Insufficient gutters and downpipes

[Cecile]

We have inherited a ton of issues with this house. This time it's the way the roof overhangs the gutters, gutter size and number of downpipes.

The person who replaced the roof about 25 years ago did a bodge job, and the roof sheets are not level with the ends of the rafters, and each sheet is a different length, giving a jagged edge. They hang over into the gutters by varying amounts, but on average the sheets cover the guttering up halfway, which makes it very difficult to clear them. Of course, when my father-in-law discovered this, the roofing company had gone bust.

Gutters are 100mm square, and every time it rains, they overflow. Evidently they are not flowing downhill to the current downpipes so the slope has to be rectified. We have three downpipes of 90mm diameter pvc on a roof that covers a house of 133 square metres, and believe we should have 6, three on a side (hip roof.)

Can anyone advise on best gutter/downpipe sizes? Roof is galvanised sheet and still in very good condition. We bring it up now because in one of our proposed renovations we will be increasing the drain of the roof on the longer axes of the house (currently 16m, extending out to 22m).

Can we get away with the current gutters and gutter guard if we increase to three or four downpipes per side?

[The Roofer]

Hi Cecile,

Have you got a sketch of the roof plan (valleys, ridges etc) including existing D/P's. Also you say you gutter is 100mm square - what is the depth from the base to the lowest side. eg a profile of the the gutter?

[Cecile]

Thanks.

Photo is of existing guttering 140mm across and 60mm at the lowest point. Plan includes existing and proposed roof.

[Danny]

The diagram shows massive non compliance. You need to firstly do two things:

1. After it next rains, wait an hour or so and get up a ladder and note where the water is pooling.
2. Find out where the storm water pipes run.

With a bit of luck, the guttering might already be sloped for additional downpipes but like l said, "with a bit of luck". There is one particularly long run between downpipes where it would be almost impossible to have any current slope.

[Cecile]

The diagram shows massive non compliance.

I think we knew this, but having been in drought for over 10 years it was not really an issue since we got so little rain

[The Roofer]

Hi Cecile,

As per Danny - but since the gutters are already off - and you're in Geelong - where it's 120mm Rainfall intensity - I would go for a minimum of six 90mm round downpipes (3 front & 3 back) and adjust your gutters to a minimum of 1:500 fall - and with the 6 D/P's would be at about 80% utilisation. And rerun extra stormwater pipe also to connect to each!

[Gaza]

just for refernce at a later date here is nsw fair trading guide to gutter and calc roof requirments

http://www.fairtrading.nsw.gov.au/pdfs/Tenants_and_home_owners/Roof_plumbing_guide.pdf

[Cecile]

but since the gutters are already off

They aren't actually off, photo is of a spare, but they might as well be. Thanks for the information, very much appreciated from you and Gaza as well.

[Danny]

G'day Gaza,

I am not having a go at you here as l know that you posted the NSW Fair Trading link in good faith and as a helping guide for Cecile but for the benefit of all who may read it, l have to warn readers of the inaccuracies contained in the document's example. It is good that it was posted as l did not realise that it had been adopted outside of the Vic. PIC. I have previously warned against the contents of the PIC document on this forum.

The document is acknowledged as based on the Victorian Plumbing Industry (PIC) Commission’s Roof Plumbing Technical Solution Sheet 0.04. It is a further sad indictment on the NSW Department of Fair Trading that they would adopt an unscrutinised document for the purpose of giving public advice given that the department had previously demonstrated an unwillingness to properly address the issue of high fronted gutters supplied with non compliant spring-clip fittings that allowed the gutters to overflow back into the house. To back that up with further tardiness is unforgivable.

Gremlin behind the walls - National - smh.com.au (http://www.smh.com.au/news/national/gremlin-behind-the-walls/2008/02/22/1203467388838.html?page=fullpage#contentSwap1)

Pages 2-4 of the 4 page document give an example and diagrams of calculating guttering and downpipe compliance for a house in Melbourne. The Melbourne 1:20 ARI is 130 mm/hr, based on a 5 minute rainfall intensity.

For compliance, downpipes must be no more than 12 metres apart. Nowhere in the document is this mentioned. If one looks at the house roof plan example given, five downpipe positions are shown and each harvests a roof area that is less than the maximum 47 sq m permitted for the given gutter and downpipe size in a 130 mm hr ARI area. So far, so good. But...

