Improving drainage from drainpipes
Locate all downspouts that empty water from gutters into areas directly next to foundation or crawlspaces. If possible, check outflow on a rainy day to determine where water flows. Your goal is to deflect water from accumulating and soaking into the ground directly next to the foundation or crawlspaces.
Purchase and install deflectors for all downspouts that empty water close to the house. Deflectors can include curved drainpipe segments, plastic rollout sleeves and drainage plates to place under downspouts. Ideally, deflectors should empty onto bare ground, rather than paved surfaces, unless the paved surface is sloped away from house walls. A rollout sleeve that sends water into a downhill driveway, for example, moves water away effectively.
Check the effectiveness of these additions on another rainy day. Extend or expand deflectors if water still accumulates close to the house during hard rain.
Creating a ground-based vapor barrier
Line ground under crawlspaces and porches with heavy plastic sheeting. Use waterproof tape to hold segments of sheeting together, to prevent vapor leakage. Sheeting should extend to cover the entire groundspace.
Anchor sheeting in place with rocks or a shallow (1 inch) layer of sand. Use only enough sand to hold sheeting down; large piles of sand will themselves retain moisture. Use landscape-cloth anchors to hold small barriers in place.
Check anchoring system after heavy windstorms or soaking rains. Repair tears with tape or an additional layer of sheeting.
Building additional drainage
Dig French drains to deflect water that may drain downhill toward the house. French drains consist of trenches, roughly 1 foot deep and 1 foot wide, lined with gravel, topped with sand and then covered with turf.
Place drains toward the bottom of land sloping down toward the foundation. This provides areas that collect water before it drains downhill to the house. Drains should be at least two feet from foundation walls or crawlspaces; length depends on the area of water flow.
Supplement downspout deflectors with French drains in areas of high water accumulation. Begin digging drain at the bottom of the downspout and extend for several feet away from the house.