Aim: Determination of the effect of varying fenestration technique, and simulated patch graft on outflow facility for Baerveldt tube. Materials and methods: Silicone tubing similar to Baerveldt implant (AMO, Santa Ana, CA) with different fenestrations techniques was connected to a digital manometer in a closed system with a fluid-filled syringe on a stand to adjust pressure. The venting slits included: (A) 4 piercings with 7–0 TG140-8 needle; (B) a 2-mm slit with a 15° blade; (C) 4 piercings with a 15° blade; (D) 9–0 Nylon on CS140-6 needle with suture stenting the fenestration. Results: For pressures of 10, 20, 30, 40 mm Hg in groups A to D, the average outflow facility (mL/min/mm Hg) were group A: 0.11, 0.20, 0.28, 0.40; group B: 0.30, 0.69, 0.98, 0.93; group C: 0.73, 0.80, 0.81, 0.88; group D: 0.58, 0.65, 0.80, 0.87. For external compression with 10 gram weights at pressures of 10, 20, 30, 40 mm Hg, outflow were group A: 0.0, 0.18, 0.20, 0.53; group B: 0.75, 0.70, 0.97, 1.21. Group C: 0.18, 0.03, 0.57, 0.04. Group D: 0.73, 0.90, 1.13, 0.91. Conclusion: Effectivity of venting slits in maintaining adequate IOP in the early postoperative period for non-valved glaucoma implant is variable, multifactorial and largely intraocular pressure (IOP) dependent. Clinical significance: This study explores methods of producing fenestration and the effects on outflow at different pressures in an attempt to determine which fenestration technique has more reproducible results that can be made applicable in clinical practice. This is also the first study to evaluate the effect of external pressures similar to scleral patch graft on the tube fenestrations.