Fixing gutter capacity issue around patio

rsmallri · Feb 04, 2024 9:47 pm

Hello,
I have a 30 y/o single story brick and tile house in Perth currently rented out. The gutters (~80 lineal meters) are corroded and need to be replaced. Outside the ktichen there is a timber frame patio with polycarbonate roof sheeting put up by a previous owner. The patio shape is complex because it follows the roofline which has a number of changes in angle and on the other side follows the paving which also changes angle. The patio roof (about 25m2) slopes into standard house gutter and during heavy rains it overflows and on a couple of occasions has come down the interior wall and window in the living room. I am hoping for some ideas on the best way to fix this.

I got some quotes to replace the patio with a skillion type sloping away from the house with its own gutter system but it will be at least $30k due to the surface area, the wastage of this shape and the need for other works like new soakwells etc. Ideally the paving and adjacent landscaping would be redone so the patio could be made smaller and rectangular but that costs more money. I don't want to go down this path at the moment and would rather do just what's required to fix the gutter issue. Some of the rafter hangers (may not be the right term) bolted to the metal fascia are fairly corroded and probably need to be replaced soon too.

Below are the ideas I've had:
1) Replace gutter like for like with slotted gutter on most of house and unslotted in patio - probably going to overflow at times
2) As above but try upgrading downpipes in patio area
3) Replace gutter in patio area with a box gutter. A couple of companies have quoted this however the rafter ends match the shape of the existing gutter and would need to be notched to house the box gutter. I have some concerns this will weaken them. The notched section would still be above where rafter hanger bolts on so maybe OK.
4) Try to re-arrange the timber packers that give the PC sheets their fall to slope away from the house gutter and run straight off edge. This isn't ideal but a lot of this would fall into garden area so could be OK
5) As above but also try and fit new gutter system to outside of Patio. Not sure how it would attach but possibly could be clipped to the edge of the timber frame. Would be a bit ugly sticking out and require a new soakwell

The tenants also say in heavy rain one of the downpipe holes overflows. A contractor has said a soakwell has probably collapsed and they want to dig up the paving an put a polypropylene one in. This costs an extra $1500 and they won't redo the paving afterwards so I am not that keen. I am also doubtful just one could have collapsed as I would have expected for a house of this era the downpipes are all manifolded together going to a couple of concrete soakwells. It seems to only happen in heavy rain so one drain overflowing occasionally doesn't seem that bad. I don't think it backs up the downpipe.

Quotes for option 3 plus installing a new soakwell for one of the downpipes have ranged from $12.5 to $23k which seems like a lot.

Any ideas would be greatly appreciated.

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SaveH2O · Feb 05, 2024 4:38 pm

I need the answer to a few questions before I can give some advice.

rsmallri

I have a 30 y/o single story brick and tile house in Perth currently rented out. The gutters (~80 lineal meters) are corroded and need to be replaced.

The tenants also say in heavy rain one of the downpipe holes overflows.
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I take it that the only gutters that overflow are those servicing the patio roof.

I notice vegetation in the patio area. Do the gutters that overflow suffer any organic accumulation?

Can you post a diagram that shows the downpipe positions in the patio area (I see one) and also the nearest ones on both sides plus the gutter sections that overflow?

Re the overflowing "downpipe hole", does this refer to the adaptor at the botton of the downpipe or something else?

rsmallri · Feb 05, 2024 11:23 pm

Thanks for your response and taking an interest.

Yes only the patio area gutters overflow.

Last time I walked around on the roof there was no organic matter in the gutter. The vegetation in the patio area is either too low or away from the gutter. There is a big tree in the vicinity that could maybe drop leaves in the right (wrong) wind conditions but we are heavily pruning it in a couple of weeks. I don't think it's a causal factors.

See sketch attached. Only one downpipe is in the patio area however there is a downpipe just past the end on one side and on the other side there is a pipe connected to the gutter going to a rainwater tank. Don't worry about the vegtation being overgrown in the photo, that is an old satellite photo which doesn't reflect current condition.

Regarding the overflowing hole, I have tried to a few times to get details from the PM and tenants and they are pretty vague. I understand water is coming out at the adaptor at the bottom of the downpipe (there is a small air gap). They said it has only happened a couple of times in very heavy persistent rain. Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅

SaveH2O · Feb 06, 2024 8:07 am

Thank you for the added information and for adding the downpipes to the photo.

I'll give a summary before I give course of action recommendations.

Roof drainage regulations

Victoria has a variation in the National Construction Code (NCC) that only permits roof drainage to be installed to the Australian Standards AS 3500 Part 3 whereas other States follow the Building Code of Australia (BCA). The BCA is part 2 of the 3 part NCC and AS 3500.3 is a Deemed To Satisfy (DTS) referenced document in the BCA but the BCA, being the primary document, has hierarchy.

