Box Gutter: The Complete UK Guide to Types, Sizing, Installation and Materials in 2026

Box gutters have been the default choice for commercial buildings, industrial units, and contemporary new-build residential projects across the UK for decades. The reason is simple: a rectangular cross-section gutter handles more water than a half-round profile of the same nominal width, it looks cleaner against a flat fascia, and when it is sized correctly it very rarely overflows.

But there is a gap between 'very rarely overflows' and 'never causes problems'. Box gutters fail regularly, and in almost every case the failure traces back to one of a small number of predictable causes. Undersizing. Bracket spacing too wide. Wrong sealant at the joints. Insufficient fall toward the outlet. These are not obscure technical failures. They are the same mistakes that come up again and again across thousands of installations.

At Online Metal Store Ltd in Chelmsford, Essex, we supply aluminium box gutters, downpipes, fascia, and the full coordinated roofline system to contractors, developers, architects, and homeowners across the UK. This guide covers everything you need to know before specifying or installing a box gutter system: what it is, how it works, the standard sizes and what each one handles, how to size a system correctly using the British Standard method, and the installation steps that separate a system that lasts 40 years from one that starts leaking in the first winter.

We are going to be direct about the things that tend to go wrong, because that is more useful than a guide that glosses over the difficulty and leaves you to discover the problems yourself.

What Is a Box Gutter?

A box gutter is a rainwater channel with a rectangular or square cross-section, used to collect and convey water running off a roof to the downpipe drainage system. The word 'box' describes the profile shape: a channel with flat sides, a flat base, and typically vertical front and rear faces, giving a rectangular cross-section when viewed from the end.

This is how it differs from a half-round gutter, which has a semicircular cross-section, and from an ogee gutter, which has a decorative curved front face. Box gutters are angular, flat-faced, and geometrically clean. That suits buildings with flat or low-pitch roofs, steel-frame construction, and contemporary architectural styles considerably better than a half-round or ogee profile.

How a box gutter works

Rainwater running off a roof surface drops onto the gutter channel, collects across the full width of the channel base, and flows under gravity toward one or more outlets connected to a downpipe.

The rectangular channel profile is hydraulically more efficient than a semicircular one of the same nominal width because the flat base and vertical sides keep the full internal depth available across the entire channel width.

In a half-round gutter, the depth tapers to zero at the edges, so a significant portion of the nominal channel width contributes almost nothing to the usable flow area.

In practical terms, a 125mm wide box gutter carries significantly more water than a 125mm half-round gutter. This is why box gutters are the standard choice wherever roof areas are large, rainfall volumes are high, or the gutter needs to be efficient in a restricted space.

Where box gutters are typically used

•      Commercial and industrial buildings: Warehouses, factories, distribution centres, retail units, offices. Large roof areas with high rainfall volumes and an architectural style that suits the angular profile.

•      Steel-frame construction: The clean lines of a box gutter suit steel-frame buildings well. Most commercial steel-frame construction in the UK uses box guttering as standard.

•      Contemporary residential new build: Architect-designed and specification-level new build residential properties where the clean profile of a box gutter coordinates better with the architectural style than a traditional half-round or ogee.

•      Flat-roof extensions: Single-storey rear extensions, garage roofs, outbuildings, garden rooms. Box gutters are practical and proportionate for the shallow fascia detail typical of these structures.

•      Concealed eaves details: When the architectural intent is to conceal the gutter behind a parapet or fascia capping system, the flat top of a box gutter allows it to sit flush with the roof level.

Types of Box Gutter: Profiles and Configurations

Box gutters are not all the same profile. Beyond the standard rectangular cross-section, several variations are used in specific architectural situations.

Standard pressed box gutter

The most widely used form. Pressed from 1.2mm to 2mm aluminium sheet into a rectangular channel profile, supplied in standard lengths with pre-drilled fixing holes on the back face at 750mm centres. One end of each section is formed into a slightly larger joggle joint spigot that accepts the plain end of the next section, providing the standard jointing method for pressed aluminium box gutter systems.

Pressed box gutters are available from 100x75mm up to 200x150mm and beyond in custom sizes. They are the default specification for most commercial and residential applications.

Shaped or bevelled box gutter

A shaped box gutter has a sloping front face rather than a vertical one, typically at 70 to 80 degrees from horizontal. The softer angle creates a more refined roofline appearance than the fully rectangular profile. This variant is popular on contemporary residential architecture where the gutter is visible from below and the specifier wants a slightly architectural rather than purely functional profile.

