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How to repair a flat roof

How to repair a flat roof

Posted on 10 March, 2022

When you have a flat roof leak, it can feel really frustrating to repair, especially considering the fact that if it’s not repaired properly then it can negatively impact your property’s value as well as its overall integrity.

Unfortunately, whilst all roof leaks are problematic, a flat roof leak can range from somewhat simple to deal with to incredibly complex, sometimes entire reconstruction is needed if the damage is too severe. 

Finding a leak

If you have a wet patch or leak on the ceiling of an upstairs room of a property, you more than likely have a leak from the flat roof. So the first thing to do if you’ve become aware of a leak from your flat roof is to find the source of the said leak in the felt. The area where the top surface has become compromised won’t directly match where the leak has appeared in the room.

You can start finding the source by inspecting the condition of your flat roof covering, if it looks in a poor state of repair and has deteriorated with splits and tears then it may be advisable to replace the whole roof, as a roof at the end of its lifespan is not economically worthwhile to keep maintaining. 

If the leak or dampness is adjacent to the wall, pitched roof, or abutting the flat roof on the interior then check the first place to check is the flashing. If not, then you need to locate the position directly above the leak or wet patch, after doing this check the slope of the roof for any cracks, splits, or blisters in the material.

For areas covered with chippings, use a brush to move them out of the way to enable a close look at the surface of the flat roof. 

Different types of damage are from different sources, cracks and splits are caused by movement of the roof structure due to heat and blisters are caused by trapped moisture or air when the roof structure moves.

Different flat repairs you can carry out

Precautions to take

No matter what type of repair you’re carrying out, it’s important to respect the height at which you’re working as well as respect ladders to get up there. A good ladder should be well made, hold your weight, and be a suitable height. It’s also important that no matter what repair you’re carrying out, make sure the area has been cleaned thoroughly beforehand of any moss or debris and any signs of fungal growth have been killed with a fungicide. 

Blister repairs

  1. For blisters, use a sharp craft knife to make a cross-cut through the top layer.
  2. Peel back the four segments and dry the area underneath using a heat gun, making sure not to overheat any area of the felt (as it will just melt)
  3. Apply a bitumen adhesive to the exposed surface and fold back the cut segments.
  4. Press down the top layer into the applied adhesive.
  5. Measure and cut out a piece of new felt, ensure it’s big enough to give above a 50mm overlap beyond the blister. 
  6. Apply adhesive over the repair and beyond it to suit the cut-out patch and apply it.
  7. Press patch firmly into place.
  8. Ensure it’s well sealed.

Split and tear repairs

  1. If the split or tear can be lifted without causing additional damage, lift it and clean the space under and around it. 
  2. Fill the cleaned space underneath with bitumen adhesive and fill any visible voids with bitumen compound.
  3. Measure and cut out a piece of new roofing felt, ensure it’s big enough to give above a 50mm overlap beyond the split.
  4. Apply adhesive over the repair and beyond it to suit the cut-out patch and apply it.
  5. Press patch firmly into place.
  6. Ensure it’s well sealed.

Repairing flashing

  1. Clean off the flashing surface.
  2. Cut purchased flashing tape to length and apply it to the required area.
  3. Smooth the tape to the surface, making sure the edges are sealed and that there are no creases.
  4. If you’re fitting a long length, have someone else gold out the far end just off the surface which will allow you to work along the length smoothening it down.
  5. If more than one strip of tape is required to build upwards, start with the lowest strip and use the next strip to overlap it, ensure the overlaps are sealed. 
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