Buying upholstery fabric

[JohnSnug]

I'm still pretty new to sewing, when buying fabric for cushions and/or curtains is there anything I need to look out for. Are some fabric made better than others, and how can I tell if it's a 'bad' or poor quality fabric (if that's a thing!). What do I need to look out for and potentially avoid.

Thanks!

[BrendaP]

It depends a lot on what sort of cushions and what sort of curtains, but generally you get what you pay for.

Upholstery fabrics which are going to be sat on need good resistance to abrasion and to pilling but cushions that are purely decorative don't need to be so robust. 
Measured in Martindale units
https://en.wikipedia.org/wiki/Martindale_(unit)
https://www.jover.es/en/news/201-abrasion-resistance-textiles

Fabrics for window curtains need to have good UV light resistance but that's less important than for internal curtains that are not exposed to sunlight.  Usually a good quality lining is sufficient.

Any fabric used for upholstery in public places - and recommended for private use - must be fire retardant.
My daughter makes curtains professionally, and does some upholstery, and she won't compromise on that one.

If you are using a printed fabric ensure that the print is on-grain widthways.  You have to cut with the design and if it's off curtains won't hang properly.

[JohnSnug]

Thank @BrendaP really useful.

For the printed fabric, is that so the pattern is straight and the fabric hangs straight too? I assume if the print is off grain, you could get it to hang correctly, but at the cost of the patterns being wonky. Have I got that right?

[BrendaP]

@JohnSnug Yes, that right. 

For fabric to hang correctly the warp threads have to be vertical - and the weft threads need to be horrizontal to the ground.  With plain, or vertical stripes you can usually coax it into the correct alignment by pulling hard across a diagonal. 

If it's a directional print you can't do that, the fabric has to be cut with the printed pattern which, if it's printed off grain,  means compromising with the vertical warp threads not being truly vertical.

[Greybird]

@JohnSnug I second everything @BrendaP has said. Beware fabric marked as "seconds". This will often include fabric with the pattern printed off grain. As far as cushions are concerned, be aware that  upholstery and curtain fabric will rarely be washable, so if your cushions are to be in frequent use, it's better to avoid pale colours and those with a white or cream background colour unless they are to be dry cleaned. You can wash the fabric before cutting to pre-shrink it, but often this will result in a change to the texture and "feel" of the fabric.

[Hummingbird]

@JohnSnug some upholstery fabrics can be heavy/thick so not ideal for softer curtains and cushions. The FR treatments on some of them can trigger skin irritation and breathing problems in those with allergies or sensitive skin, especially if you're working with them for a long time on a big project.

There are more interiors fabrics available now that are supplied as cool wash, look for that before purchasing if it's important. But, remember if you're making interlined or handmade curtains they will never be washable, and that linings (and other materials involved) may behave differently to the face fabric (even if all are listed as washable!)