Cast Iron Gutter Fall, Outlet Positions and Fixing: A Specifier’s Guide

Technical summary

A cast iron gutter should be laid with a slight, continuous fall towards its outlets — typically 1:600 (about 1.7mm per metre), and no steeper than about 1:350 (about 3mm per metre). Capacity is governed far more by the number and position of outlets than by fall: a central outlet draining two ways carries roughly twice the flow of an end outlet, while long runs and corners close to outlets reduce capacity. Because cast iron outlets are fixed fittings and downpipe offsets come in fixed projections, outlet positions must be set out against the drains and the wall line before the order is placed. A G46 moulded gutter can be fixed on fascia brackets, screwed directly through its back above the waterline, carried on rafter brackets, or supported on rise and fall brackets driven into masonry at no more than 915mm centres.

Introduction

Questions about gutter fall, outlet positions and fixing come to us more often than almost any other technical subject — usually from a contractor or stonemason who is about to set the work out and wants to get it right first time. A cast iron gutter should be laid to a slight, even fall of around 1:600 towards its outlets, with outlets positioned over the drains and close to corners, and with every bracket set to a single string line from the high point to the low point. Get those three things right and almost everything else follows.

This guide is written for the people who design, approve and build cast iron rainwater systems: architects, building surveyors, conservation officers, main contractors, roofers and stonemasons. It applies equally to listed and period buildings and to new work, including extensions built against historic fabric. We use the moulded G46 gutter as our worked example because its flat back and deep section raise particular fixing questions, but the principles apply to every cast iron gutter profile.

Mistakes in setting-out are common because cast iron is less forgiving than lighter materials. Outlets are cast fittings rather than holes cut on site, downpipe offsets come in fixed projections, and the weight of a full gutter demands sound fixings. A decision made casually at ordering stage can leave a downpipe that will not meet its gully, or a gutter that holds water along half its length.

Why setting-out matters more with cast iron

Setting-out matters more with cast iron because the system is made of fixed, cast components that cannot be adjusted on site in the way lighter systems can. The outlet positions, the offset projections and the fall are effectively decided when the order is placed.

Cast iron gutters arrived in quantity in the late eighteenth century as a mass-produced alternative to lead, and the moulded profiles — ogees, the G46 and its relatives — became the standard eaves detail on Georgian, Victorian and Edwardian buildings. Their joints are socket and spigot, sealed and bolted. Their outlets are separate cast fittings, either a short section of gutter with an integral nozzle or a stop end with a nozzle cast into it. Their downpipe offsets are single castings with a set projection.

That has three practical consequences. First, an outlet cannot simply be moved a few hundred millimetres on site; the gutter lengths are cut to suit the fittings, so the fittings must be in the right place from the outset. Second, the distance between the outlet and the wall must match one of the standard offset projections, or be adjusted to do so. Third, the gutter weighs considerably more than a lightweight equivalent, and more again when full of water or snow, so the fixing background and bracket spacing must be designed rather than improvised.

The reward for getting it right is a system that, with periodic painting and clearing, routinely lasts a century or more. Cast iron does not sag or creep, so a fall that is set accurately on day one stays accurate.

How much fall does a cast iron gutter need?

A cast iron gutter needs a slight, continuous fall towards its outlet — 1:600 is the usual figure, equivalent to about 1.7mm per metre, and the commonly quoted range runs from 1:600 to a maximum of around 1:350, or about 3mm per metre. Historic building guidance in Northern Ireland recommends a minimum of 1 in 360, which sits at the steeper end of the same range.

The standard method for designing gutters, BS EN 12056-3:2000, does not mandate a particular fall for eaves gutters. Its capacity calculations treat an eaves gutter as level, and a gentle fall of the order of 1–3mm per metre is recommended where it is practicable. In other words, fall is not what makes a gutter big enough; outlets and section size do that. Fall is what makes the gutter drain completely, clear itself of silt and dry out between showers.

Why a dead-level gutter is a false economy

A gutter laid dead level will carry its design flow, but it will not empty. Water stands along the sole of the gutter after every shower, leaves and silt settle into it, and the inside of the casting stays wet for days at a time. On cast iron that is the single most common cause of internal corrosion. A modest fall costs nothing and means the gutter drains dry.