Valley gutter

A valley gutter is installed in the internal angle where two roof slopes meet, collecting the concentrated water flow from both sides and channelling it down to the drainage system. Valley gutters are wider and shallower than standard box gutters, sized to handle the combined effective roof area of both slopes draining into the valley. They are a specialised form of box gutter and need careful sizing because the concentrated flow from two roof slopes in heavy rain can be substantial.

Parapet gutter

A parapet gutter runs along the inside face of a parapet wall, draining water from the flat roof area behind the parapet through an outlet at the base of the parapet or through a through-wall drainage point. Because a parapet gutter is effectively internal to the building envelope, any failure or overflow routes water into the structure rather than harmlessly over the outside edge. Correct sizing and reliable joint sealing are especially critical for parapet gutters.

Wide sump gutter

On large commercial roofs where very high water volumes need to be managed, wide sump box gutters with large-diameter outlets are used. These are typically fabricated rather than pressed, custom-sized to the drainage design requirements, and installed as part of a co-ordinated roof drainage engineering design.

Box Gutter Materials: Aluminium, Steel, uPVC and Lead

Box gutters are available in several different materials, and the choice affects longevity, fire compliance, colour options, and installation ease. Here is an honest comparison of the main options used in UK construction.

Aluminium box gutter

Aluminium is the dominant material for box gutters on UK commercial buildings and on specification-level residential new build. The combination of light weight, genuine corrosion resistance, any-colour powder coating, and non-combustible fire classification makes it the natural specification choice for a wide range of applications.

•      Non-combustible: Aluminium achieves A2-s1,d0 reaction to fire under BS EN 13501-1. Required for external materials on buildings above 18m under Approved Document B. uPVC and most plastics are classified combustible and are not compliant for these applications.

•      Any RAL colour, batch-matched: Powder-coated aluminium can be specified in any RAL classic colour. When ordered as part of a co-ordinated system (gutter, downpipe, fascia, soffit), all components are batch-coated in the same production run, guaranteeing colour consistency across the whole roofline.

•      40 or more years service life: A properly installed aluminium box gutter with an external-grade powder coat should perform for 40 or more years without replacement. The aluminium itself does not rust; the coating does not fade noticeably for 20 or more years.

•      Light and easy to handle on site: Aluminium at 1.5mm gauge is significantly lighter than galvanised steel, which makes installation faster and reduces the physical difficulty of working at height with long gutter sections.

•      No theft risk: Unlike lead, aluminium has a low scrap value. Lead theft from commercial rooflines is a well-documented problem in the UK that aluminium eliminates entirely.

Online Metal Store supplies aluminium box gutters from our production facility in Chelmsford. Full range details are on our aluminium box gutter product page. Our range coordinates with aluminium fascia boards, soffits, copings, and downpipes for a fully matched roofline system.

Galvanised steel box gutter

Galvanised steel is used extensively on agricultural buildings, industrial units, and profiled sheeting roofs where the gutter is specified as part of the cladding package. It is cost-effective, structurally strong, and well-matched in appearance to galvanised or plastisol-coated profiled sheeting. The limitation is the corrosion risk if the galvanised zinc coating is damaged, and the heavier weight compared to aluminium.

uPVC box gutter

uPVC box gutters are widely used for standard domestic installations where the initial cost is a primary consideration and fire compliance for regulated building heights is not a requirement. For buildings above 18m, uPVC is not compliant under Approved Document B. The colour range is limited compared to powder-coated aluminium, and precise colour matching with other roofline components is more difficult.

Box Gutter Sizes and Hydraulic Sizing: Getting the Calculation Right

Sizing a box gutter correctly is the single most important step in the whole specification process. An undersized gutter will overflow in heavy rain, no matter how well it is installed. An oversized gutter costs more than it needs to and may look out of proportion on smaller structures. Getting it right means working through the hydraulic calculation rather than guessing from what the previous gutter looked like.

Standard size range

Aluminium box gutters are available in a range of standard profiles defined by their external width and internal depth. Common UK standard sizes include:

The effective roof area figures above are approximate and based on South East England rainfall intensity with a single end outlet. They are planning guides only. For any commercial project or any project where overflow could cause property damage, carry out a formal BS EN 12056-3 hydraulic sizing calculation using the actual roof area, pitch multiplier, and local rainfall intensity data for the specific site.

