Green roof problems and how to solve them

Green roofs are dynamic, living things so they can sometimes do unexpected things.  Here are some common green roof problems and some suggestions how to solve them.

Grass on a sedum roof

In cool, moist weather, it’s not unusual to see patches of grass growing amongst the sedum plants. This tends to happen on newly installed sedum roofs whilst the vegetation layer is being watered regularly to establish it.

Grass is normally an indication that a sedum roof is getting more water than it really needs. Firstly, check that all drainage outlets are clear and excess rainwater can escape easily from the roof.

Hand weeding will remove unwanted plants but the easiest thing to do is wait for summer. A long period of drought will kill the grass plants.  When they’re brown, crispy and dormant remove them from the roof so that they can’t regenerate.

Moss on a sedum roof

Moss is a sign that conditions on the roof are not quite right for sedum plants.  It could be too damp, too shady or low in plant nutrients.

As with grass on the roof, check that drainage outlets are working properly before beginning any remedial work.

If the problem is shade from an overhanging tree, prune some of the branches to let in more light.  If the roof is shaded by other buildings then you may need to accept that it’s going to grow moss instead of sedums.   This is something that should have been considered at the design stage.

 

Grass growing in sedum matting often indicates that a roof has been watered a bit too well

On a roof with a deep substrate layer – 5cm or more – re-visit the planting plan and introduce shade tolerant plants to keep the moss at bay.

If moss is appearing on a green roof that is not shaded, then the most likely problem is that the sedum plants have been weakened by hunger.  Apply a specialist green roof feed in spring or early summer and the problem should resolve itself.

You may need to repeat the feeding 6-8 weeks later and if the sedums still look sparse, apply cuttings to thicken up the sward.

Red foliage

Red foliage on a sedum roof is entirely normal in winter time and during very hot dry weather.  If the roof doesn’t turn green again in spring or autumn then your roof is either hungry or thirsty – or both.

Red-coloured foliage on a living roof is not always a problem. Here, however the red foliage is interspersed with bald patches indicating a lack of nutrients or water – or both

If the leaves look wrinkled and baggy like a deflated balloon then they need water.  If they’re turgid and look full of water, they’re OK and just waiting for better weather.

Sparse foliage

Bald patches on a sedum roof indicate that the plant layer has been stressed for quite some time.  Is there enough sunlight on this part of the roof?  Are you up to date with your feeding program?  Is it too dry?

Poor drainage

If you have puddles that sit on your green roof for more than a day or so, you have a problem with drainage.  Likewise if you have large, roughly circular areas where sedum plants won’t grow, excess water probably isn’t leaving the roof quick enough.

Check drainage outlets very carefully and if that doesn’t fix it, then contact the installers for advice.  The structure of the roof should not be bowed or uneven enough to cause this sort of problem – not under any circumstances!