Why a steep fall causes its own problems

Too much fall is equally unhelpful. On a long run, a steep fall drops the low end of the gutter well below the roof edge, so water can overshoot it in heavy rain, and it makes the gutter visibly out of line with the fascia, the eaves course or the brick and stone coursing below. On a formal elevation — a Georgian front with a strong horizontal cornice line, for example — a gutter that visibly runs downhill can look wrong. This is one reason we favour more outlets and a gentle fall over fewer outlets and a steep one.

Fall over typical run lengths

The table below converts the usual range into millimetres over typical run lengths, measured from the high point to the outlet.

Whatever fall is chosen, the outer edge of the gutter should sit slightly lower than the inner edge, never the reverse, so that any overflow in an extreme storm spills outwards rather than back against the fascia or into the wall head.

BS EN 12056-3 and Approved Document H: what the standards say

BS EN 12056-3:2000 is the design standard for roof drainage, and in England and Wales Approved Document H, Section H3, is the Building Regulations guidance that references it. Together they set how gutters and downpipes are sized; neither prescribes a single fall for eaves gutters.

BS EN 12056-3 sizes a gutter by comparing the run-off from the effective roof area with the tested capacity of the gutter and its outlet. Approved Document H gives a simplified route for smaller roofs, using a design rainfall intensity of 75mm per hour, a table of effective areas adjusted for roof pitch, and a table of capacities for half-round gutters laid level with an outlet at one end. It notes that a fall, a more capacious section or a round-edged outlet can allow a smaller gutter or pipe to be used.

In Scotland the equivalent guidance is in Section 3 of the Building Standards Technical Handbooks, and in Northern Ireland it is Technical Booklet N. For listed buildings, Historic England, Cadw, Historic Environment Scotland and the Historic Environment Division all make the same essential point: the existing system should be repaired or matched like for like wherever possible, and alterations to rainwater goods on a listed building may need Listed Building Consent. Our listed building questions page covers the consent position in more detail.

For a new extension, the drainage design submitted for Building Control should show the gutter and downpipe sizes, outlet positions and the roof areas they serve. Where a moulded profile such as the G46 is specified, ask us for capacity information for the size and outlet arrangement proposed, rather than assuming half-round figures apply.

Where should gutter outlets be positioned?

Gutter outlets should be positioned directly over, or within reach of, the below-ground drains, as close as practicable to corners, and so that no length of gutter has to carry water further than it can handle. Outlet position affects capacity more than fall does.

End outlets and central outlets

An outlet at one end of a run means every drop of water travels the full length of the gutter. A central outlet, with the gutter falling towards it from both ends, halves the distance and roughly doubles the flow the same gutter can carry. It also halves the visible drop at each end. Where the drainage layout allows, a central outlet is usually the better design.

Long runs and the 50-times-depth rule

Guidance based on BS EN 12056-3 reduces the capacity of an eaves gutter once its length to an outlet exceeds 50 times its overall depth. The length is measured from the stop end to the outlet, or as half the distance between two outlets. The reduction factors are set out below.

For a 100 x 75mm G46 the 50-times threshold is reached at around 3.75m to an outlet; for the 125 x 100mm G46 it is around 5m. Runs can be longer than this, but each extra metre costs capacity, so a long elevation is usually better served by an additional outlet than by a larger gutter.

Corners near outlets

An angle in a gutter disrupts the flow. Where a corner is less than 2m from an outlet, guidance applies a reduction factor of 0.8 to the gutter’s capacity; between 2m and 4m, the factor is 0.9. On a building with returns and bays, outlets placed sensibly in relation to the angles avoid having to upsize the whole system.

Outlet sizes for the G46

The G46 is produced in 100 x 75mm and 125 x 100mm sizes. Standard outlets are for 65mm and 75mm downpipes, and 100mm round and 100 x 75mm outlets are also available. Larger outlets and downpipes carry more water but should be considered against the building’s proportions; on a modest period house an oversized downpipe can look as wrong as an undersized one performs.

Outlets and rainwater heads

Where a gutter discharges into a rainwater head rather than straight into a downpipe, the head provides a buffer that helps with capacity and can absorb a small misalignment between the outlet and the pipe. The head must be sized to suit the downpipe; we will not recommend a head whose outlet socket leaves too little metal around it to support the pipe.