The BS EN 12056-3 sizing method

BS EN 12056-3:2000 is the British Standard governing roof drainage design. The sizing method it sets out involves three main steps:

1.    Calculate the effective roof area: The plan area of the roof section draining to the gutter, multiplied by a pitch multiplier to account for the increased surface area of a sloped roof. A flat roof has a multiplier of 1.0. A 30-degree pitch has a multiplier of approximately 1.15. A 45-degree pitch has a multiplier of approximately 1.41.

2.    Find the design rainfall intensity: UK design rainfall for gutter sizing is based on a 2-minute storm event with a 1-year return period. For the South East of England, this is typically 0.022 litres per second per square metre. Scotland and the north and west of England have higher intensities. Site-specific rainfall data is available from the Met Office.

3.    Calculate the required flow rate: Multiply the effective roof area by the design rainfall intensity to get the required flow capacity in litres per second. Match this against the published flow data for the gutter sizes you are considering. The selected gutter must have a rated flow capacity at least equal to the calculated required flow.

Example: A 200 m2 effective roof area in Essex at 0.022 l/s/m2 requires a flow capacity of 200 x 0.022 = 4.4 l/s. From the table above, a 150x115mm box gutter at 4.6 l/s provides the required capacity. The 150x100mm at 3.8 l/s would be marginal and should not be used.

Using a centre outlet rather than a single end outlet effectively doubles the drainage capacity for the same gutter profile, because each half of the run only needs to carry flow from its own section of roof. For long runs where a larger profile would otherwise be required, a centre outlet arrangement can allow a smaller (and less visually dominant) profile to be used.

Thermal expansion and expansion gaps

Aluminium expands by approximately 0.000023mm per millimetre per degree Celsius. Over the temperature range a UK roofline experiences (roughly minus 5 degrees in winter to plus 40 degrees on a south-facing fascia in direct summer sun), a 3m section of aluminium box gutter moves by approximately 3.1mm.

This is why a 3 to 4mm expansion gap must be left at every joggle joint between adjacent gutter sections. If sections are pushed up hard against each other with no gap, the system will buckle in summer as the aluminium expands. This is one of the most common and entirely avoidable installation failures on aluminium box gutter systems.

Box Gutter Installation: Step-by-Step Guide

This guide covers the installation of a standard pressed aluminium box gutter on a fascia-fixed residential or commercial application. All work at height must comply with the HSE guidance on working at height. For most roofline work, a scaffold tower or appropriately secured ladder with standoff is required.

What you will need

•      Aluminium box gutter sections in the specified profile and length

•      Fascia brackets at 750mm maximum centres (additional bracket within 375mm of every joint and fitting)

•      Running outlets positioned over the downpipe connection points

•      Stop ends for gutter terminations

•      90-degree angle fittings for corners (do not try to cut and mitre on site)

•      M6 x 20mm aluminium bolts, neoprene washers, and nuts for joggle joints

•      Low modulus silicone sealant (Arbosil 1096 or equivalent)

•      Stainless steel countersunk screws M5 x 25mm for direct-fix applications

•      Acetone and clean cloths for surface preparation

•      Spirit level and chalk line

•      Tin snips or fine-tooth aluminium cutting disc for cutting to length

Step 1: Survey the fascia and check its condition

Before any brackets go up, check the condition of the fascia boards. They need to be sound enough to carry the gutter and the water load in heavy rain. Probe any suspect sections: soft or spongy timber means rot, and that needs addressing before new guttering goes on. A gutter fixed to a rotted fascia will fail within a couple of winters regardless of how good the gutter system is.

On replacement jobs, check whether the old gutter was the correct size for the roof area. If it was overflowing, the replacement needs to be correctly sized, not just like-for-like.

Step 2: Snap the chalk line and set the fall

Snap a chalk line along the fascia to mark the top fixing line of the brackets. Set the fall toward the outlet at 1 in 350 minimum. A fall of 1 in 200 or steeper is better: it clears debris faster and reduces standing water between rain events. Check the starting height at the high end gives adequate clearance under the bottom tile course or roof edge membrane.

