As shown. This is the latest import tariff rate published in 01/2011 by the Mexican Ministry of Economy.
The top left corner is the customs code and the top right corner is the product category. The yellow piece in the center is some of the countries that are exempt from import duties. The bottom piece is the part of the country that charges import duties.
The product you want is building materials. The unit is "kilogram", the import tariff rate of 7%. The export tariff rate is 0%.