From outlet to drain: downpipe offsets and projection

Because the gutter sits in front of the wall and the downpipe is fixed to it, most outlets need an offset to carry the water back to the wall face. In cast iron, offsets are single castings with fixed projections, so the outlet position must be chosen to match a standard offset size.

The projection is the horizontal distance from the centre of the gutter outlet to the centre of the downpipe where it sits against the wall. In practice we measure from the wall face to the centre of the outlet, then subtract the distance from the wall face to the centre of the pipe. Our guide to measuring a downpipe offset explains the method with diagrams.

Where the measured projection falls between two standard sizes, there are three options: move the gutter line slightly in or out where the eaves detail allows, adjust the outlet position along the run so that it sits over a point where the projection works, or pack the downpipe off the wall with 30mm spacer plates. Round offsets can also be rotated slightly in plan to take up a small lateral difference; square and rectangular offsets cannot, so they need the correct front or side version.

The outlet must also line up with the drain in plan. If the gully is not directly below the outlet once the offset has returned the pipe to the wall, either the gully moves or a second offset is needed lower down. At ground level, a projecting plinth or string course may require a plinth offset, typically at 135°. All of this is far cheaper to resolve on a drawing than on a scaffold.

Fixing a G46 moulded gutter

A G46 moulded gutter can be fixed in four ways: on fascia brackets, by screwing directly through its flat back into a fascia, on rafter brackets, or on rise and fall brackets driven into masonry. The right choice depends on the eaves construction, not on preference.

The G46 is a deep, flat-backed moulded ogee designed to sit flush against a fascia, which gives it a crisp, architectural line and makes it a long-standing choice for both heritage and new work. Its history and relationship to the other ogee profiles are covered in our history of the ogee gutter.

How many brackets does a run need, and which type suits your eaves? Our guide to cast iron gutter bracket spacing gives worked bracket counts for typical run lengths, set-out drawings and a profile-by-profile bracket table.

Brackets are mild steel, supplied galvanised or zinc plated, and should be painted with the gutter. Screws should be stainless steel or the zinc-plated gutter screws supplied for the purpose, sized to suit the substrate. The fixing background must be sound: a cast iron gutter full of water, ice or snow will expose a decayed fascia very quickly, so fascias and rafter feet should be inspected and repaired before the new gutter goes up.

Whichever method is used, the gutter should be positioned so that its centre line sits beneath the edge of the roof covering, close enough to catch run-off in light rain and far enough out that it is not struck by sliding snow.

Fixing cast iron gutters to stone and masonry eaves

On masonry or stone eaves with no fascia, the gutter is usually carried on rise and fall brackets driven into the joints of the wall, so the fall line and bracket positions need to be agreed with the stonemason before or during the stonework, not after it.

The spike of a drive-in bracket needs a sound bed joint to take it. If the coursing is set out without reference to the gutter, brackets may land on the face of a stone rather than in a joint, forcing either drilling into the stone itself or bracket spacings that no longer suit the gutter lengths. On new stonework the simplest approach is to mark the high point, the low point and the bracket positions on the setting-out drawings, so the mason can leave or rake out joints in the right places.

The threaded stem of a rise and fall bracket allows the height of each support to be adjusted, which is how the fall is achieved on a level course. The usual method is to set the two end brackets first, run a tight string line between them at the chosen fall, and then fix every intermediate bracket to that line.

The same thinking applies to the downpipes. Pipe holdfasts, pipe nails and built-in holderbats should land in joints at sensible heights, and the downpipe line should avoid quoins, window and door dressings, string courses and plinths wherever possible. Where it cannot, the offsets and plinth offsets discussed above need to be identified early. On historic masonry set in lime mortar, fixings should be made in the joints rather than the stone, using lime-compatible methods, so that they can be renewed without damaging the masonry.

Joints, sealant and the direction of lap

Cast iron gutter joints are socket and spigot, made with gutter sealant and a nut and bolt, and they should lap in the direction of fall so that water runs over each joint rather than into it.

