyk
    Apr 19 at 15:43
  • The wall itself would not be a bearing wall. The ridge beam which would still need to be built up so it can handle the load over the span. Also the "point load", the only place that is really providing any "bearing" in that wall, the very spot the post bears on the bottom plate will need solid framing under that point. If the pier post and the floor beam is in that spot on either end, then it is a matter of insuring the floor joists or band joist are thick enough in that given spot to aid in support of the roof. Perhaps just doubling the joist in that area for about 4" or so.
    – Jack
    Apr 20 at 6:27
  • Not to nit pick, and perhaps you know what I mean by a ridge beam, but you still have a ridge pole drawn, and not a ridge beam. I understand with Sketchup it may be difficult to change it. But do understand it will need more than a single member to support the span without sagging under its own weight without collar ties or ceiling rafters. What kind of snow and/or ice do you get out your way? I just noticed you added a center post. It is a big help, I was figuring a multiple piece beam to go clear span with no center support. A center support will require less structure at the ridge.
    – Jack
    Apr 20 at 6:33
  • Thanks for clarifying. Rafters are 2x4s and the “ridge beam” is 2x6. I am now thinking of going without a ridge beam for 2 main reasons. One, due to the low angle of the roof I don’t have a lot of space to mount efficient collar ties under the rafters. Two, I don’t want support poles inside the shed, and using a 2X ridge beam will require a lot of manpower to bring up there (to clarify, I do use a doubled up support (in red) for the beam in my visual). So I think I’ll go with roof trusses reinforced with thick plywood collar ties, like in the last image in my message. Does that make sense?
    – Moyk
    Apr 20 at 7:48
  • Collar ties are only required for a ridge pole, a single 2X6. A ridge beam does not need collar ties or a mid span support if constructed heavy enough out of multiple layers, say for example 2 or 3 ganged up 2X10s or 2 pieces of 9 1/2" X 1 3/4 LVL so it does not sag over the 18 ft length. that way you can get the slope of the rafters and the beam at the red posts will not need the short red post above the top plate.
    – Jack
    Apr 20 at 14:38
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The main problem with extremely low sloping or flat roofs is actually the risk of leaking. This is the reason why historically, it was really only in arid climates where flat roofs were widely used. In most places, pre modern (and even some contemporary) roofs tend to be much steeper, particularly at high latitudes where winters often bring lots of snow. The steep roofs allow snow to largely just slide off onto the ground, not to mention a lot of people in these latitudes with steep roofs used the attic as essentially another floor (or two)!

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