How to determine pitch and rafter size on shed roof?

[Jayrox]

 

Hi, I am building an outdoor shed and have found lots of great info here. So much I’m potentially over thinking it.  The shed will be 4.2m x 3m with a set of barn / French doors on the 3m side as the only opening.  As height is important (I can’t go higher than 2.5m off the ground including the base it will be on) my questions are:

1. What is the minimum in cm fall / pitch I can have to accomodate a colourbond tin roofing. I believe it’s 5 or 6 degrees they recommend as the minimum.  Would 240cm on the tall end down to 220cm on the short end meet the requirement for Flat roof / Skillon. 

2. If I decrease the spacing between the rafters can I decrease the rafter size. Again with height being tight the narrow rafter will help. Would I get away with a 90x35 spaced closer than a 140x35. The span will be max 3m. 

3. I don’t have a nail gun - what screws do you recommend for building the frame. Do they go straight in at the end or angled at the sides like you would with a nail gun. 

just for reference I found this photo in here and is closest example of what I am looking at however my opening will be on the narrower side. 

many thanks. 

[EricL]

Hello @Jayrox 

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your shed.

Let me try and answer your first question. Your roof drops 20 cm from front (240 cm) to back (220 cm). Whether that gives you a 5-degree slope depends on how long the roof is from front to back.

Here’s a way to check:

A 5-degree slope means the roof should drop about 8.7 cm for every 1 metre of length. So, if your drop is 20 cm, divide that by 8.7 cm: 20 ÷ 8.7 = 2.3 metres.

That means your roof can be about 2.3 metres long from front to back to have a 5-degree slope. If your roof is longer than 2.3 metres, your slope will be less than 5 degrees, not enough for standard corrugated Colorbond. If it’s 2.3 metres or shorter, you’re fine. So, yes, a 240 to 220 cm fall works only if the roof depth is about 2.3 metres or less.

In regards to your second and third question, I propose engaging the services of a draftsperson or engineer to draw up a plan for you. They will be able to tell the correct size of timber to use and what screw size and length to join it all together. Because the plan has been drawn up by professionals it will comply with the rules and regulations of your local council. Your proposed shed size is bigger than the standard 10 square meters, I suggest checking your local council's rules and regulations in regards to shed size limits. 

I also recommend looking at this discussion - How to build a wooden shed? by @mikebarker. 

Let me call on our experienced members @mikebarker, @Dave-1 and @Nailbag for their recommendations.

If you need further assistance, please let us know. 

Eric

[Nailbag]

Hi @Jayrox 

Just a couple of initial thoughts:

Have you got a building permit for this? If not I would inquiry with local council as it will most likely need one. If needed then proper working drawings will need to be provided which as @EricL mentioned a draftsperson will be the best and cheapest solution here. This will also solve your questions on spans and falls. Your footing requirements and sub-floor will be specified and the plans most likely will need to have included what is required for the water running off from the shed. 

And I would hire a nail gun or compare cost of multiple days hire vs buying one ion the DIY models. This will make it far easier to construct with. But if using screws, again the draftspersons plan will determine.

Nailbag

[Jayrox]

Thanks @Nailbag for response - have checked with council and no permit needed in SA as long as:

maximum roof span 3m

Maximum height 2.5m

under 15sqm. 

Hope I haven’t missed anything as basing not requiring a permit on that. 

[Jayrox]

Thanks @EricL appreciate that response.  Roof length will be 3 metres so based on above I make that as 26.1cm drop. 

in SA and my council - shed size can be up to 15sqm so as long as I haven’t missed anything was hoping on completing without a permit.  

[Dave-1]

Good Morning @Jayrox 

I cant really add to what @EricL and @Nailbag have siad. They both have pretty much covered the bases.

Only thing would be to take lots of photos as you construct the building/shed, It would be an interesting project to see up here and also helps with referencing where things are down the track.

Dave

[Nailbag]

Hi @Jayrox 

Great news, you have it easy over there! I would still recommend getting the plans professionally drawn up. Its not that expensive and will give you all the detailed information to follow as working plans.

Nailbag

[Jayrox]

Thanks again - still working away on my pitch.  Does Bunnings sell a roof sheeting that is suitable for a lower pitch. I have seen some info on minimum 1 degree sheeting. 

as I’m needing to keep it under 2.5m height every cm is super important. 

currently looking at a width including overhang of 3.4m so just to double check as an example a 1degree fall on 3.4 m length would be a 6cm drop?  (Adding 6cm drop for degree I want to increase the pitch) 

Thanks again. 

[EricL]

Hello @Jayrox 

Yes, Bunnings does sell roof sheeting that can work on very low roof pitches, though it depends on the exact product and how it’s installed. Some metal roofing options, such as COLORBOND® Klip-Lok 700 (special order), are designed for very shallow roofs and can be used with a minimum slope of about 1 degree. This type of sheeting is often used for flat or near-flat roofs because it’s designed to prevent leaks even when water drains slowly. Bunnings also carries products like the Fielders Spanform steel roofing (special order), which works on pitches as low as 3 degrees, and certain polycarbonate sheets, such as Palram’s EZ Glaze panels (special order), which are described as suitable for low-pitch installations.

In simple terms, a 1-degree roof pitch means that for every metre of roof length, the height drops slightly so water can run off. Over a roof length of 3.4 metres, a 1-degree slope would result in a drop of about 6 centimetres from one end to the other. So, if one side of your roof starts at 2.5 metres high, the opposite side would be about 2.44 metres high. If you increased the slope to 2 degrees, that drop would roughly double to about 12 centimetres.

Because every centimetre matters in your setup, it’s good that you’re checking this carefully. Just keep in mind that while some roofing sheets are technically rated for a 1-degree slope, that’s the absolute minimum, and proper installation is critical. Low-pitch roofs can collect water more easily, so using the correct fixings, overlaps, and sealing methods is essential to prevent leaks. It’s best to confirm the minimum pitch requirements for the exact sheeting you choose and follow the manufacturer’s instructions closely.

I understand your need for space, but I would encourage you to stay within the 3-to-5-degree slope for proper drainage. You can imagine how difficult it will be to correct the fall should you discover that the 1-degree slope is not effective. Please remember to double check with your engineer.

If you need further assistance, please let us know.

Eric