I'm using 2×4 timber cleats to support the treads for a set of stairs going up to a deck. The stringers are 2×10 and both the stringers and 2×4 are pressure treated pine. What would be the best size and type of fastener to use? I was planning on using M10 carriage/coach bolts.
What’s the best way to attach timber cleats to stair stringers
fastenerstairs
Related Solutions
Toenailing through the baluster on the uphill side with a finishing nail gun should be quite sufficient. The plowed channel holds them laterally. If you wanted extra strength, you could add glue just before nailing.
16 or 18 gauge nails about 1 inch to 1 1/4 long should suffice. The hole will likely be nearly invisible.
Additionally, plowed railings usually come with a filler strip that is angle cut to sit between the balusters after they are attached. If you use the nailing gun (and glue if you wish) to attach those as well, they would further lock in the baluster.
You can use 2 pcs of 5/4X6" to replace each tread. It will keep the same dimension as your originals, 5 1/2" + 5 1/2" + 1/4" gap = 11 1/4" This is a very typical detail. You should have an overhand at the front corner, call a "nose". Your originals may already have that, if so, simply follow the same detail, but using the two pieces per tread.
Edit added sketch of treads, etc.
Best Answer
You're supposed to figure that out from the load table mumbo jumbo.
TABLE 9.14 LAG SCREW (BOLT) CAPACITY FOR SINGLE SHEAR CONNECTIONS (LBS.), page 69, Chicago deck code, PDF.
3/8ths (M10) is the middle of the road; it's better than using 1/4" or 5/16". Use stainless steel fasteners. You must pre drill your holes for cleats. Ideally it's an actually cut stringer; notice how cleats aren't even an option.