How to Make Perfect HST Blocks
Modern quilting with layer cake HSTs is a fun and challenging project. Do you use Half Square Triangle (HST) blocks in your quilts? More importantly do your HSTs sometimes miss their mark with a snubbed point or two? They are an incredibly versatile block and always result in a wonderful quilt design.
But they are triangles and triangles are fussy to work with. I tended to stay away from them for that reason. Fortunately, I stuck with it and found some answers to a couple common problems.
All About the Points
Essentially, a good looking HST quilt comes down to the points. It’s not as simple as you’d think. With accurate cutting and the right seam allowance, it should be easy. That’s what works on other quilts, so why not on HSTs?
There are two primary factors to consider when making half square triangles. First is a consistent seam allowance and second is accurate cutting.
Also remember a big difference is that you are working with triangles. That means you are cutting and sewing on the bias, which can get messy without a few precautions. Here are two tricks that work great to get perfect HST blocks that fit together accurately. Focusing on these two steps as you’re sewing your blocks will help to make your HSTs fit together so much better.
Cutting & Seam Allowances for Perfect HST Points
My recent HST quilts taught me some important lessons. My Fat Quarter Chevron quilt was made with all the same size HSTs. They were very large blocks that look awesome. The Off-Set Star quilt combined two HST sizes which became more intricate to put together. What I learned is that consistent block sizes matter, therefore accurate seam allowances are important. No surprise that going back to the basics helps me to clarify where my problem lies. But I found it goes even a bit deeper than that.
The best way to reduce how much you need to trim is to eliminate the variances that cause HSTs to be unequal in size. Here’s how I worked around this challenge.
HST Seam Allowances
How to Adjust HST Seam Allowances
As a lifelong sewer, I understand consistent seam allowances are important. But quilt assembly is unique for two different reasons. First quilting uses only a ¼” seam allowance. Plus, the seam allowances are pressed to one side. Why does this matter?
You know that every time we add a seam to a block, we need to increase the size we cut that block to accommodate the required seam allowance. The more seams a block has, the more chance that your blocks may vary in size. That’s simply due to our sewing techniques and whether we strive for accuracy on every block. I’ll be honest, I do not. Sewing for years has given me a pretty good eye for straight seam allowances, so it generally works out for me. Triangles are not so forgiving.
Any block with many smaller pieces means lots more seams. If your seam allowance isn’t consistent, your blocks will not all be the same size. That makes sense and we all deal with that on every quilting project. But when I sewed my HSTs with super accuracy, they were still off enough to need a trim.
How to Sew Multiple HSTs At Once With Accuracy
That’s where the HST block trimming comes in which I find to be incredibly tedious. The more HSTs we get from a block, the more chance there is for inconsistency. While I love the Magic 8 method, I found it frustrating to square up all those smaller blocks. It’s a great time saver to sew, but my block sizes seemed to be all over the place and required lots of trimming.
Additionally, my challenge was to figure out how to cut two different size HST blocks that would fit together for the design I wanted without trimming every single one of them. When sewing triangles, it’s not only important to take the time to sew very accurate seams, it’s also how you adjust your seam allowance. But a ¼” is a ¼” right? Well yes, and no. If you’ve ever measured your seam allowance with a ruler in different spots along a seam line, you may find some inconsistencies. Some places are a bit more while others are a bit less.
These seam allowance variances aren’t a huge factor in blocks with squares and rectangles. You can generally ‘fudge’ any troublesome spot when needed. But triangles demand a bit more attention. You could call them ‘high maintenance’ blocks that result in a beautiful quilt. Once you see how to conquer the challenge, you’ll be sewing HSTs without a second thought!
Time for A Seam Allowance Adjustment
The solution dawned on me when I was ironing my first blocks. When the seam is pressed to one side, the top fabric is folded over the seam. That little bit of fold takes up some extra fabric. Not much, but enough to make a slight difference. Add a few slight differences and you have a tangible difference you can not only see, but also measure. This is unique to quilting because we press the seam allowance to one side. Whereas most sewn items require the seams to be pressed open which equalizes how the fabric lays.
My solution was to adjust my seam allowance so when the HST seam is pressed to one side, the block size remains more consistent. Here’s what I did.