DP 1. This harvests two roof areas but the downpipe is positioned 2 metres from the top corner, away from the greater harvested area on the larger of the two rooves. The downpipe's position also appears to be a bit more than the compliant maximum distance of 12 m from DP 2. Although not required by the regulations, the downpipe should be positioned 2 metres lower on the plan so as to more efficiently harvest the most concentrated flow at its source.

DP 2. Moving DP 1 two metres down would also allow DP 2 to be positioned closer to the roof valley. For compliance, a downpipe must be positioned within 1.2 m of a roof valley. This is however an often unrealistic figure that needs amending but the diagram shows a distance of nearly 4 metres, clearly non compliant. Moving the downpipe 2 metres closer to the valley would also position the DP to harvest the most concentrated flow from the largest catchment area on the roof.

DP 3. The distance between DP 3 and DP 4 appears to be about 16 metres, clearly non compliant.

DP 4. Is poorly positioned away from the area of most concentrated flow. While this is not required for compliance, it would nevertheless position this downpipe closer to DP3.

DP 5. Is also poorly positioned away from the greater roof area drained.

The total gutter length as shown as 60 metres. 6 downpipes should have been fitted and their positions should have been given better thought. Some high points (HP) also need changing. It should also be noted that the distance between downpipes influences the fall between the high point and the downpipe (low point). Eaves gutters must have a minimum fall of 1:500 (2 mm per metre).

Tens of thousands of new homes are build every year in Australia that have non compliant roof drainage. Most are passed as compliant.

Cecile,

To be able to suggest the best downpipe locations for your home, you need to post a roof plan that has the sections marked , distances shown plus all other relevant information. Reading the document that Gaza linked and reading this post as well as the page referenced by my signature should give you the information you need. Good luck.

[Moondog55]

Thank-you for your help roofer//Danny
We are going to be adding an extension to the back, and the roof at the rear may be changed to a gable end, thus increasing the capture area.
As far as rainfall goes;-( the pattern seems to be changing to scattered downpours of much greater short term intensity, so without having to change the gutters would increasing the number of downpipes to 4 per side ( or more ) have much extra effect??
I fear we need to dig trenches and re-do the storm drainage completely.

[Danny]

We are going to be adding an extension to the back, and the roof at the rear may be changed to a gable end, thus increasing the capture area.


The slope of the roof is factored into the catchment area as the harvest area is greater than a flat roof when the roof is facing the weather. This is why abutting walls also have to be used in the calculations. I posted a photo of a unit that has three abutting walls draining to the one downpipe on this thread:
http://www.renovateforum.com/f194/shell-right-mate-102658/

When you look at the photo, you will see how heavy rain will substantially increase the amount of water draining to the gutter if the walls face the weather. With the example shown in the photo, the walls do face the usual weather pattern plus there are additional issues as well.

The roof's pitch is used as a multiplier when calculating the factored roof area. Your gable roof 'plan' area would have a larger multiplier than a usual 23 degree pitch roof which would in turn has a larger multiplier than a (near) flat roof. To calculate the gutter high points (which determine the roof catchment areas), you need to show a plan drawing that shows the roof area, ridges, lengths, valleys, abutting wall areas and pitches.



As far as rainfall goes;- the pattern seems to be changing to scattered downpours of much greater short term intensity

Absolutely. The calculations used for eaves gutters are also factored for a 1:20 ARI. In theory, the actual calculations are over engineered but in practice, the (common) poor positioning of downpipes (which is not factored into the calculations/regulations) can and often does cause problems. See my previous post.

It staggers me to constantly see so many houses with abysmal roof drainage and know that possibly (read probably) most new homes are built and sold with non compliant roof drainage.


without having to change the gutters would increasing the number of downpipes to 4 per side ( or more ) have much extra effect??


The optimal positioning of the downpipe has the greatest influence and should have priority. This is particularly important for steep pitch rooves. If the downpipes are positioned for maximum efficiency and the distance between them is compliant, you will have sufficient slope to handle storms of much greater intensity than a 1:20 ARI. The thing that you have to do is determine the best positioning and adjust the gutter slope to set your high and low points. This is why l first suggested that you need to know your existing slopes (by looking for pooling). You might get lucky with the existing guttering as it may need minimal adjustment but you need to do this first. The new guttering on the extension is easily worked out.

There is another photo next to the unit on the other thread. Have a look at where the water from the upper roof drains to (look for a short downpipe) and then look for the downpipe position on the lower roof. The lower downpipe's location is absurd but the drainage is actually compliant thanks to the inadequacy of the applicable standards. If a plumber came along and fitted a bigger downpipe, it would do diddly. Correct positioning is the key.