You are in Perth and so your gutters were almost certainly installed to the BCA regulations which, unfortunately, are lesser than the AS 3500.3 requirements, the reason why Victoria, where I am, has never ever adopted them.

I mention this because the BCA requires a gutter low point at a roof valley whereas AS 3500.3 requires a high point. The BCA also requires downpipes to be fitted as close as possible to the roof valleys for this reason.

Unfortunately, I don't know whether your gutter's high or low points are at the roof valleys but this is something that you should know when it comes time to replace the gutters but given that the downpipe in the patio area is close to the valley, I would be confident in assuming that the low point is at the valley. If so, then +80% of the additional roof area is flowing to the nearby downpipe.

The adaptor at the bottom of the downpipe is left unsealed to allow for excess water to escape rather than backflow into the gutter and then overtop from the gutter. The 'overflow' from the adaptor may however be splash from the water flowing down the downpipe's wall hitting internal flow barriers where the flow path changes shape.

I assume that the gutters near the downpipe adjacent to the patio do not overflow.

Unfortunately, WA building practices that sit the gutter on top of the fascia requires a side exit for the downpipes. Side exits have decreased drainage capacity when compared to vertical discharge gutters.

rsmallri

Below are the ideas I've had:
1) Replace gutter like for like with slotted gutter on most of house and unslotted in patio - probably going to overflow at times

The gutters will have to be replaced in line with the current regulations but WA roof drainage practices are really in the Wild West!

Are the current gutters slotted? If not, you need to be aware that a slotted gutter has less carrying capacity than an unslotted gutter with the same profile.

Regulations now require overflow provisions to be fitted but again, you are in WA and provisions such as having a 10 mm gap between the back of the gutter and the fascia cannot be applied. Overflows are overflows and ideally, you want to, if possible, eliminate any overflows including from the downpipe adaptor.

rsmallri

2) As above but try upgrading downpipes in patio area

Overflows rarely happen at the downpipe, the exception being when the downpipe or auxillary pipework is flow restricted and water backs up into the gutter. However, the patio downpipe has an approximate 20 sq m roof catchment area to contend with plus roof catchment areas are factored with a roof slope multiplier to account for wind driven rain. If your roof has a standard 22.5 degree slope, the multiplier is 1.21 which effectively now makes the approximate additional roof area draining to the downpipe 24.2 sq m.

If your 1:20 Average Recurrence Interval (ARI) on which roof drainage regulations and calculations are based is 150 mm/hour, your additional roof area draining to the downpipe is an additional 60.5 L/m during a minimum 1:20 ARI. Note that the L/h figure is based on an average rain intensity over a 5 minute duration.

Water flows slowly along a gutter and every bend serves as a flow restriction plus water falling into a gutter from opposite sides generates turbulence that also slows the flow. With the ridiculous side drainage outlet being additional to the affect of increased turbulence in the gutter plus the gutter bends (which are wrongly not accounted for in roof drainage regulations/calculations), you are guaranteed overflows.

rsmallri

3) Replace gutter in patio area with a box gutter. A couple of companies have quoted this however the rafter ends match the shape of the existing gutter and would need to be notched to house the box gutter. I have some concerns this will weaken them. The notched section would still be above where rafter hanger bolts on so maybe OK.

Quotes for option 3 plus installing a new soakwell for one of the downpipes have ranged from $12.5 to $23k which seems like a lot.

This is major work with high expense but whoever quoted $23k didn't want the job.

rsmallri

4) Try to re-arrange the timber packers that give the PC sheets their fall to slope away from the house gutter and run straight off edge. This isn't ideal but a lot of this would fall into garden area so could be OK

Wouldn't that create a gap whereby rain could blow in if the weather was coming from that direction?

rsmalli

5) As above but also try and fit new gutter system to outside of Patio. Not sure how it would attach but possibly could be clipped to the edge of the timber frame. Would be a bit ugly sticking out and require a new soakwell

See number 4 plus add additional expense.

IF POSSIBLE, THE BEST SOLUTION IS BELOW

I am going to suggest that you fit two of my Supa Gutter Pumpers that were designed by myself for the sole purpose of serving as an eaves gutter emergency overflow provision during heavy rain provided the overflows are not caused by blockages. This would be far and away your best possible solution but the issue possibly preventing fitting these is with the guttering sitting on the fascia board. Do you have a flat bottomed gutter and how much flat area (mm) is there between the extremity of the gutter's exposed flat sole and the fascia?

Basically, the Gutter Pumper is a siphonic inverted nozzle that is technically an unpowered gravity induced vacuum pump. When the gutter's water depth exceeds 25mm, the anti vortex baffle prevents a vortex delivering air to the pipe which allows a full flow of water to be sucked into a 20mm pvc pressure pipe at high velocity/high flow rates.

