Above: Members of the Border Regiment in their front-line
trench, Thiepval Wood, the Somme, August, 1916.
(photo pinched from Douglas Sutherland's "Tried and Valiant")
The image above is an excellent example of shelters used in the trenches during the Great War. At the top, a soldier rests under a ground sheet that has been staked or pinned to the trench parapet. This type of shelter is perfectly described by Sergeant Earnest Boughton of the 15th London Regiment:
"[France, March 27, 1916], A peg of wood will help suspend a groundsheet against the dripping clay wall and a man may flatten himself under this looking like part of the trench. (Housman, p 201)."
Other shelters were also employed as well. Also shown in the photo are several soldiers sitting in "funk holes" cut into the trench wall. Note that the shelters are against the front wall of the trench -- as evidenced by the firing steps just left of the center of this image.
Shelters were also made of sandbags, which sometimes included a corrugated iron roof: "Off go our packs, myself and four others have been lucky enough to get a sandbag shelter with a little straw on the ground [Pvt. Frank Bignall, 11th British Columbia Regiment, CEF, Flanders, April 18, 1915] (Housman, p 51).
The Field Service manual suggests several makeshift shelters, including a four-man tent made by lashing two ground sheets or blankets together (see illustration below). The remaining blankets or ground sheets are used by the four occupants to keep warm. (Field Service, pp 44 & 47)
Dugouts were also common. An excellent description of one of these comes from another letter by Sergeant Boughton:
"... a long, narrow chamber hewn out of chalk by the French originally, with ceiling of pine pit props. In it were extended some fifteen men. Along the walls were niches made to hold candles, and across the narrow entrance sacking and a waterproof sheet kept the reflection of light from the enemy some few hundred yards away. Rifles and such parts of the equipment as could be discarded lined the walls. (Housman, p 198)"
Unfortunately, in rainy weather these dugouts often became catch basins for the rainwater, flooding out the occupants and floating all of their belongings off in the current (Housman, pp 194-195)