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How big?
Backing rod with caulk / silicone is the answer for up to around 50mm
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silicon sealant
Has poor unprimed adhesion to most commonly encountered building substrates.
Backer rod ensures that whatever sealant is used it is bonded on only two sides.
This allows the sealant to cope with any change in the joint width,
normally due to different coefficients of thermal expansion.Any damp material, (plaster, building rubble, etc., between the brick wall and door frame), needs to be removed. IS the door frame only anchored by a couple of screws/wedges?
You may consider filling the bulk of the void with anything you have to hand, (end of a bag of plaster etc.)Soudal Fix All High Tack Adhesive & Sealant, (Toolstation 84704),
has much better unprimed adhesion than any silicone sealant, I have used.A 30mm wide joint will need a bead of sealant at least 15mm deep to give a longlife waterproof seal.
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Go old school with oakum to fill the void, then burnt sand mastic to seal.
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Soul
mespilus
NotThamesWater
@Howard
Product advice pls
Where I’ve got a big gap between brick and wooden door frame what kind of product should I fill it with? Previous just slapped silicon to seal it and that’s fucked the frame and the cill where it’s let water in then trapped it.
Was thinking some kind of filler to back whatever seal that I then put on top.