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How to re-roof a flat patio structure with enough run for drainage?

Helping my son and daughter-in-law with a roofing problem on their "patio' which leaks like a sieve due to poor roof installation.

Problem: It was originally intended to be more like a simple, open pergola with no roof. The structure's rafters are dead flat, ie there is no run built into the patio, but the previous homeowner put a polycarbonate roof on it with no purlins / battens and therefore no slope for drainage. It's attached to the house along its Northern and Western sides, so when I re-roof it I need to make a North to South run in the roof so rain can run off into a gutter or into the garden at its southern end.

Apparently (per the Suntuf installation guide) this should be a 5 degree slope. Which means a 235 cm drop along its 2670mm length.

MY MAIN QUESTIONS:

1) Do I really need so much of a slope it seems excessive!? If not what is the smallest I can get away with?

2) What is the best method or achieving it:

Do I add "purlins" of the appropriate heights perpendicular to the existing rafters (rafters run north : south)...this sounds challenging because the purlins would need to be very wide, ie the northern most purlin would need to be 23.5cm higher than the top of the patio's rafters (!!), then successively narrower purlins to achieve the slope needed until I reach the southern end of the patio?

OR

Do would I be better off adding a tapered beams running along the top of the existing rafters to create the slope I need, then add standard purlins on top of those beams to attach the roofing sheets to?

I don't love the sound of either of those options so maybe someone with far more knowledge than I have can give me a better idea!!!

Any advice welcomed! Photo taken from the South East corner of the structure attached

Cheers
Tim

Patio Roof 1.jpg

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Comments

MitchellMc

You are right to question the 5 degree figure @TandT, because while it is what Suntuf specifies for warranty and ideal performance, it is often more than is strictly necessary for a small patio roof if everything else is done correctly. In practical backyard builds, many polycarbonate and metal patio roofs work reliably at around 2 to 3 degrees, provided the sheets are installed with the correct fall direction, adequate support, proper fixing and an effective gutter or run off point. At 2670 mm, a 2 degree fall is roughly 93 mm and a 3 degree fall is about 140 mm, which is far more manageable visually and structurally than a 235 mm drop. You would be taking on more maintenance risk the flatter you go, but for a covered patio rather than a habitable roof, 2 to 3 degrees is commonly accepted and generally performs well if installed neatly.

In terms of how to achieve the fall, adding stepped purlins of different heights perpendicular to the existing rafters would technically work, but as you have already worked out it becomes clumsy very quickly. Deep build ups at the high end are awkward to fix securely, look bulky, and can introduce flex and fixing issues over time. It is usually the least elegant option and one that is best avoided unless the fall required is very small.

The cleaner and more structurally sound approach is to create the slope by running tapered beams in the same direction as the fall, sitting on top of the existing rafters. These can be achieved by ripping treated pine to a taper. Once that fall is established, you then install standard purlins or battens across those supports in the normal way to fix the polycarbonate sheets. This spreads the load properly, keeps fixings simple, and avoids extreme build ups at the high end.

One final thing to keep in mind is that polycarbonate really does not like being installed dead flat. Even a modest fall combined with correct purlin spacing, allowance for expansion, and proper fixing washers will dramatically improve drainage and longevity. If you aim for around a 3 degree fall, create it with tapered supports running north to south, and then install conventional purlins over the top, you will end up with a roof that drains properly, looks intentional, and is far less likely to leak or give trouble down the track.

Let me tag @Nailbag for his thoughts.

Please let me know if you have any questions.

Mitchell

TandT

https://www.workshop.bunnings.com.au/discussion/comment/61393#Comment_61393

Hi Mitchel,.

That is fantastic confirmation of what I HOPED, but wasn't confident about. Thankyou.

Do you have a suggestion about how I should attach the tapered beams to the rafters: Would galvanized batten screws through the tapered beam and into the rafter (well sealed to keep moisture out) be okay? That seems like the neatest option.

Thanks again
Tim

Nailbag

Hi @TandT

There is plenty of insightful information @MitchellMc has provided and I would run with the tapered timbers on top of the existing rafters he's suggested. It's the cleanest and easiest solution in my mind, though will require some considerable long rips with a circular saw and sharp blade. A 24 toothed blade is ideal here as it wan't bog down too much.

Be very cautious with these cuts as the saw will have a tendency to kick back as you get longer into it. The trick is to wedge the timber slightly behind to prevent the ply closing and pinching the blade. Move the wedge closer as you proceed through the cut. Also lift the saw's height so the tip of the blade is only just through the underside. This will help reduce friction. If using a cordless saw, it will increase run time. You will need a decent high-capacity battery or two and take your time. Let the saw do the work and keep the revs up to help prevent stalling.

And since you're taking the sheets off and essentially starting from scratch in creating the fall, I would go with the recommended 5%. It's no more work your end, but has the benefit of increased run off during storm events with high winds.

I would use a range of suitably length batten screws with the heads recessed flat to surface which will provide the best strength. Drill pilots holes in the tapered timbers to ensure max fastening strength into the rafters. You may need to simply use long nails at the thin ends.

Nailbag

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/93179#Comment_93179

Hi @TandT,

Batten screws would be a good approach, but I'd also use Pryda Unitie 170 x 32mm Right Hand.

Mitchell

TandT

https://www.workshop.bunnings.com.au/discussion/comment/112812#Comment_112812

Hi Mitchell, Thanks for the advice, I've been finishing my planning for this project and will be doing it soon.