[SilentButDeadly]

It staggers me to constantly see so many houses with abysmal roof drainage and know that possibly (read probably) most new homes are built and sold with non compliant roof drainage.


Alternatively...
"It staggers me to constantly see so many houses with abysmal [insert topical subject matter here] and know that possibly (read probably) most new homes are built and sold with non compliant [insert topical subject matter here].

No reason to think that roof drainage is special after all!

As I've been told by a franchised builder not long ago..."...but that's what the customers want!" or "...but the customers don't want to pay for it!".

[The Roofer]

SilentButDeadly,

Totally agree - but compliance must be paid for - it is not negotiable in construction! But unfortunately with downpipes - the designers or architects place them for looks not for the use they are intended for! Sad when looks and money over ride good practice!

[Cecile]

Fabulous answers. Thanks everyone.

[Danny]

Alternatively...
"It staggers me to constantly see so many houses with abysmal [insert topical subject matter here] and know that possibly (read probably) most new homes are built and sold with non compliant [insert topical subject matter here].

No reason to think that roof drainage is special after all!


SBD,
You posted no facts or references yet claim that in doing so you have established that other areas of the housing industry also suffer a high incidence of non compliance.



As I've been told by a franchised builder not long ago..."...but that's what the customers want!" or "...but the customers don't want to pay for it!".

Do you base your claims of endemic non compliance in other areas of housing construction on your discussion with just the one alleged builder you have quoted?

Do you also believe that home buyers "want" non compliant workmanship and/or fittings as per your discussion with the alleged builder that you quoted?

This forum gives persons the opportunity to inform and educate the trade and public about all manner of subject matter. You will note that l have done this in this thread by explaining best practice and detailing why a linked industry reference document contains non compliant directions and examples. I have also referred to my knowledge of deficiencies within the code and given reasons for those beliefs.

ringtail has recently started the new thread: http://www.renovateforum.com/f196/decking-collapses-103877/ that deals with a serious ongoing problem that has resulted in a number of serious injuries and deaths. There are also a number of tutorial threads on a number of subjects started by forum members that have been of positive benefit to other forum members, site visitors and the forum itself. The opportunity is there for you to likewise make a positive contribution by alerting and educating everyone to these as yet not identified areas of high incident non compliance that you claim to have privy knowledge of. Unless you have confused minimum standards with non compliance, your social conscience should dictate that you do so.

[SilentButDeadly]

With all due respect, Danny, I'm not necessarily implying non-compliance with a Code by the building industry. What I am implying is that the building industry doesn't necessarily go for site specific, location specific and best practise design and construction techniques. And the reason often given is that this is not what the customer wants or alternatively it costs more to do and the average person isn't prepared to pay. Sometimes the building company is simply not prepared to play either...

The upshot is that we still get houses that are inadequate for their environment and location but still typically built to Code.

What I really resent is your implication that I have a social conscience. If I had one then I wouldn't find the whole sorry show anywhere near as amusing as I do.

[Danny]

What I am implying is that the building industry doesn't necessarily go for site specific, location specific and best practise design and construction techniques. And the reason often given is that this is not what the customer wants or alternatively it costs more to do and the average person isn't prepared to pay. Sometimes the building company is simply not prepared to play either...

The upshot is that we still get houses that are inadequate for their environment and location but still typically built to Code.



Now, that is something l do agree with. No question.

My references have been directed towards the common industry acceptance of non compliance and inadequacy of some standards in some areas in which l have an involvement. It's like a cancer really.

[The Roofer]

SilentButDeadly,

This is true in what you say:

The upshot is that we still get houses that are inadequate for their environment and location but still typically built to Code

This is the real world and regardless of the standards required - the customers end up with a non-conforming house - its illegal! How is this addressed when BSA, ASA etc? they haven't got a clue to the real standards required in each area. In 1968 - I was involved in a house built in North Queensland - the builder was very experienced and old - he decided the contruction detail - the inspector then just agreed - as he did with all the new houses in that area - Cyclone Althea came though and this house was the only one left standing of about 20!

The inspectors (or certifiers now) today have had no experience in the art of physical building (involving plumbing, roofing, carpentry, electrical etc etc etc ). I think Danny is simply saying the checking mechanism for any trade in building today is regulated by desktop people that are not hands on! It doesn't matter if it's a gutter, downpipe or a structural beam - they all should be checked by real people not the customer!