On long runs with a single end outlet, the fall over the full length can create a significant height difference. If the gutter needs to sit higher at the non-outlet end than the fascia depth allows, a centre outlet arrangement may be the practical solution: each half of the run falls toward the centre, keeping both ends at a sensible height.

Step 3: Mark and fix the brackets

Mark bracket positions at 750mm centres along the chalk line. Add extra bracket positions within 375mm of every planned gutter joint position, outlet, and angle fitting. Fix brackets to the fascia using stainless steel screws. All fixings into aluminium gutter systems must be stainless steel: zinc-plated or galvanised screws will cause bimetallic corrosion at the fixing point within a few years, leaving rust staining on the fascia and progressively weakening the connection.

Fix the outlet bracket first, since this position is fixed by the drain connection below. Set the outlet bracket to the chalk line height and work outward from there toward the high ends.

Step 4: Fit the stop end and first gutter section

Fit the stop end to the first section of gutter before offering it up to the brackets. Apply sealant to the interior face of the stop end, fit flush with the gutter end, and secure with the supplied fixings. Offer the gutter section up to the brackets and clip or screw it in position.

Step 5: Joint the gutter sections correctly

This is the step that most leaking box gutter installations get wrong. The correct procedure for a pressed aluminium joggle joint is:

1.    Clean both mating faces thoroughly with acetone on a clean dry cloth. Allow to dry. Any contamination at all will prevent the sealant bonding correctly.

2.    Apply two continuous 6mm beads of Arbosil 1096 Low Modulus silicone sealant along the plain end of the gutter, parallel to the length and clear of the bolt holes.

3.    Slide the joggle spigot end of the adjacent section over the prepared plain end. Leave a 3 to 4mm expansion gap between the butt ends. Do not push sections up tight against each other.

4.    Insert M6 x 20mm aluminium bolts through the pre-drilled holes in the joggle, fit neoprene washers on both sides, and tighten nuts until a small bead of sealant is visible around the joint edge. Apply a small bead of sealant around each bolt head.

Do not use standard DIY-store silicone sealant for this application. High-modulus silicone cracks at gutter joints within one to two seasons as the aluminium cycles through thermal movement. Low modulus sealant stays flexible throughout the service life of the gutter.

Step 6: Fit angle fittings at corners

Use factory-formed 90-degree angle fittings at all corners. Do not try to cut and mitre gutter sections on site: a site-cut mitre joint is harder to seal, more likely to leak, and looks noticeably rougher than a factory fitting. Apply sealant at both legs of the angle fitting in the same way as a standard joint, and remember the additional bracket within 375mm on each side of the fitting.

Step 7: Install the downpipes

Once the gutter run is complete, install the aluminium downpipes from the outlet to the drain connection. Fix downpipe brackets at 1500mm maximum centres. Use matching offset bends to bring the downpipe in against the building face if the outlet projects forward of the wall. Leave expansion gaps at each downpipe joint and apply sealant.

Step 8: Test the system with water

Before finishing, run a hose into the gutter at the highest point of the run. Confirm: water flows toward the outlet without pooling; all joints are watertight; the outlet and downpipe convey water away without backing up; the downpipe discharges correctly at the drain connection. If a joint is leaking during the test, let the area dry fully before removing the bolt, cleaning all old sealant away, and reapplying correctly.

Annual maintenance: clear debris from the gutter channel each autumn after leaf fall. Check that outlet screens, if fitted, are clear. Inspect joints on older installations for any sign of sealant degradation. An aluminium box gutter kept clear of debris will perform reliably for 40 or more years.

Common Box Gutter Failures and How to Avoid Them

Most box gutter problems in the UK come from a short list of recurring causes. If you understand them, you can avoid them on new installations and identify them quickly when inspecting a failing system.

Insufficient fall toward the outlet

A box gutter that is level or running slightly away from the outlet will have areas of permanent standing water. Standing water accelerates sealant degradation at joints, encourages algae and debris accumulation, and means that in the first heavy rain after a dry period, the debris that has dried out gets washed into the outlet and blocks it. Minimum fall is 1 in 350 toward the outlet; 1 in 200 is better.

Bracket spacing too wide

750mm is the maximum. On many poorly installed systems, brackets are at 1000mm or more, which produces visible sag between brackets when the gutter is full of water. Sag in the fall direction means the gutter may actually slope away from the outlet in the sagged section, creating a low point that never drains. Additional brackets within 375mm of every joint and fitting are not optional.

