Get answers to the most commonly asked questions here – these FAQS are designed to provide a better understanding of metal gutters and their various applications.
If you cannot find the answer to your question here, please contact MGMA on 01633 891584 or via the helpline.
Q1 Should I use traditional or contemporary products?
Generally the style of an eaves rainwater system is governed by the style of the building, but it is up to the specifier to decide. Traditional styles are available in cast iron conforming to BS 460 or aluminium to BS 8530. Contemporary hybrid traditional styles with modern jointing methods are also available in aluminium and steel.
Q2 Doesn’t aluminium require regular maintenance?
Only periodic maintenance is required in accordance with good maintenance procedures. Polyester-coated aluminium requires only a warm soapy water solution to bring back the original appearance. Twice a year (once in the autumn and again in the spring) we recommend that gutters and joint integrity are checked and that any collected gutter debris is removed.
Q3 Doesn’t aluminium corrode when left unprotected?
The correct grade of uncoated aluminium will resist corrosion when exposed to the environment. Like copper that turns green, the surface of exposed aluminium will oxidize becoming a dull grey colour and this patina protects the aluminium from further corrosion. However, low grade aluminium will corrode, become pitted, particularly in a marine environment and coatings will only give limited protection. Furthermore, unprotected aluminium must not come into contact with electrostatically non compatible materials and fixings or rainwater ‘run off’ from material such as copper or even cedar shingles. If in doubt, seek advice from an MGMA member.
Q4 I am currently working on an Edwardian project that requires the cast gutter to be replaced. The existing gutter has patterns embossed on the front of it, can you manufacture this?
MGMA members who specialise in cast iron and pressed aluminium rainwater systems can provide both standard ranges and bespoke gutter styles, so if the type of gutter you have is not available as a standard product a copy of the original can be made however, this will be subject to a cost premium.
Q5 I am currently working on a large Victorian house, which rainwater system should I use?
Victorian and Edwardian buildings generally adorned the building facades with decorative gutters and rainwater pipes. The two favourites are Moulded Ogee No 46 gutter profile with square or rectangular pipes with large eared joint sockets, or Victorian Ogee gutters with round pipes with large eared sockets. Both products are available in cast iron conforming to BS 460 or aluminium to BS 8530. MGMA rainwater specialists will also be able to advise the correct size of gutter/downpipe combination.
Q6 I am confused about the rainfall intensity to be used. I have always specified 75mm/hr, is this correct?
BS EN 12056, the current roof standard, requires that you use a one-year event for all external gutters, which means that the rainfall intensity varies depending on where you are in the country. For example, a project in the north of Scotland will have a rainfall intensity less than half that used in the South East of England. Please refer to MGMA Guidance Documents GD03 and GD21 for more information.
Q7 The powder coating is coming off my cast aluminium gutters after only one year. They are not near the coast, but next to a main road. Could you explain why this is happening?
If paint is delaminating then it is either not architectural grade paint, has not been correctly applied or the aluminium substrate is not of a marine grade. It can be anyone of these or a combination of two, or even all three.
All paints will have a marginally shortened life expectancy in terms of colour and gloss retention when subject to aggressive atmospheric conditions and/or attack by salt spray. However, if the cast aluminium is marine grade and the polyester powder paint is an architectural grade applied in accordance with BS EN 12206-1:2004, then it should not come off and certainly not after one year.
Damage caused to the powder coating by installers must be repaired; the method of repair will depend on the paint finish and the manufacturer must be consulted for advice.
Q8 Should I fit balloon leaf guards or gutter guard type products?
We do not recommend the fitting of any leaf guard products, as they can have a detrimental effect on performance, especially when blocked with leaves.
BS EN 12056 calls for a 50% reduction in design capacity if balloon leaf guards are fitted.
Q9 I am currently installing a cast aluminium gutter system, what should I use to seal the joints?
We would recommend that the gutter joint be cleaned thoroughly and that a low/medium modulus neutral cure silicone sealant is used, together with aluminium nuts, bolts and washers.
MGMA gutter manufacturers generally publish their own joint details and should be consulted prior to installation.
Q10 The contract that I am currently working on is in an area prone to vandalism. Do you have a system that can help?
Gutters generally only become damaged if the rainwater pipes are climbable. Therefore, ensure the pipe is a non-climbable pipe with a flat back fixed back against the wall with concealed supports plates/fixings. This will ensure there is no gap behind the pipe enabling the pipe to be gripped by hand and climbed. The pipe must also not have a shoe at the base as this provides a means by which the end of the pipe can be levered away from the wall. The base on the pipe should be firmly grouted into an appropriate drain gully connection. A further consideration is to increase the thickness of the pipe material for the bottom length only, to protect against surface denting etc.
