While some like trimming down 3-tab shingles for a starter strip (or add tar), I generally use starter strip & make sure it is laid out at the angle I need. (At the top of the valley you should have a 2+” gap & it should increase by 1/8” of an inch for every foot of valley). Carpenters don't refer to the angle of a roof, such as 30° or 45°, but to the pitch of the roof, which is just the ratio of vertical to horizontal measurements. I say this because it's quite an important distinction. Angles are pretty hard to measure, but roof pitch is simple. An average and easy roof pitch to work with is a 5-12.
With regards to the pitch, the building standard code BS 5534 recommends a minimum pitch of 200 for slate roofs. But since there is still a wide variety of slate roofing systems, there are occasions when the minimum pitch can go down to as low as 15 0 depending on the uniqueness of the roof.
Follow manufacturers recommendations for roof pitch, fastener spacings and type, support spacings and other laying procedures when installing products, Avoid the use of black carbon ('lead') pencils for marking sheets, Use metal cutting discs or shears to cut sheets rather than angle grinder discs. Use only neutral cure silicone rubber sealants, VersaTube manufactures Do-It-Yourself steel building kits. No cutting, welding or heavy equipment required and every framing component can be lifted with average human strength and assembled with common household tools. Save time and money on your next building project with VersaTube Building Systems. Slip-Fit Technology