Missing or wrong expansion gaps

Sections installed hard against each other with no expansion gap will buckle in summer heat. Sections installed with excessive gaps that were then filled with sealant that cannot accommodate the movement will develop cracked sealant and leaking joints. The correct gap is 3 to 4mm, left as a physical gap, accommodated by flexible low modulus sealant at the joint.

Wrong sealant type

Using high-modulus silicone, grab adhesive, or any sealant that does not accommodate repeated thermal movement at box gutter joints is one of the most common causes of leaks on systems that were correctly designed and sized. Always specify low modulus silicone (Arbosil 1096 is the industry standard). Arbosil or equivalent. Nothing else.

Steel fixings against aluminium

Zinc-plated or galvanised steel screws in contact with aluminium in the presence of moisture set up bimetallic corrosion at the contact point. Within a few years the fixing shows rust staining on the surrounding aluminium and the fixing itself starts to corrode and weaken. Always use stainless steel fixings (A2 grade minimum, A4 for coastal or highly exposed sites) with aluminium box gutter systems.

Ignoring the fascia condition before fitting new gutters

Fitting new box gutters to a rotted or inadequately fixed fascia is extremely common and extremely predictable in its outcome. The gutter is only as secure as what it is fixed to. Replace any damaged fascia sections before fitting new guttering. This is the step that most budget-minded customers want to skip, and the one that is most reliably regretted.

Design Uses: Box Gutters Across Different Building Types

Commercial and industrial buildings

The majority of UK box gutter installations are on commercial and industrial buildings. Steel-frame warehouses, distribution centres, retail units, office buildings, and industrial factories all typically specify aluminium box guttering as the default, co-ordinated with the cladding and fascia package.

On these buildings, the gutter, downpipe, fascia, and soffit are specified together as a co-ordinated system in matching powder-coated colours. Online Metal Store supplies the full co-ordinated aluminium roofline system for commercial projects, including aluminium box gutters, round and rectangular downpipes, aluminium fascia boards in Types 1 to 4, and vented aluminium soffit boards, all available in batch-matched RAL colours.

Residential new build and renovation

On specification-level new build residential, box gutters are increasingly specified as a contemporary alternative to half-round. In anthracite (RAL 7016) or jet black (RAL 9005), a box gutter co-ordinates cleanly with dark window frames, charcoal render, and the overall architectural palette of modern UK residential construction.

On Victorian and Edwardian renovation projects where the property has an ogee or half-round gutter system, replacing with a box gutter can look out of character. In these situations, either retaining the existing profile in aluminium or specifying an aluminium ogee profile is usually the better design decision.

Flat-roof extensions

Box gutters are the standard choice for flat-roof rear and side extensions. For these applications, the gutter is typically installed at the very base of the fascia board, with the overall roofline depth kept as shallow as possible to maintain maximum head clearance at doorways and windows below.

A 100x75mm or 115x75mm box gutter handles the relatively modest roof areas typical of domestic extensions. For larger extensions or extensions in high-rainfall areas, move up to the 125x100mm profile. The aluminium drip trim and roof flashing range from Online Metal Store co-ordinates with our box gutter system for a complete flat-roof edge detail.

High-rise residential and fire compliance

On UK residential buildings above 18m, all external materials including rainwater goods must meet A1 or A2-s1,d0 fire classification under BS EN 13501-1. Aluminium box gutters and downpipes achieve A2-s1,d0 as standard. uPVC rainwater goods are classified as combustible (Class D) and are not appropriate for use on regulated high-rise buildings.

For the regulatory context, see Approved Document B (fire safety) on gov.uk.

Box Gutters from Online Metal Store Ltd

Online Metal Store Ltd supplies aluminium box gutters, downpipes, and a full co-ordinated roofline system from our production facility in Chelmsford, Essex. We supply trade and retail customers across the UK with fast lead times and nationwide delivery direct from the factory to site.

Our aluminium box gutter range covers standard profiles from 100x75mm to 200x150mm and bespoke sizes for projects with unusual drainage requirements. All products are powder-coated in any RAL classic colour, batch-matched with associated fascia, soffit, and downpipe components ordered at the same time.

Full co-ordinated roofline system

Beyond box gutters and downpipes, Online Metal Store supplies the complete aluminium roofline system:

•      Aluminium box gutter range: Standard profiles 100x75mm to 200x150mm. Any RAL colour. 1m, 2m, and 3m lengths.