[Moondog55]

OK If it is built to code surely it has to be "LEGAL" otherwise what is the point of having a code??
Is this a design and compliance problem or is this a lack of "Design for ultimate performance " problem??
We know the roof does not perform, we also know it was passed at the time of installation; we do not know how or why but that is irrelevant in our situation as all we want to do is fix the problem.
I think we have been given enough information to work out what we need to do, and I'll start by finding and repairing all the old drainage replacing the old concrete pipes with plastic.
So how much rain should we allow for?
20 year event or 100 year event? because it seems as if we get 100 year events every year or so now.

[Danny]

G'day Moondog55,

I hope that l haven't given the impression that l was criticising Cecile or yourself when l stated that "It staggers me to constantly see so many houses with abysmal roof drainage and know that possibly (read probably) most new homes are built and sold with non compliant roof drainage". It was posted as an overview to alert persons reading the thread that the issue of overflowing gutters and stormwater non compliance also effects new homes and is a common problem that many are not aware of.

Unfortunately, the high incidence of non compliant roof drainage is endemic across Australia and the BCA code and AS3500.3, which is deficient in critical areas, also allow for poor performance even though the installation may be compliant as per one common photo example (bull nose verandah) l previously referred to.



So how much rain should we allow for?
20 year event or 100 year event? because it seems as if we get 100 year events every year or so now.

Your (Geelong) 1:20 ARI is 120 mm/hr which is worked out over a five minute intensity of 10 mm in 5 minutes. A 1:100 ARI is roughly 50% greater. My area had a 1:500 ARI a year ago and the only way you can cater to this is to install additional 'non continuous' overflow provisions as recognised by the code unless you want to have very large gutters and big downpipes that are closer together than the maximum compliance distance of 12 metres apart and their location is roughly equidistant between the high points. Most but not all 'non continuous' overflow provisions as recognised by the code still overflow as per gaps between the back of the gutter and the facia.

You already have three 90 mm round downpipes. These drain better than the alternative 100 X 50 mm rectangular downpipes. If you position the new downpipes to optimum effect, you can easily cater to a 1:100 ARI. Simply...

1. Have sufficient slope. The minimum for eaves guttering is 1:500 (2 mm every metre).

2. Do not exceed the maximum compliance 12 metres apart rule. Having the downpipes a little closer will also allow for additional slope.

3. Try to position the downpipes at or near the area of greatest concentrated flow for maximum efficiency (refer to my comments re the deficiency of the example given in the Fair Trading/PIC document). The greater amount of water drained will then have the least distance and time to travel to the downpipe.

4. Drain as close as feasible to the centre of the gable roof run off. Do not position the downpipe at the end of the wall. The end corners should be the high points for greatest efficiency.

5. Install the highest point of the gutter's back wall at least 20 mm below the top of the facia.

6. Make sure that the water cannot overflow over the facia and back into the house if you use high fronted gutters as the front of the gutter may be higher than the facia. Use flashing or other provisions to prevent this.

7. Make absolutely sure that the gutter's floor at the high point is well below the top of the gutter's back wall at the low point (where the downpipe is fitted). A build up of water MUST be allowed to flow over the high points and drain to the neighbouring downpipes.

Unless the downpipe or the gutter is obstructed, nearly all gutter overflows caused by heavy rain happen away from the downpipe but people will blame the downpipe and not the downpipe's position. I inspected a place last week where a plumber had fitted a bigger downpipe and a rainhead to 'fix' an overflowing gutter but the gutter still overflowed. The problem and remedy is not well understood and l see these failed solutions all the time.

This document is worth a read (but some information is outdated).

http://www.lysaght.com/files/dmfile/RainwaterSolutionsVICTAS21October2009.pdf

[Moondog55]

Criticism, of course not!
I am wondering though if I really should get up on the roof with a nibbler or metal cutting blade and cut all the roof sheets to a better overhang to allow us to put in the gutter guard.
Having too many downpipes really isn't an issue then? the way we see it now there needs to be 2 extras on each side where the catchment area is greatest, but in the future we will need to add one more where the new roof meets the existing roof when it is modified to a gable rather than the existing hip
I have found where the gutter low points are, hard to miss actually as the water is still there in the form of mud.
Can I use standard aluminium flashing with a zincalume roof??

[The Roofer]

Moondog

Can I use standard aluminium flashings with a zincalum roof?

Yes - direct contact and drainage are not a problem!