How to Perfectly Adjust HST Seam Allowances
Sewing the larger HST blocks required a single diagonal cut from one corner to the other. So, when I sewed the center seams on either side of my diagonal line, I took a generous ¼” seam allowance. We’re only talking a couple threads wider at most, but a slight adjustment makes a difference. Next, when I sewed the Magic 8 block, I sewed all the seams with a scant ¼’ seam allowance.
Essentially, I took a slightly larger seam allowance on the larger block so it would be just a bit smaller. Then I took a slightly smaller seam on the smaller blocks to make them just a bit bigger. Remember, the more seams the more chance for skewed blocks. So by adjusting the 8 HSTs seam allowance that were cut from one block, a bit of a ‘fudge factor’ was added. Those seam allowance adjustments worked fabulously!
I took the brave step of not trimming the blocks and everything fit together wonderfully. There was an occasional block that was a bit off, but it was easily adjusted when sewing the blocks together. This really got me excited and by the end of this quilt, I found that this method works great.
Modern Quilting with Layer Cake Half Square Triangles
How to Make 8 Half Square Triangles with Layer Cakes
This quilt is made using 10” layer cake blocks with some extra fat quarters for the background. When making the small HSTs from a 10” block, take a scant ¼” seam. When sewing and cutting 8 HSTs from a single 10” block, your 4-patch will measure 9”. Your finished block will be 8 ½”. This 4-patch equals the full size HST block.
When sewing and cutting 2 larger HSTs from a single 10” block, you’ll first cut it down to a 9” square. When making the large HSTs from a 10” block, take a generous ¼” seam. Once your diagonal seams are sewn and cut, your block will measure 9”. Now both size blocks will fit together for sewing.
Cutting HST Blocks Accurately
While the seam allowance adjustment works great, there is one other step you can take to improve your accuracy. When cutting your 8 HSTs from one block, place your ruler straight and precisely at the center. If you’re off a smidge in one direction, you’ll have 4 HSTs that are a bit larger than the others. Also, if your ruler is not laid perfectly straight but at a slight angle, you’ll have the same problem.
You can see in the photo above my cutting mat has a 10” square in the center which made this very easy. The first cuts I made were from top to bottom, then from side to side. By lining up the block within the 10” square, I easily placed my ruler exactly in the center. Then I cut the diagonal seams. Since they are already sewn, you really can’t miss. But if the vertical and horizontal cuts are off, you’ll have some wonky HSTs in your quilt!
These minor seam adjustments keep your block sizes more uniform. Remember, even if your seam is perfect, there is fabric fold-over. It may be just a thread or two, but with multiple seams it does add up when precise measurements count.
Sewing the Perfect HST Points.
Tricks to Perfect HST Points
Now that your blocks are uniformly sized, you need to get your points sewn perfectly as well. My recent HST post included my sewing techniques to solve that problem. Not surprisingly it’s essentially the same process. Chainstitch all your quilt blocks together in one direction with a generous ¼” seam first. Then sew the opposite seams with a scant ¼” for consistently perfect points. When you take that larger seam first, you are setting the point just a smidge farther away from the seam allowance. Then when you take a slightly narrower seam allowance to join those blocks, the points don’t get cut off.
I Love That the Same Solution Solved Both Problems!
This method drastically reduced my need to trim HSTs. Very few are off enough to worry about, which makes me happy. The last thing I want to do when I’m sewing a quilt, is trim every square after it’s been sewn. This has saved me so much time!
Once your seam allowance is adjusted and your HSTs are more uniform, your blocks will line up so much better. That makes perfect points even easier!
Now look above at these spectacular blocks lined up nice and square with perfect points demanding everyone’s attention! These two methods have not only simplified how I make HSTs but also gives me consistently great results. And that’s what this is all about. I want to make beautiful quilts which don’t include wasted time trimming blocks just because my seam allowance needed a slight adjustment. Here’s the beautiful, floral batik fabric I used for this quilt.
So, there you have it. Two simple seam allowance adjustments can make your next HST quilt a breeze to make. Your blocks with fit well without all that extra trimming and your points will look amazing! Have fun with your next HST quilt!