In practice that means the spigot end of the upstream length sits inside the socket of the next length downstream. Apply sealant to the socket, bring the lengths together, insert the bolt and tighten until the joint is firm — not so tight that the sealant is squeezed out of the joint. Clean off surplus sealant from the inside of the gutter so that it does not form a dam. Cut ends should be de-burred and touched up with paint before fitting.

Union clips, where used, follow the same principle. Every joint, outlet and angle should be supported by a bracket close to it, as noted above, so that the weight of the gutter is never carried by a joint alone.

New work against historic buildings

Where a new extension meets an existing building, the gutter levels at the junction usually decide the fall for the whole new run, so the existing gutter should be surveyed before the new one is designed.

Older gutters were often laid very close to level, and many have settled over the years. If the new gutter is designed to discharge into the old one, it may add water to a system that already struggles. Wherever possible, we recommend that the new run falls away from the historic building to its own outlet and downpipe, with a stop end at the junction. That keeps the two systems independent and protects the older fabric.

If the new gutter is to read as a continuation of the old, the profile, size and height should match. Where the existing profile is an older moulded section that differs subtly from current patterns, our copy casting service can reproduce it from a sample or measured drawing. Where the host building is listed, the extension and any changes to its rainwater goods will normally fall within the scope of the Listed Building Consent, and the conservation officer may want to see the gutter profile, finish and downpipe positions on the drawings.

Finish matters on mixed old and new work too. Linseed oil paint gives a breathable, low-sheen finish that sits comfortably beside historic paintwork; factory black and primed finishes are also available.

Setting out a cast iron gutter: step by step

Setting out a cast iron gutter means fixing the outlet positions and fall on paper, transferring them to the building, and fixing every bracket to a single line. The sequence below is the one we recommend.

1. Survey the drains and the eaves. Confirm the position of every gully or drain connection, the eaves construction (fascia, rafter feet or masonry) and any existing gutter the new work must meet.
2. Fix the outlet positions. Position outlets over or close to the drains, near corners and so that no run exceeds its capacity. Prefer central outlets on long runs.
3. Check the offset projections. Measure from the wall face to each outlet centre, subtract the wall-to-pipe-centre distance, and confirm a standard offset fits. Adjust the outlet or gutter line if it does not.
4. Decide the fall. Use 1:600 as the default, up to about 1:350 where the eaves detail allows. Calculate the drop over each run from high point to outlet.
5. Set the end brackets. Fix the bracket at the high end and the bracket at the outlet end at the calculated levels, within 150mm of the ends and fittings.
6. Run a string line. Stretch a tight line between the end brackets and fix every intermediate bracket to it at no more than 900–915mm centres, with extra brackets at outlets and angles.
7. Hang and joint the gutter. Place the lengths loosely, check the fall and the outlet positions, then make the socket and spigot joints with sealant and bolts, lapping in the direction of fall.
8. Fit the downpipes and test. Fit the offsets and downpipes, then pour water in at the high end of each run to confirm it drains fully with no standing water or leaks.

Common mistakes in gutter fall, outlets and fixing

Most problems we are asked to solve come from a handful of avoidable errors, almost all of them made before the first bracket goes up.

• Ordering before setting out. Outlets and offsets are fixed castings; ordering them before the drains and wall line are confirmed leads to misaligned downpipes and wasted fittings.
• Laying the gutter dead level. It carries the water but never drains dry, so silt builds up and the casting corrodes from inside.
• Too much fall on a long run. The low end drops below the roof edge and the gutter visibly runs downhill against the building’s horizontal lines.
• Eyeballing intermediate brackets. Small errors at each bracket create flat spots and back-falls. Every bracket should be set to a string or laser line.
• Too few outlets. A long run to a single end outlet is the most common cause of overflowing. An extra outlet usually solves more than a bigger gutter.
• Ignoring corners near outlets. An angle close to an outlet reduces capacity and should be allowed for in the design.
• Brackets that miss the joints. On masonry eaves, coursing set out without reference to the gutter leaves drive-in brackets nowhere sound to go.
• Joints lapped the wrong way. A joint lapped against the fall drives water into the sealant line rather than over it.
• Fixing through the back below the waterline. On a directly fixed G46, fixings below the waterline create a leak path.
• Overloading an old gutter. Discharging a new roof into a historic gutter that is already laid flat or undersized transfers the problem to the older building.