Fitting two including the fittings and pipes would cost less than $100 all up.They would also decrease the volume of water going to the patio downpipe, thereby also helping fix the adaptor water loss issue. If there is enough space available for them to be fitted, I can give you more details.
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Jack-the-roofer · Feb 06, 2024 12:36 pm

rsmallri

Thanks for your response and taking an interest.

Yes only the patio area gutters overflow.

Last time I walked around on the roof there was no organic matter in the gutter. The vegetation in the patio area is either too low or away from the gutter. There is a big tree in the vicinity that could maybe drop leaves in the right (wrong) wind conditions but we are heavily pruning it in a couple of weeks. I don't think it's a causal factors.

See sketch attached. Only one downpipe is in the patio area however there is a downpipe just past the end on one side and on the other side there is a pipe connected to the gutter going to a rainwater tank. Don't worry about the vegtation being overgrown in the photo, that is an old satellite photo which doesn't reflect current condition.

Regarding the overflowing hole, I have tried to a few times to get details from the PM and tenants and they are pretty vague. I understand water is coming out at the adaptor at the bottom of the downpipe (there is a small air gap). They said it has only happened a couple of times in very heavy persistent rain. Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅

I Like the Save H2O reply.
His gadget looks interesting.

I am not as smart - and like to keep things simple.

It looks like you have a large roof area feeding into the valley and the volume of water for this catchment is too big for the poor patio downpipe.
It also looks like you have a complicated downpipe route that runs from the back of the gutter across the eaves lining then into the exposed downpipe. This is prone to blockages and bad water flow.

This is made worse by the support framing for the patio roof structure that is bolted through the fascia and sits below the base of the gutter.

Then when you replace the gutters, you have to remove the polycarbonate roof....

Looks like you have a challenge ahead.

My preferred option is to hang a new mini-rainwater head on the support frame to the patio roof and cut the front of the new gutter to drain into the rain head. Pipe the rain head to the existing downpipe ground inlet (join onto the existing downpipe if not rusted?).
This will allow the full cross sectional volume of the gutter to dump into the rainwater head - and not backflow into the eaves (and cause window leaks).

A good Perth roofer can figure out the finer details based on this solution.

I have a picture. Apologise upfront for the poor quality of the drawing..

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SaveH2O · Feb 07, 2024 8:52 am

rsmallri

The tenants also say in heavy rain one of the downpipe holes overflows. A contractor has said a soakwell has probably collapsed and they want to dig up the paving an put a polypropylene one in. This costs an extra $1500 and they won't redo the paving afterwards so I am not that keen. I am also doubtful just one could have collapsed as I would have expected for a house of this era the downpipes are all manifolded together going to a couple of concrete soakwells. It seems to only happen in heavy rain so one drain overflowing occasionally doesn't seem that bad. I don't think it backs up the downpipe.

I'll comment further on the above.

The contractor is either an idiot or an unethical opportunist!

Water doesn't fall down a downpipe, it flows down the walls but an adaptor's flow transition from a rectangular pipe to a round pipe brings it in contact with flow impediments unless the adaptor is well designed...which many are not. The resultant hydraulic rebound can then flow through the gap between the rectangular downpipe and the adaptor's inner wall during heavy rain.
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The above adaptor has near flat overhanging deadplates where the water transitions from a rectangle downpipe to a round downpipe. These cause a rapid transition from supercritical to subcritical flow, resulting in a sudden increase in water depth and energy dissipation.
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You can see why the above adaptor has a smoother flow transition.

I suggest that you seal the gap between the adaptor and the downpipe but make an air gap further up the downpipe to serve as an emergency overflow should the downpipe ever back up.

rsmallri · Feb 11, 2024 11:29 pm

Thanks to both of you for your detailed and well thought out responses. The Supa Gutter Pumper and mini rainwater head options both sound interesting and more pragmatic than anything else I had been considering.

I have some doubts the Supa pumper would fit in my case. The gutter profile seems to be Longline which looks like it has a 75mm span across the bottom. In the patio area the waling plate blocks the underside of the gutter as you said. For the areas adjacent to the patio I don't know exactly how much sits proud of the fascia but I would guess it's only ~25mm as that rib isn't visible with the gutter installed and the fascia profile I found shows the flat section as being 26mm wide. Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅
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rsmallri · Feb 11, 2024 11:47 pm

I just found this Stratco document which has some ideas as well.

Stratco Gutter Overflow Solutions

SaveH2O · Feb 12, 2024 6:20 am

rsmallri

I have some doubts the Supa pumper would fit in my case. Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅
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Unfortunately, you are correct.

