There is no customs duty on building materials, but there is VAT. You must declare the goods at the border if you are bringing goods over the value limit.
If you purchase goods with a total value below the value limit, you do not need to pay VAT or other fees. The value limit is NOK 3,000 or NOK 6,000, depending on how long you have been outside Norway, and applies when you personally travel with the goods.
If you have shopped over the value limit, you will need to pay 25 percent VAT on the purchase amount.
Value Limit
More than one day
If you have been outside Norway for more than 24 hours, you can bring goods worth up to NOK 6,000 without paying customs duty and other fees. This value limit is the total sum of the goods you have purchased, except for quota goods.
Less than one day
If the stay abroad is shorter, you can once within 24 hours bring goods worth up to NOK 3,000 customs and duty-free. This value limit is the total sum of the goods you have purchased, except for quota goods.