How to Re-roof a FLAT patio structure with enough run for drainage

[TandT]

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

[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]

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]

Hi @TandT,

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

Mitchell