diff --git a/3rdparty/CMakeLists.txt b/3rdparty/CMakeLists.txt
index 51900e92..39b8217c 100644
--- a/3rdparty/CMakeLists.txt
+++ b/3rdparty/CMakeLists.txt
@@ -2,7 +2,14 @@ if(CMAKE_SYSTEM_NAME STREQUAL Emscripten)
   set(BOOST_INCLUDE_LIBRARIES random stacktrace math)
   set(BOOST_ENABLE_COMPATIBILITY_TARGETS 1)
   add_compile_options(-fdeclspec)  # math doesn't build otherwise
-  add_subdirectory(boost EXCLUDE_FROM_ALL)
+
+  # Use out-of-tree sources for boost, since we can't use pre-built
+  # libraries for the web build
+  if(BOOST_SOURCE_ROOT)
+    add_subdirectory(${BOOST_SOURCE_ROOT} boost EXCLUDE_FROM_ALL)
+  else()
+    message(FATAL_ERROR "For web builds BOOST_SOURCE_ROOT must be provided")
+  endif()
 
   # TODO: make this available via find_package so we wouldn't have to
   # special-case it in every library
diff --git a/package/web/package.sh b/package/web/package.sh
index ce5a5a85..3cdb20db 100755
--- a/package/web/package.sh
+++ b/package/web/package.sh
@@ -13,6 +13,7 @@ rm -rf ./*
 # Use emsdk wrapper for cmake - it sets some env variables to select the
 # right toolchain.
 emcmake cmake \
+  -DBOOST_SOURCE_ROOT=$(realpath ../source)/psemek/3rdparty/boost \
   -DCMAKE_BUILD_TYPE=RelWithDebInfo \
   -DPSEMEK_PACKAGE_MODE=ON \
   -DPSEMEK_PACKAGE_HOST=OFF \