here's an effective way to carry out gutter union joint repairs
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you'll need roof repair compound and weed control fabric. do not use bitumen, it is cheaper but it is not the best product as it is not sticky enough.
a ladder stand off is needed
for a gap like a window just strap on a piece of wood as shown
cut the fabric to fit the gutter joint
clean the joint, plain water is just fine
previous pointless sealing with silicon
joint clean and dry.
keep the sealant in a bucket to prevent spillage
joint clean and ready for sealant application
apply the sealant generously, wear gloves as it is very very sticky
then apply the piece of fabric pressing it down, the sealant must penetrate the fabric
then apply another coat of sealant on the top. job done.
this method is also useful to repair rips in a poly tunnel, you need to apply the garden fabric on both sides (inside and outside)
as shown earlier, it does a good job.