A manifold assembly configured for integration with a joint cover apparatus in a ducting system to detect leaking gas, the manifold assembly comprising: an elastomeric manifold (503, 703) block having a substantially circular inlet aperture and one or more gas passages having upper ends, said one or more gas passages being fluidly coupled to the inlet aperture, wherein the upper ends of said one or more gas passages are configured to direct leaking gas toward one or more respective gas detectors; and a flow control valve (505, 705) disposed within the manifold block between the inlet aperture and the one or more gas passages, said flow control valve configured to maintain a closed state in which a component of the flow control valve sealingly engages with the inlet aperture to preclude gas at the inlet aperture from flowing into the one or more gas passages until said gas reaches a pressure corresponding to a pressure threshold; characterized in that said manifold assembly further comprises at least one aperture plate (504, 704) having a substantially circular opening, said at least one aperture plate being formed from a rigid metallic material, said at least one aperture plate being rigid for maintaining the substantial circularity of the inlet aperture and being disposed substantially proximate to a lower end of the manifold block, said circular opening of the at least one aperture plate being in substantial alignment with the inlet aperture of the manifold block, said elastomeric manifold block including an annular portion (601, 801) extending radially inward of the circular opening that is configured to deform and compress against the circular opening of the aperture plate by said component of the flow control valve, to yield a fluid tight seal between the inlet aperture and the one or more gas passages until said gas reaches said pressure threshold, wherein the diameter of the inlet aperture of the elastomeric manifold block at the annular portion is less than the diameter of the circular opening of the aperture plate. an elastomeric manifold (503, 703) block having a substantially circular inlet aperture and one or more gas passages having upper ends, said one or more gas passages being fluidly coupled to the inlet aperture, wherein the upper ends of said one or more gas passages are configured to direct leaking gas toward one or more respective gas detectors; and a flow control valve (505, 705) disposed within the manifold block between the inlet aperture and the one or more gas passages, said flow control valve configured to maintain a closed state in which a component of the flow control valve sealingly engages with the inlet aperture to preclude gas at the inlet aperture from flowing into the one or more gas passages until said gas reaches a pressure corresponding to a pressure threshold; characterized in that said manifold assembly further comprises at least one aperture plate (504, 704) having a substantially circular opening, said at least one aperture plate being formed from a rigid metallic material, said at least one aperture plate being rigid for maintaining the substantial circularity of the inlet aperture and being disposed substantially proximate to a lower end of the manifold block, said circular opening of the at least one aperture plate being in substantial alignment with the inlet aperture of the manifold block, said elastomeric manifold block including an annular portion (601, 801) extending radially inward of the circular opening that is configured to deform and compress against the circular opening of the aperture plate by said component of the flow control valve, to yield a fluid tight seal between the inlet aperture and the one or more gas passages until said gas reaches said pressure threshold, wherein the diameter of the inlet aperture of the elastomeric manifold block at the annular portion is less than the diameter of the circular opening of the aperture plate.