•      Aluminium fascia boards: Types 1 to 4 in 2.5m and 3m lengths. From £49.85 per length.

•      Aluminium soffit boards: Solid and vented, any width. Any RAL colour.

•      Aluminium copings: For parapet walls and flat roof details. Sloping and flat profiles in 2mm and 3mm gauge.

•      Aluminium roof flashing and drip trim: For flat roof edge terminations and wall abutment details.

For technical advice on sizing, specification, or installation details, contact our team at Sales@OnlineMetalStore.co.uk or call 07907 239290.

When ordering a box gutter system from Online Metal Store, provide the full roof area calculation, the local rainfall intensity for your site, and the outlet positions. We can verify the gutter profile selection against the BS EN 12056-3 sizing method before your order is placed, at no extra cost.

2026 UK Box Gutter Cost Guide

Indicative supply-only costs are listed below for planning purposes. Installation costs vary with roof complexity, access, and regional labour rates.

Frequently Asked Questions About Box Gutters

What is the difference between a box gutter and a half-round gutter?

A box gutter has a rectangular cross-section with flat sides, a flat base, and typically vertical front and rear faces. A half-round gutter has a semicircular cross-section. The box profile carries more water than a half-round of the same nominal width because the full internal depth is maintained across the whole channel width. Half-round gutters taper to near-zero depth at the edges, so a significant portion of their width contributes little to the usable flow area. Box gutters suit contemporary and commercial architecture; half-round suits traditional and heritage styles.

What size box gutter do I need?

The correct size depends on the effective roof area draining to the gutter and the local design rainfall intensity, calculated using the method in BS EN 12056-3. For South East England (0.022 l/s per m2 design intensity), a 100x75mm box gutter handles up to about 60 m2. A 125x100mm handles up to about 130 m2. A 200x150mm handles up to about 410 m2. These are approximate planning figures for single-outlet systems. For commercial projects, always carry out a formal calculation. Online Metal Store can help verify the sizing before your order is placed.

How far apart should box gutter brackets be?

Maximum 750mm centres. An additional bracket must be placed within 375mm of every gutter joint, outlet, and angle fitting. In high-wind or snow-load locations, reduce to 600mm. Too-wide bracket spacing is one of the most common causes of visible sag in box gutter installations, which in turn creates low points that trap debris and can reverse the fall toward the outlet.

What expansion gap should I leave between sections?

Leave a 3 to 4mm physical gap between the butt ends at every joggle joint. Aluminium expands approximately 1.4mm per metre over a typical UK temperature range. Without expansion gaps, the system will buckle in summer. This gap is accommodated by the flexible low modulus sealant at the joint, which remains watertight while allowing the movement.

Are aluminium box gutters fire rated?

Aluminium achieves A2-s1,d0 reaction to fire classification under BS EN 13501-1. This is the non-combustible classification required for external materials on residential buildings above 18m under Approved Document B. uPVC box gutters are classified as combustible (Class D) and are not appropriate for regulated high-rise buildings.

How long do aluminium box gutters last?

A quality powder-coated aluminium box gutter, correctly installed and maintained with an annual debris clear-out, should perform for 40 or more years. The aluminium does not rust. The external-grade polyester powder coat is rated for 20 or more years without significant fading or peeling.

Can I use any silicone sealant for box gutter joints?

No. You must use a low modulus silicone sealant specified for aluminium rainwater systems. Arbosil 1096 is the industry standard. High-modulus silicone cracks at gutter joints within one to two seasons as the aluminium goes through repeated thermal expansion and contraction cycles. Standard DIY silicone is almost always high-modulus. Check the product specification before use.

Can I fit box gutters myself?

Yes for a single-storey domestic installation with appropriate safe access equipment. The jointing procedure requires careful preparation and the correct sealant. For two-storey or commercial buildings, scaffolding is typically required under HSE working at height regulations. If you are unsure about working at height safely, engage a qualified roofline contractor.

What is the correct fall for a box gutter?

Minimum 1 in 350 toward the outlet. A fall of 1 in 200 is better practice. Steeper falls clear debris faster, reduce standing water, and ensure the system self-drains effectively between rain events. On long runs, the height difference between the high end and the outlet can become significant: a centre outlet arrangement keeps the maximum height difference manageable.