一、天沟外排水系统的组成
　　外排水系统是用屋面明沟(称为天沟)收集并排泄屋面雨雪水至设于外墙上的立管，并通过该立管特此部分雨雪水排入室外雨水明沟或地下暗管的雨水排除系统。或者是用屋面成天沟集水而通过设于屋面上的雨水到室内悬吊管输送至室外墙的立管再排入室外面水道的系统。因此天沟外排水系统应由天沟，雨水斗、排水立管等组成。
　　二、天沟的设置及作法
　　采用天沟外排水系统时，应以建筑的伸缩缝或沉降缝作为反面分水线，为了防止天沟末端处积水，应在天沟的末端设置溢流口，蹬流口的下缘比天沟上绦低50一 100mm，天沟过水断面视屋面的情况，可以是矩形，梯形等。天沟坡度不宜太大，以免屋顶垫层过厚而增加结构的荷载重量：但也不宜太小，以免屋顶漏水和施工困难，坡度一般以0.003~0.006为宜，且天沟长度应小于50m。
　　天沟的作法：天沟的作法一般是在屋面板上铺泡沫混凝土及炉渣，用二毡三油作为防水层，表面上再铺一层沥青玛脂，上面撒一层绿豆砂。为减小天沟表面的粗糙度，改善水力条件，也可以用水泥砂浆抹面。目前，天沟多用预制钢筋混凝土槽板，表面用1：2砂浆抹面。天沟出流措施是延长天沟穿出山墙，然后连接立管沿外墙排出。

Composition of the outer drainage system
The outer drainage system is a rainwater removal system that collects and discharges roof rain and snow water to a riser on the outer wall using the roof Minggou(known as the sky ditch), and through which part of the rain and snow water is discharged into the outdoor rainwater Minggou or underground dark pipes. Or it is a system that uses the roof to collect water in the ditch all day and sends it through the rainwater on the roof to the riser of the outer wall of the room and then discharged into the waterway outside the room. Therefore, the outer drainage system of the trench should consist of the trench, the rain bucket, and the drainage Guandeng.
Establishment and Practice of Tiangou
When adopting the outer drainage system of the trench, the extension seam or settlement seam of the building should be used as the opposite watershed. In order to prevent the accumulation of water at the end of the trench, a spillway should be set at the end of the trench, and the lower edge of the spillway should be lower than the trench. 50100mm, The water section of the trench may be rectangular, trapezoidal, etc. depending on the roof. The slope of the trench should not be too large to prevent the roof cushion from being too thick and increasing the load weight of the structure: But it should not be too small to avoid leakage and construction difficulties on the roof. The slope is generally suitable for 0.003 to 0.006, and the trench length should be less than 50m.
Tiangou's approach: Tiangou's approach is generally to spread foam concrete and slag on the roof of the house, use two felts and three oils as a waterproof layer, and cover the surface with a layer of asphalt grease, sprinkled with a layer of mung bean sand. In order to reduce the surface roughness of the trench and improve the hydraulic conditions, cement mortar can also be used to rub surface. At present, Tiagou mostly uses prefabricated reinforced concrete slabs with a 1:2 mortar surface. Tiangou outflow measures are extended Tiangou through the gable, and then connected to the riser along the external wall discharge.