Paradoxically, a lot of emergency overflow solutions discharge to atmosphere whereas the Gutter Pumper diverts water.

WA gutters are sometimes fitted with small side spouts colloquially called 'pissers' but they also discharge to atmosphere as does the large inverted nozzle described in the Stratco doc.

I have assumed that the gutter overflows on both sides of the downpipe but not at the downpipe itself. Am I correct?

I am envisaging a possible small custom design that could be made at a sheet metal shop. My initial thought is to have a 10-12mm wide slot 80-100mm long cut into the gutter's exposed flat underside on both sides of the downpipe. A flanged fitting would then be silicone sealed and pop riveted around the slot. The fitting would need to flange out on its sides so it it could be fitted at one end with either a 15mm (18.3mm ID) or 20mm (23.7mm ID) pressure pipe which have ODs of 21.3mm and 26.7mm respectively.

The pvc pipe would then be plumbed to the downpipe which would be 3-4 metres away or a couple of metres away if the hanging pipe was unable to be secured. Alternatively, the pipe could be plumbed to a large ornamental urn or similar that was fitted with a low drain outlet.

I don't envisage you being able to have a slot much more than 10mm wide due to the fitting needing a flanged perimeter but due to the design being a longitudinal sump, some coarse mesh or a perforated zinc sheet may be needed. This could be easily fitted inside the gutter and separate to the fitting.

The flow rates should be quite good because the pipe should full flow once the water level exceeds critical submergence which in this situation would be about x3 the pipe's ID but testing would be needed to verify this. Pushing the pipe 30-50mm or so into the fitting with the bottom half of the pipe inside the fitting (sump) removed will also prevent a free surface vortex forming.

Do you know someone who has a 3D printer and can do files?

rsmallri · Feb 12, 2024 10:52 pm

Thanks again for your reply. The overflow definitely happens away from the downpipe but whether it happens at the downpipe as well I can't say for sure. The property is tenanted and I only get 2nd hand info with limited detail.

I have a 3D printer and have made some files before. I can see what you are saying with the gutter design and it could work well in some cases. In my case there are a couple of issues, one is that the patio downpipe is between a sliding door and a fairly large L shaped window. To access a section of gutter that doesn't have the patio waling plate under it would be quite far from the downpipe on the other side of the window (probably at least 4 lineal metres and 2x 90 degree bends away) so may not necessarily mitigate overflow closer to the downpipe. The other thing is being tenanted I can't really do trial and error and giving a prototype to a contractor to go and install isn't ideal.

The rainhead idea sounded pretty good. What about just doing that and maybe upgrading the downpipe and adaptor size and possibly a hole in the downpipe say 200mm off ground level? If that doesn't work I can try some of the other solutions.

SaveH2O · Feb 16, 2024 9:53 am

This is a very challenging situation, made much harder by not being on site.

Firstly, I think that I originally overestimated the % coverage of the additional roof areas to some gutter sections as the greenery shown in the overhead photo is (I assume) unlikely to overhang the patio. Is this correct?

I don't know for sure where the gutter's high and low points are or whether the gutters pool after it stops raining. Pooling would indicate that the gutter's slope was levelled for aesthetics when the patio roof was constructed as is often the case.

If there is a slope, the patio area gutter's low point would be either at the valley close to the patio downpipe as per the NCC Part 2 (BCA) regulations or even at the downpipe itself.

The two high points would logically be at the ridge to the left of the downpipe adjacent to the patio area and also at the ridge to the left of the patio downpipe.

Unfortunately, I can't see how the flow of water down each valley communicates with the water in the gutter but it would be rare for all of the water flowing off the valley to be directed towards the section of gutter flowing towards the downpipe.

rsmallri

The rainhead idea sounded pretty good. What about just doing that and maybe upgrading the downpipe and adaptor size and possibly a hole in the downpipe say 200mm off ground level? If that doesn't work I can try some of the other solutions.

My experience with gutter overflows is that unless the downpipe or subsurface drainage pipe is flow restricted, the overflows happen away from the downpipe. In many cases, fitting a larger downpipe is not the answer.

If you have overflows from both sides but away from the downpipe, you will need to attend to both sections of gutter. You need to find out (if possible) whether more than one section of gutter overflows.

I also don't know how bad the discharge from the bottom of the downpipe is but there are two scenarios for its cause.

1. The subsurface pipe is flow restricted.
2. It is the result of the water flowing down the downpipe hitting the internal deadplates in the adaptor and the resultant hydraulic jumps resulting in water splashing up the space between the adaptor and the downpipe walls.

Aesthetics aside and provided the discharge from the bottom of the downpipe is rectified, you could first plumb, as a trial run with the current guttering, an additional outlet to the gutter, divert the water the short distance to the downpipe and wait to see what happens. You might be lucky.