Lead times, site surveys and specification

Standard G46 gutters, fittings and brackets are stock or standard items, so lead times are short; bespoke or copy cast profiles take considerably longer, and on complex sites a chargeable site survey is the surest way to get the setting-out right before anything is ordered.

Stock items are typically despatched within 5 to 7 working days of order, and non-stock standard items within 7 to 10 working days — please confirm current availability when you enquire. Bespoke and copy cast components typically require 12 to 14 weeks from pattern approval, and a linseed oil paint finish adds further time. We state lead times at quotation and will not promise a turnaround we have not confirmed.

Where the drainage layout is uncertain, the eaves detail is unusual, the new work meets a historic building, or access is difficult, we recommend our site survey service. It is a chargeable service, and it exists to protect the project: a surveyed set of outlet positions, projections and fixing details avoids the far greater cost of reworking cast iron on a scaffold. For straightforward work, a marked-up elevation and a few photographs of the eaves and drains are often enough for us to advise.

When writing a specification, we suggest stating the profile and size, the outlet positions and sizes, the fall (for example, ‘laid to a continuous fall of 1:600 towards outlets’), the bracket type and maximum centres, the joint method, and the finish. We are glad to help with specification wording.

Historic building examples

The examples below are illustrative of the setting-out problems we are commonly asked about on historic buildings. They are representative cases rather than descriptions of specific Tuscan Foundry projects.

Georgian terrace, Bath — Grade II

A long front elevation had been drained to a single end outlet, and the original moulded gutter overflowed above the entrance in every heavy storm. Rather than enlarge the gutter and lose the historic profile, a second outlet was introduced at the party wall line with a matching rainwater head. Each run was relaid at 1:600 on refurbished brackets. The overflowing stopped, the cornice line was unchanged, and the scheme required no alteration to the historic profile.

Victorian chapel, Powys — Grade II

The chapel’s eaves are coursed stone with no fascia, and the gutter had been carried on drive-in brackets set wherever a joint happened to fall. Spacings of over a metre had let the gutter sag between supports, holding water. During repointing the mason was given a setting-out drawing of bracket positions and the fall line, joints were raked out accordingly, and new rise and fall brackets were set to a string line at regular centres.

Edwardian villa, Harrogate — conservation area

A new rear extension in matching stone was to meet the original gutter at a re-entrant corner. Survey showed the existing gutter was laid almost level and already near capacity. The new G46 gutter was set to fall away from the historic house to its own outlet and downpipe, with a stop end at the junction, so the older system received no additional water and both could be maintained independently.

Former mill offices, Lancashire — Grade II

Replacement downpipes had been ordered from a measured drawing, but on site the outlets sat further from the wall than the drawing showed and the offsets did not reach. Rather than introduce improvised bends, the outlet positions were remeasured, one outlet was moved along the run to sit over the existing gully, and offsets of the correct standard projection were supplied. Spacer plates took up the final small difference at the wall.

Estate farmhouse, Aberdeenshire — Category B

Rhones on this granite farmhouse were carried on rhone hooks built into the wall head, and two hooks had failed, leaving a back-fall towards the gable. New hooks were set into the joints during wall head repairs, the fall was re-established at the steeper end of the usual range to clear heavy leaf fall from nearby trees, and the joints were remade lapping towards the outlet.

Frequently asked questions

How much fall does a cast iron gutter need?

A cast iron gutter should be laid to a slight, continuous fall of around 1:600, which is about 1.7mm per metre or 17mm over 10 metres. The commonly accepted range runs from 1:600 to a maximum of about 1:350, or 3mm per metre.

Can cast iron gutters be laid level?

They can carry their design flow when laid level, because the standard capacity calculations assume a level gutter, but we do not recommend it. A level gutter never drains dry, so water and silt stand in it and the casting corrodes from the inside. A gentle fall of around 1:600 avoids this.

Is 1:600 or 1:350 the better fall for a gutter?

Use 1:600 as the default. Move towards 1:350 only where the eaves detail allows it and a steeper fall is useful, for example where heavy leaf fall needs clearing. On long runs, adding an outlet is usually better than increasing the fall.

