Joist Hangers

Not always – it is project-specific. That being said, every joist does need proper support. 
Joists can be installed by building their ends into the masonry, by bearing on a wall plate or steel beam, or joist hanger. The right choice depends on the structural design

Timber-to-timber joist hangers should be nailed using the fixings specified.  Catnic metal joist hangers are designed to be fixed with 3.75mm sherardised square twist nails, 30mm long for hand nailing or 35mm for nail guns, driven through every pre-punched hole. The hanger's performance depends on using these fixings in these positions.
For timber-to-masonry hangers, the top flange is built into the masonry bed joint as the wall goes up. It doesn't need fixing into the blockwork, but the back plate of the joist hanger must sit flush against the supporting masonry, and a minimum of 675mm of cured masonry must be in position above the joist hanger flanges before any load is applied. The masonry must have a crushing strength of at least 3.5N/mm². 
Check the product datasheet for your hanger or contact Catnic's technical team if you're unsure.

For timber-to-masonry hangers, yes. The hanger's top flange is built into the mortar joint as the wall goes up, so it must be positioned at the correct height during bricklaying. 
For timber-to-timber hangers, you must fix the hanger to the supporting timber first, then drop the joist in and nail it.

Yes, the joist must sit tight into the hanger. 

Yes, when the correct hanger is selected for the joist and the load, and it's installed according to the provided instructions. Joist hangers are a standard, well-established way of supporting timber floors and ceilings. The load capacity of the joist hangers depends on the hanger type and size. And, for timber-to-masonry hangers, the strength of the masonry it's built into. Using the specified fixings in every hole of the joist hanger is also a very important factor in achieving its SWL (safe working load).  Each Catnic hanger's load rating is given on its product page and datasheet, along with the conditions it applies to.

No, joist hangers don't stop damp getting into a wall, but they can help protect your floor timbers from it.
Joists installed on steel hangers are kept clear of the wall, which reduces the risk of penetrating damp. There is also no need for cutting the blockwork around each joist, meaning no gaps that require sealing.

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