I have two more questions I'd really appreciate your help with.

1) what dimensions should I use for the timber purlins? I see that 42 x 35 is one recommendation but I was hoping to stick with standard size timber available at bunnings and I can't see 42x35 in outdoor framing treated pine. Should I just use 70 x 35 for simplicity or could I rip down some 90 x 35s to make 42 x 35s?

2) Sealing the rafters and purlins after cutting them:
The rafters and purlins need to be painted to match the existing patio timbers. Other than the usual primer undercoat and topcoat of paint, Do I need to do any special treating of the cuts first given that it's treated pine? If I build the roof well there shouldn't be any standing water on the timbers, so I'm hoping I don't have to go to too extreme a process to protect them, BUT I don't want thbe whole thing rotting and needing to be done again.

Thanks for your help
Tim

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/124087#Comment_124087

Hi @TandT,

Assuming the span between the rafters is around 600mm, either 70x35 or 90x35 that has been ripped down the middle would work. Personally, I would just go with 70x35 for simplicity's sake, but you could certainly rip down some 90x35 if you wanted to.

As you will be painting the timbers, there is no need to add any additional treatment. You would simply prime and paint them with 2-3 coats of a suitable exterior paint, ensuring you get solid coverage with no gaps. This will seal the timber so no water can be absorbed.

Let me know if you have any further questions.

Jacob

TandT

https://www.workshop.bunnings.com.au/discussion/comment/131437#Comment_131437

Thanks Jacob,

This forum is so useful, honestly it's one of the best services Bunnings offers.

Thanks for your thoughts. I agree about the 70x35s being the easier option and will go with that AND am happy to hear about the sealing - that's good news.

I do have a couple more questions regarding flashing if it's okay. I'll be using the Suntuf wall flashing for the high end-wall.

1) Regarding the top edge of the flashing: The profile of the flashing (see photo below) has a little turn-back at the top which leaves me a bit confused re how to attach it to a flat sheeted wall. How do I deal with the turn back bit (circled in the photo)?? I'm thinking that I'd cut that turn back off, then put a bead of sealant (or maybe a strip of Butyl tape would be better??) between the flashing and the wall, and screw the flashing to the wall (where there are studs). Do you have advice about that?

2) Regarding the bottom (ie the corrugated) edge of the wall flashing: I was assuming that I should NOT put any silicone sealant or butyl tape between this edge and the roofing sheets themselves, because it'd cause issues with the polycarbonate sheet's tendency to expand and contract with the heat That's a point that gets stressed in a number of documents about installing Suntuf (ie don't use silicon sealant due to expansion issues). However on one guide for installing Suntuf roofing, butyl tape is recommended for use with the wall flashing (see photo below)?!?! Can you advise me on this please?

Sorry for all the questions. Hopefully I'm getting pretty close to the end of them now.

Thanks
Tim

e !! ?? End wall flashing.jpg Butyl tape on flashing.jpg

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/135909#Comment_135909

Hi @TandT,

I would use butyl rubber in both places, behind the top edge at the wall and where the flashing meets the roofing sheets. Butyl has enough flexibility to accommodate the expansion and contraction of the polycarbonate sheets, whereas silicone is too rigid and could cause problems over time.

Regarding the return or turn-back on the top edge of the flashing, the simplest approach is to trim it off so the flashing sits flush against your wall. You can then place a bead of butyl tape behind it before screwing it to the wall. In many professional installations, the installer might just screw the return as-is and compress it against the wall, but that can distort the sheets slightly, so trimming it off first is a cleaner solution. This way, the flashing sits snugly, is sealed properly, and still allows for the movement of the roofing sheets.

Mitchell

TandT

https://www.workshop.bunnings.com.au/discussion/comment/138991#Comment_138991

Thanks Mitchell,

That's good to know.

Can you advise an actual butyl product I should use. I was looking at the Butyl adhesive strip that is 1.5mm x 20mm x 10m. Will that work or do you think I should go thicker and wider? (I was thinking this small size for aesthetics, but will go bigger if you think I should. This is not a product I've ever used before so advice would be appreciated.

Thanks again (product photos below),

Tim

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/141053#Comment_141053

Hi @TandT,

That butyl strip size should be perfectly sufficient for this application, especially if you’re aiming to keep things neat visually. A 1.5 mm thick by 20 mm wide strip will compress nicely once the flashing is fixed in place and will do the job of sealing out water without being overly bulky.

The key thing is surface prep. Make sure both the wall and the flashing contact areas are clean, dry, and free of dust or loose material before you apply the strip. I’d suggest running the butyl along the very top edge of where the flashing will sit, and again along the lower contact point with the roof sheetin, so there’s no path for water or debris to track in behind the flashing.

Also worth keeping in mind that while it’s an adhesive strip, it shouldn’t be relied on as the primary fixing. Think of it as a gasket rather than glue. Once the flashing is positioned, make sure you mechanically fix it with enough fasteners to properly compress the butyl. When you’re installing it, press the flashing firmly against the wall, then fix it off while maintaining that pressure.

Mitchell

TandT

https://www.workshop.bunnings.com.au/discussion/comment/142590#Comment_142590

Thanks for that great advice Mitchell,.

I think I am all out of questions!!

Let the building begin....(first weekend in February)

Cheers
Tim

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