In answer to the question of pipes being vandalised; pipe systems in both square and diameter are available for anti-climb and anti-vandal situations. Pipes are extruded with rear legs to the profiles; these hide the fixing plates and fit tight back to the building’s facade. This makes them difficult to climb and difficult to remove from the building. These systems are approved for prisons, hospitals, schools and any other vulnerable buildings that require a secure pipe system.
Q11 I am currently working on a commercial project and I am limited to the amount of rainwater pipe drops on the scheme. Do you have a gutter section that would be suitable?
The size of the gutter will be determined by the flow capacity from the roof and relative to the number of pipes available. All MGMA members provide a complimentary gutter design service and will be able to advise on the correct size of gutter and pipes required.
If downpipe positions are restricted then it is possible to increase the system size of both gutter and pipe. It is also possible to produce sumped or tapered outlets. Two outlets side by side fed into a hopper and pipe – all these offer a means to increase the flow capacity of a system.
Q12 I am currently working on a commercial project, which rainwater system should I use?There are basically two rainwater systems that that can be considered. An eaves gutter fixed to the external perimeter of the building, so that should it overflow, water will not enter the building. The size of such a gutter system will be determined by the design criteria in accordance with EN 12056. However, if this building is to have an ‘internal gutter system’; then the gutter may have either a gravity pipe system, or a siphonic pipe system. Although the gutter system to both would be the same, the pipe systems are completely different.
The design of a gravity system can be carried out by any of the MGMA members in accordance with EN 12056; however a siphonic system need to be designed by an accredited specialist siphonic drainage company. Please refer to MGMA Guidance Document GD06 for more information.
Pressed gutters are usually specified for their easily variable size and capacity – these are pressed from various thicknesses of material in steel or aluminium, dependent on size.
Q13 Should external rainwater pipe joints be sealed or not?
There are two schools of thought regarding this subject and neither is wrong. Traditionally, spigot and socket pipe joints were left unsealed. Plumbers will say that the reason for this is that should the underground drain block, the unsealed pipe joints allow the water run out rather than building up to the level of the gutter causing the gutter to overflow and water to cascade down the walls of the building.
However, pipes with fully sealed joints are more efficient, because when the pipe is trying to run full bore, it sucks air into the pipe via the open joints, creating turbulence within the pipe which reduces the flow of water. If joints are sealed, a siphonic affect is created which accelerates the flow of water through the pipe increasing the flow of water through the gutter outlet.
However, regardless which option is adopted, the pipe joint to the gutter outlet should always be sealed as this improves the flow of water though the outlet.
Q14 Do all gutters have to be installed to a “fall” or can they be installed level?
Small domestic gutters are generally installed to falls of 1:600 to enhance the flow as the gutters are generally quite shallow. For example, a 112 mm half round gutter fitted to falls will achieve a flow rate of 1.27 litres per second compared to 1.06 litres per second if installed level.
However, gutters that form a decorative feature to a building are generally larger and deeper, hence capable of accumulating the rainwater ‘run off’ and are therefore installed nominally level, ensuring the aesthetic horizontal lines of the building façade are maintained.
Q15 How long can a gutter run be, before an outlet is required?
This will depend on the size (capacity) of the gutter relative to the maximum anticipated flow from the roof area. If the gutter capacity corresponds to the design flow rate from the roof; the maximum gutter length from either the end of a gutter run or the highest point, must be no more than 50 x the depth of gutter for example, a gutter 50mm deep x 50 = 2.5 metres. Thereafter additional factors such as frictional resistances for corners etc will proportionally reduce that length. Placing a gutter outlet centrally within a gutter run will drain twice as much water as the same length of gutter with an outlet at one end of the gutter run. We recommend that you consult an MGMA member for advice.
Q16 Do metal gutters expand and contract like PVC?
Yes, all metal gutters expand and contract and the amount of thermal movement will depend on the metal used, but metal does not expand as much as PVC systems. All correctly designed metal gutter systems will have features generally at each joint to allow for thermal movement without breaking the joint seal. We recommend that you follow the individual installation instructions published by each MGMA member.
Q17 How can I protect my gutter system from damage caused by sliding snow?
The best solution is to ensure the gutter is fitted lower to allow the snow to slide off the roof. In countries where heavy snow fall is frequent that is precisely how gutters are installed. With roofs now being heavily insulated, snow build up is becoming more frequent. Snow guards are normally installed on roofs to protect pedestrians or lower level structures.
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