How to install gutters on a flat roof

Most modern flat roofs are covered by a membrane that prevents water leaking into your property. But if your roof has a slight slope it will need gutters to ensure proper drainage. Here are three steps to take before gutters should be installed on a flat roof…

1 Ensure there is proper drainage

Flat roof gutters installationIf a flat roof is well constructed, rainwater will be able to drain quickly without it forming ponds that lead to a build-up of water.

This is of particular importance when outside temperatures dip below 0C and it is cold enough to freeze any standing water.

This means the roof needs to be built with the necessary drop that lets the rainwater drain off the roof and into the gutter outlets.

You may find that a small amount of pooling will occur but this shouldn’t affect the overall performance of the roof covering.

2 Choose an appropriate design

A flat roof drainage system can include gutters, downspouts and scuppers – openings through the perimeter walls that let water to drain into the exterior downspout. Good roofing practice includes primary and secondary scuppers that act in an emergency.

Metal flashing can be used if there’s a steep roof above the flat one, the top of the metal being overlaid with the tiles. You may want to install roof drains with strainers or flexible sleeves to catch any debris.

3 Select the best materials

You can opt for gutters made from galvanised steel, copper, aluminium or vinyl. Galvanised steel gutters need to be soldered together so you may need professional gutter technicians if you don’t have the right tools and materials to get the job done. They are prone to rust and need to be painted periodically, but are basically a good option.

Aluminium gutters are available in a variety of colours, and they don’t rust!

Vinyl gutters are easy to self-install and are also impervious to rust and rot. However they can warp in extreme weather conditions and be prone to splitting.

Copper gutters will last for years without needing any maintenance except for keeping then free-flowing.

Seamless gutters are an ideal solution as they don’t leak. However, you’ll need a contractor to help you because they’re custom made on a portable machine.

Flat roof gutter installation step-by-step guide

Measure the roof and decide where you’re going to position your downspouts. Cut the gutters using a saw, but remember to wear protective clothing to avoid any injuries.

Next, place the gutters together on the ground with an overlap of about 2in.

Drill about six evenly spaced holes at each overlap – make them 1/8 inch and drill a fraction away from the edge. Put in rivets and secure with a rivet gun. Then, using a caulk gun, seal the underside of each gutter section with caulk. Cut the downspout outlet holes with tin snips and line with gutter sealant.

It’s a good idea to use string to line up the gutters as you attach them. Measure the length of the gutter and calculate the slope. This is normally ¼ inch allowance for every 10 feet of gutter. Mark the line with a pencil and fix the gutter to the sloped line. Use hook hangers and screw them into the fascia boards, every 18 inches or so.

Install the downspouts into the outlets and attach them to the side of the property with hangers. Finally, seal with gutter sealant for a waterproof fit. Aluminium mounting straps will add extra security.

Prevent weathering problems

If you’ve got cavity walls you’ll need to fit cavity trays with weep holes. To prevent water seepage you’ll also need to fit upstands and waterproof membranes. Flashing will provide a good, waterproof joint when it covers the membrane.

Lead can be used to overlap the horizontal mortar joints and the top edge of the roofing materials. When you have all this in place your drainage system should be fully functional.

Keeping your gutters clean is essential for maintaining a functional drainage system. Blockages and obstructions will cause the gutters to back up with water and then your problems will start.

The weight of the pooling water may pull the gutters away from the fascia boards, and overflowing water will quickly damage brickwork, rot window frames and destabilise the foundations.

Don’t forget, you can get your gutters cleaned on a regular basis by hiring a specialist company.