Where should the outlet go on a long gutter run?

On a long run, a central outlet with the gutter falling towards it from both ends is usually best. It roughly doubles the flow the same gutter can carry compared with an end outlet and halves the visible drop at each end.

Does a corner near the outlet reduce gutter capacity?

Yes. Guidance based on BS EN 12056-3 applies a reduction factor of 0.8 where a corner is less than 2 metres from an outlet, and 0.9 where it is between 2 and 4 metres away.

How far can a gutter run before it needs another outlet?

Capacity starts to reduce once the length to an outlet exceeds 50 times the gutter’s depth. For a 125 x 100mm G46 gutter that is around 5 metres. Longer runs are possible, but each additional metre reduces capacity, so a further outlet is often the better answer.

How do I work out which downpipe offset I need?

Measure from the wall face to the centre of the gutter outlet, then subtract the distance from the wall face to the centre of the downpipe. The result is the projection. Choose the standard offset that matches it, and adjust the outlet position or use spacer plates if it falls between sizes.

What sizes do cast iron downpipe offsets come in?

Round cast iron offsets for 65, 75 and 100mm pipes come in projections of 75, 115, 150, 230, 305, 380, 457, 533 and 610mm. Square and rectangular offsets run from 56mm to 610mm, and square and rectangular plinth offsets are available at 75, 115 and 150mm.

How far apart should brackets be on a G46 cast iron gutter?

Fix the first bracket within 150mm of each end, then space brackets at no more than 900 to 915mm centres, with an additional bracket at every outlet and angle. Rise and fall brackets driven into masonry should also be at no more than 915mm centres.

How do you fix a cast iron gutter where there is no fascia board?

Where there is no fascia, use rafter brackets fixed to the rafter feet, or rise and fall drive-in brackets driven into the joints of the masonry. The threaded stem of a rise and fall bracket lets each support be set to the correct height to achieve the fall.

Can a G46 gutter be screwed directly to the fascia?

Yes. The G46 has a flat back and can be screwed directly through it into a sound timber fascia without separate brackets, provided the fixings stay above the waterline so they do not create a leak path.

Which way should cast iron gutter joints lap?

Joints should lap in the direction of fall, with the spigot end of the upstream length sitting inside the socket of the next length downstream. Water then runs over each joint rather than into it.

Should a new extension gutter drain into the old building’s gutter?

Where possible, no. We recommend that the new run falls away from the historic building to its own outlet and downpipe, with a stop end at the junction, so the older system is not overloaded and each can be maintained independently.

Do I need Listed Building Consent to change gutter outlets on a listed building?

Often, yes. Altering the profile, outlet positions or downpipe arrangement on a listed building can affect its character and may need Listed Building Consent. Like-for-like repair usually does not, but check with the local conservation officer before work starts.

What is the lead time for G46 cast iron gutters?

Stock items are typically despatched within 5 to 7 working days and non-stock standard items within 7 to 10 working days, subject to current availability. Bespoke and copy cast components typically take 12 to 14 weeks from pattern approval.

When should I book a site survey for cast iron gutters?

We recommend a site survey where the drainage layout is uncertain, the eaves detail is unusual, new work meets a historic building or access is difficult. It is a chargeable service that fixes outlet positions, projections and fixing details before anything is ordered.

Conclusion

The essentials are simple. Lay cast iron gutters to a slight, even fall of around 1:600, never dead level and rarely steeper than 1:350. Put outlets where the drains are, near corners, and often enough that no run is overloaded — and prefer central outlets on long elevations. Check every outlet against a standard offset projection before ordering. Fix every bracket to a single string line, at no more than 915mm centres, with extra support at fittings, and agree bracket positions with the stonemason wherever the gutter is carried on masonry.

We supply the full G46 moulded gutter range in both sizes, with outlets, angles, stop ends, union clips, fascia, rafter and rise and fall brackets, and the complete range of cast iron gutters and downpipes, in primer, factory black or linseed oil paint. If you would like us to check a layout, confirm capacities for a moulded profile, or help with specification wording, please contact our team or call 0333 987 4452. Where the setting-out is complex, our site survey service will fix the details before anything is cast.