Ohalot 15:1 says that a pile of garments, where the uppermost garment is at least a tefach off the ground, counts as an ohel. This is presumably because the uppermost garment is considered the 'roof' and the lower ones, being themselves susceptible to tumah, don't block the tumah and so we treat them as if they're not there.
However, 15:1 goes on to say that the same applies to a pile of wooden boards. But wooden boards are not susceptible to tumah, so how come they don't block it? I think the Tiferet Yisrael explains this but I can't follow his explanation. Can someone help?